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Habegger, Maria Laura.
Bite force in two top predators, the great barracuda, Sphyraena barracuda and bull shark, Carcharhinus leucas, during ontogeny
h [electronic resource] /
by Maria Laura Habegger.
[Tampa, Fla] :
b University of South Florida,
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Thesis (M.S.)--University of South Florida, 2009.
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ABSTRACT: Functional morphologists have extensively used measurements of performance to investigate the relationship among form, function and ecology through ontogeny. Among different measurements of performance bite force play a crucial role influencing fitness. Although, bite force has been thoroughly investigated among vertebrates, the majority of the studies on fishes have been concentrated only in small species. Consequently, this is the first study that compares the bite force performance in two large marine predators, the great barracuda (Sphyraena barracuda) and bull shark (Carcharhinus leucas). Values of posterior bite force in S. barracuda varied from 3 258 N for an ontogenetic series of 27 individuals (23 130 cm, TL). Bite force as well as the majority of the biomechanical variables that contribute to it scaled with isometry in S. barracuda. Values of posterior bite force in C. leucas varied from 170 5,914 N for and ontogenetic series of 16 individuals (73 285 cm, TL). Bite force at the most anterior bite point scaled with positive allometry as well as the majority of the subdivisions of the adductive musculature that greatly contribute to bite force.Bite force performance in this two species showed strong differences, where S. barracuda has one of the lowest relative values of bite force among fishes and C. leucas has one of the largest ones. Additionally, the scaling patterns for bite force and most of the biomechanical variables investigated in this study differed among these two species. These results suggest that predatory success may be acquired by different strategies, and that the same ecological role in a marine ecosystem may be reached by having different bite force performance.
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Advisor: Philip J. Motta, Ph.D.
t USF Electronic Theses and Dissertations.
Bite F orce in Two Top Predators, the Great Barracuda, Sphyraena barracuda and Bull Shark Carcharhinus leucas, D uring Ontogeny. by Mara Laura Habegger A thesis submitted in partial fulfillment of the requirements for the degree of Master of Scien ce Department of Biology College of Arts and Science University of South Florida Major Professor: Philip J. Motta, Ph.D. Steven Deban, Ph.D. Daniel Huber, Ph.D. Date of Approval: April 3, 2009 Keywords: biomechanics, performance, functional morph ology, scaling, ecomorphology Copyright 2009 Mara Laura Habegger
Dedication To my parents, for their unconditional support and love, for being by my side even from
Acknowledgments space expressing my gratitude to all the people that helped me through these years, but there is one person that made this dream come true and I will never forget his generosity. I want to express my most sincere gratitude to this person, Dr. Philip Motta for giving me the opportunity of my life, for believing in my work. Thank you for having the ability to make each day an amazing learning experience, for having the unique quality of being an excellent professional and an amazing human being. I really don support. I also would like to thank my committee, Dr. Deban and Dr. Huber who deserve a committee, I have been honored to have the opportunity of being surrounded by such knowledgeable people. Thank you for your support and continuous feedback. To all the people that contributed with specimens: Capt Dave Zalewsky, Jack Morris Dr. Gelsleichter, Armando Ub eda. Special thanks to Dr. H e u pel not only for the specimens provided but also for being one of the first people to believe in me. To the lab for their constant help and support. To Kyle for being a life saver many times. To Lisa for being always there wil ling to make my life easier and off course for showing me Staples and so sharing many ta lks, laughs and hugs. To my very good friend Pilar for giving me the Spanish breaks that sometimes I need so badly when my English was tired, for finding
always a few minutes to share with me. To Cruz and Julia for being my best friends for almost ten year s for making distances shorter and showing me there are not boundaries for true friendship. To my sister Ianina for always being there. To Jose for sharing this whole experience since the beginning. To my best friend Tomy for 15 years of unconditional com pany and love where words where unnecessary. To Martin, I really generosity, for living everything to share your live with me, and for becoming every day a little bit more a b iologist. For putting my research and passion, so many times, on the with you.
i Table of Contents List of Tables iii List of Figures iv Abstract vi Chapter 1 : Introduction 1 Functional Morphology 1 Bite Force 2 Ontogeny 6 Feeding Performance in Top Predators 7 Chapter 2 : Feeding Biomechanics in the Great Barracuda ( Sphyraena barracuda ) During Ontogeny 10 Introd uction 10 Materials and Methods 13 Electromyography 14 Results 16 Anatomy 16 Prey C apture and Muscle Activity 17 Theoretical Calculations of Bite F orce 18 Discussion 19 Conclusions 25 Ch apter 3 : Feeding Biomechanics of the Bull Sharks ( Carcharhinus leucas ) During Ontogeny 40 Introduction 40 Materials and Methods 43 Results 46 Discussion 47 Conclusions 52 Chapter 4 : General Discussion 64 References 70
ii Appen dices 89 Appendix 1: Values of bite force of five tooth positions in S. barracuda 90 Appendix 2: Lever distances in S. Barracuda 91 Appendix 3: In lever distances (cm) for each subdivision of the adductor mandibulae 95 Appendix 4: Values of bite force of five tooth positions in C. leucas 93 Appendix 5: Lever distances in C. leucas 94 Appendix 6: In lever distanc es (cm) for each subdivision of the adductor mandibulae 95
iii List of Tables Table 1 A bsolute values of bite force in 27 individuals of S. barracuda 30 Table 2 Scaling patterns of bite force in S barracuda 36 Table 3 Values of anterior bite forces (ABF), mass removed bite force for 20 species of fishes obtaine d from the literature 37 Table 4 Absolute values of bite force in 16 individuals of C. leucas 54 Table 5 Scaling patterns of bite force in C leucas 61 Table 6 Values of anterior bite forces (ABF) for nine chondrichthyan species and one bony fish gathered from the literature 62
iv List of Figures Figure 1 Drawing of Sphyraena barracuda 26 Figure 2 a Left lateral view of the adductive muscles of S. barracuda 27 Figure 2b Left lateral view of the adductive muscles of S. barracuda 28 Figure 3 Composite diagram for the muscular activity of S. barracuda 29 Figure 4 Bite force (N) vs. total length (cm) in S. barracuda 31 Figure 5 Log transformed values of cross sectional area (cm 2 ) of the adductor mandibulae muscle divisions of for S. barracuda linearly regressed against log transformed values of total length (cm). 32 Figure 6 Log transformed va lues of mechanical advantage for S. barracuda linearly regressed against log transformed values of total length (cm) 33 Figure 7 Log transformed values of weighted in lever (cm) for S. barracuda linearly regressed against log transformed values of total length (cm) 34 Figure 8 Log transformed values of out lever distances (cm) for S. barracuda linearly regressed against log transformed values of total length (cm) 35 Figure 9 Residual values of anterior bite force plotted against body mass for 20 species of fishes. 38 Figure 10 Premaxillary rotation in S. barracuda 39 Figure 11 Illustration of Carcharhinus leucas 53 Figure 12 Bite force (N) vs. total length (cm) in C. leucas 55 Figure 13. Log transformed values of cross sectional area (cm 2 ) of the adductor mandibulae muscle divisions of C. leucas linearly regressed against log transformed values of total length (cm) 56
v Figure 14. Log transformed values of cross s ectional area (cm 2 ) of the adductor mandibulae muscle divisions of for C. leucas linearly regressed against log transformed values of total length (cm). 57 Figure 15. Log transformed values of mechanical adv antage of for C. leucas linearly regressed against log transformed values of total length (cm). 58 Figure 16. Log transformed values of weighted in lever (cm) of for C. leucas linearly regressed against lo g transformed values of total length (cm ) 59 Figure 17. Log transformed values of out lever distances (cm) for C. leucas linearly regressed against log transformed values of total length (cm). 60 Figure 18. Residual values of anterior bite force plotted against body mass for 10 fish species. 63
vi Bite F orce in Two Top Predators, the Great Barracuda, Sphyraena barracuda and Bull Shark Carcharhinus leucas, D uring Ontogeny Maria Laura Habegger ABSTRA CT Functional morphologists have extensively us ed measurements of performance to investigate the relationship among form, function and ecology through ontogeny. Among different measurements of performance bite force play a crucial role influencing fitness. Although, bite force has been thoroughly investigated among vertebrates, the majority of the studies on fishes have been concentrated only in small species. Consequently, this is the first study that compare s the bite force performance in two large marin e predators, the great barracuda ( Sphyraena barracuda ) and bull shark ( Carcharhinus leucas ). Values of posterior bite force in S. barracuda varied from 3 258 N for an ontogenetic series of 27 individuals (23 130 cm, TL). Bite force as well as the major ity of the biomechanical variables that contribute to it scaled with isometry in S. barracuda Values of posterior bite force in C. leucas varied from 170 5,914 N for and ontogenetic series of 16 individuals (73 285 cm, TL). Bite force at the most ante rior bite point scaled with positive allometry as well as the majority of the subdivisions of the adductive musculature that greatly contribute to bite force. Bite force performance in this two species showed strong differences, where S. barracuda has one of the lowest relative values of bite force among fishes and C. leucas has one of the largest ones. Additionally, the scaling patterns for bite force and most of the biomechanical variables investigated in
vii this study differed among th ese two species. These results suggest that predatory success may be acquired by different strategies, and that the same ecological role in a marine ecosystem may be reached by having different bite force performance.
Chapter 1: Introduction Functional Morphology Functi form and function for at least five decades (Alexander, 1968; Arnold, 1983; Wainwright, 1989; Huber et al., 2005). Diverse anatomical aspects from many organisms have been associated wit h particular functions and subsequently linked to their ecology (Wainwright, 1988, 1987; Hernandez and Motta, 1997; McBrayer, 2003; Verwaijen et al., 2002). Consequently, functional morphology has become a powerful explanatory tool for organism ecology (Wa inwright, 1994). Among different anatomical systems, cranial morphology has been extensively studied due to its strong association with food acquisition (Motta and Wilga, 1995; Herrel et al., 2001; Verwaijen et al., 2002; McBrayer, 2004; Huber et al., 2005 ) and consequently survival. Moreover, skull morphology has been demonstrated to have a strong association with organismal ecology (Herrel et al., 1999; McBrayer, 2004). Nevertheless the link among form, function, ecology and fitness is sometimes difficult to quantify; measurements of performance can be a useful tool for investigating these associations and serve as a surrogate for fitness. Moreover, the limits of performance are determined by the morphology of an organism. Thus, design may constrain the ab ility to perform many behaviors (Wainwright, 1994).
2 Bite Force Bite force plays a crucial role as a measurement of feeding performance because it is related to food acquisition and therefore organismal survival (Binder and Van Valkenburgh, 2000; Herrel e t al., 2001; Huber et al., 2004, 2005, 2006). Bite force is also an important tool for understanding the diversity of cranial form among vertebrates (Herrel et al., 2001, 2007; Huber and Motta, 2004). Consequently, bite force has been investigated among el asmobranchs (Huber and Motta, 2004; Huber et al., 2005; Wroe et al., 2008; Huber et al., 2009), bony fishes (Hernandez and Motta, 1997; Korff and Wainwright, 2004; Grubich et al., 2007), reptiles (Herrel et al., 2001, 2002; McBrayer and White, 2002), birds (Herrel et al., 2005), and mammals (Binder and Van Valkenburgh, 2000; Dumont and Herrel, 2003; Wroe et al., 2005) Measurements of bite force may be obtained by using different approaches. In vivo measurements are commonly obtained by the use of force tr ansducers, two parallel metal plates connected by strain gauges that are able to quantify the amount of deformation exerted in each bite (Anderson et al., 2008). The design and utility of this force transducer is related to several factors such as animal s ize, habitat or beh avior (see Erickson et al., 2003 for an example). To obtain in vivo measurements several approaches may be used: voluntary bites ( in situ ), restrained bites, or electrical induced bites (Huber et al., 2005). In situ bite force is obtain ed when animals naturally bite the transducers, and may be elicited by natural aggressive behaviors (Herrel et al., 2001) or by feeding stimulation (Huber et al., 2005). Restrained bite force, however, involves the isolation of the organism and the placeme nt of the transducer between the jaws (Huber et al., 2005; Erickson et al., 2003 ). For stimulated bite force, animals are generally sedated
3 while the adductive musculature is electrically stimulated to tetanic contraction with a transducer between the jaw s. Bite force can also be estimated by theoretical calculations. These are obtained by the creation of 3 D force vectors generated by the adductive musculature in static equilibrium conditions. This approach has proven to be statistically similar to some o f the techniques previously described (Huber and Motta, 2004). Moreover the use of theoretical calculations overcomes the logistical difficulties associated with the study of bite force in large or dangerous animals and also allows the calculation of mecha nical variables that determine performance capacity (muscle force, leverage). In addition to the variety of techniques used to measure bite force, several proxies related to bite performance has been described to allow an easy estimation of bite force from simple morphological measurements These, m orphological measurements including head length, head width, and head depth have been established as good predictors of bite force in bony fishes and elasmobranchs (Carothers, 1984; Wainwright 1987; Huber et al. 2006) In the blacktip shark Carcharhinus limbatus a strong relationship of bite force to prebranchial length and head width was found, (Huber et al., 2006). Different head morphologies may be related to bite force; wider heads may provide more space to accommodate larger jaw adductor muscles without interfering with the space occupied by other organs (Herrel et al., 2001; Huber et al., 2009). Besides cranial geometry, several other variables may affect bite force. Mechanical advantage (MA) involves two opposite measurements of performance, force and velocity, represented by the ratio of the in lever (distances from the different points of muscular insertion to the jaw joint) and out lever (distance from each bite point to the
4 jaw joint). High values of m echanical advantage are characteristic of a force efficient jaw and low values are associated with a more speed efficient jaw. Low values of mechanical advantage were reported in ray finned fishes (e.g. 0.03 in Strongylura incise anterior mechanical adva ntage ) (Westneat, 2004). High values of mechanical advantage have been found in Chondrichthyes (e.g. 1.45 in Hydrolagus colliei posterior mechanical advantage ) (Huber et al., 2008). However, general values of mechanical advantage found in bony fishes rang ed from 0 to 0.7 (Westneat, 2004). From this we infer, if muscle insertion is consistent, that long jaws are more speed efficient whereas short jaws are more force efficient (Westneat, 2004). ecology, especially during prey capture. Prey capture method may be, in some cases, predicted by mechanical advantage. Speed efficient jaws may be more frequently found in predators that feed upon elusive prey, for example silversides (Atherinidae) prey u pon plankton and have characteristic fast jaws (Westneat, 2004). Force efficient jaws may be frequently found in predators that prey upon hard or less elusive prey types. In hard prey specialist such as the spotted ratfish ( Hydrolagus colliei ) or horn shar ks ( Heterodontus francisci ) high values of MA ( 1.45 and 1.26 respectively) were reported (Huber et al., 2008; Kolmann and Huber, in prep ). However it is important to address that even with low values of mechanical advantage, bite force performance can stil l be high for example in Carcharodon carcharias low values of mechanical advantage characterize its jaw as a speed efficient jaw however, the presence of large adductor muscles were primarily responsible for large values of bite force (Wroe et al., 2008)
5 Another variable that governs bite force is cross sectional area (CSA) of the adductive musculature. An increase in CSA is associated with higher bite force, other variables remaining constant (Herrel et al., 2005). Additionally, diversity of muscle fibe r architecture such as pinnation may influence force production as well. Pinnation is a way to pack a larger amount of muscle fibers into the available space and provide a more precise insertion (Gans and Gaunt, 1991). Bite force has been strongly associat ed with feeding ecology (Hernandez and Motta, 1997; Herrel et al., 2001; Huber et al., 2006) as well as territorial interactions (Anderson and Vitt, 1990) and defensive behaviors in lizards (Hertz et al., 1982; Lailvaux et al., 2004). Studies on the ecolog ical consequences of changes in bite force and head morphology in Anolis lizards (Herrel et al., 2006) suggested that disproportional increase in bite force during ontogeny may be associated with differences in morphology and then linked to shifts in diet. Bite force was found significantly correlated to the size of consumed prey and also to its hardness and increases prey capture efficiency by reducing processing time (Verweijen et al., 2002; Herrel et al., 2006). Moreover, bite force has been strongly lin ked to the success of larger lizards in aggressive interactions with conspecifics. The b enefits of high bite force in male lizards arose by sexual selection as well as by their success in holding the female during copulation (Herrel et al., 2001).
6 Ont ogeny Interspecific variation in form and function has been thoroughly studied by functional morphologist (Wainwright, 1988; Herrel et al., 2001; Huber et al., 2009) However intraspecific variation in form function complexes also occurs through ontogeny, resulting in changes of performance with age and size. Variation in form and function through ontogeny may dramatically influence organismal performance (Herrel and Gibb, 2005) and have strong consequences on the ecology, behavior and physiology of organi sms (Peters, 1986; Calder 1984; Brown and West 2000). Consequently, when our interests are focused on these associations in a particular organism, ontogen y should be taken into account. During organismal ontogeny drastic changes in anatomy, physiology and behavior may occur (Schmid example, tooth morphology in Alligator mississippiensis changes through ontogeny, from a needle like shape into a more robust spike like shape and these changes are ass ociated with change s in diet (Erickson et al., 2003 ). Ontogenetic changes in bite force have been used as a measure of performance to explain this association through ontogeny (Huber et al., 2006; Herrel et al., 2006; Vincent et al., 2007). Juvenile and ad ult bite performance are expected to differ (bigger individuals will bite harder than smaller ones), since bite force is proportional to the length (to the second power) and mass (to the third power) of Allometric chan ges in bite force during ontogeny may be associated with dietary shifts, variations in prey type or size. For example, in lizards, prey hardness was highly
7 2006). Larger liz ards incorporate in their diet not only larger but harder prey through dietary specialization and reduce dietary breadth as well (Wainwright, 1988). In an ontogenetic seri es of sheepshead Archosargus probatocephalus, an ontogenetic increase in crushing performance was significantly correlated with an increase in the amount of hard prey consumed (Hernandez and Motta, 1997). This species was trophically constrained by bite fo rce during ontogeny since a clear switch in diet occurred from soft prey to primarily hard prey (Hernandez and Motta, 1997). Increases in crushing performances through ontogeny have been associated with switches in diet and reduction in dietary breadth in labrid fishes as well (Wainwright, 1988) Feeding Performance in Top P redators Marine top predators like sharks or large bony fishes play a vital role influencing the structure and function of marine communities (Heithaus et al., 2007). Through trophic cascades, top predators may exert profound effect influencing lower trophic levels, even regulating primary production (Carpenter et el, 1985). Changes in abundance of key predators have been shown to have severe consequences for community structure (Shea rs and Babcock, 2002) by the loss of trophic cascades (Pace et al., 1999) or the increase in abundance of mesoconsumers, and consequently the decline of resource species (Myers et al., 2007). Moreover, the removal of apex predators can lead to total degrad ation of ecosystems (Myers, et al., 2007) and affect several trophic levels as well as fisheries (Heithaus et al., 2007)
8 The majority of the studies on bite force in fishes are confined to relatively smaller species or those from lower trophic levels (e.g sheephead Archosargus probatocephalus Hernandez and Motta, 1997; hogfish Lachnolaimus maximus Clifton and Motta, 1998; striped burrfish Chilomycterus schoepfi Korff and Wainwright, 2004; spiny dogfish Squalus acanthias, Huber et al., 2004 and horn sha rk Heterodontus francisci Huber et al., 2005 ). Surprisingly, few studies have focused on larger predatory fishes (lemon sharks Negrapion brevirostris (Huber, 2006) and white sharks Carcharodon carcharias (Wroe et al., 2008)) and just one study has descri bed the bite performance of an apex predator during ontogeny, the blacktip shark Carcharhinus limbatus (Huber et al., 2006). Top predators such as C. carcharias have been reported to undergo ontogenetic shifts in diet, where small sharks prey primarily upo n fishes but larger individuals change their diet almost exclusively to mammals (Klimley, 1985). In general, larger sharks commonly consume larger prey so larger absolute values of bite force are expected to occur in these predators (Huber et al., 2006, 20 09). Additionally, larger predatory fish may be able to exploit larger or harder prey resources that cannot be exploited by smaller conspecifics, reducing competition. For example, only large sharks such as white sharks Carcharodon carcharias, tiger sharks Galeocerdo cuvier and bull sharks Carcharhinus leucas have been found to predate on cetaceans (Long and Jones, 1996; Cliff and Dudley, 1991; Heithaus, 2001a, 2001b). In this study I compare the bite performance, feeding biomechanic s and feeding ecology d uring ontogeny of two marine top predators, the great barracuda ( Sphyraena barracuda ) and the bull shark ( Carcharhinus leucas ). Like other top predators (Shears and Babcock, 2002) the great barracuda and bull shark may exert top down influence on
9 their res pective fish communities. The study of bite force through ontogeny in these top predators may provide insights into their feeding ecology and prey selection. Consequently, values of bite force in apex predators may be used as an informative measure of thei r foraging capabilities providing further information on food webs dynamics and use of resources in marine ecosystems. Finally and more importantly, this study provides a unique opportunity to evaluate the performances of two organism occupying similar tr ophic levels (at least in part of their life history) in the food web while being associated with two different marine ecosystems (reef and coastal environments). In this manner this study can reveal if convergence in bite performance occurs between the tw o apex predators, or determine if similar positions in the trophic chain can be reached by utilizing different biting strategies.
10 Chapter 2: Feeding Biomechanics in the Great B arracuda ( Sphyraena barracuda ) During O ntogeny Introduction Seve ral factors affect the use of resources including competition, energy consumption or risk of predation (Wainwright, 1991). However two may be crucial, prey availability and predator performance, the latter including how good a predator is at locating, cap turing and handling different prey types (Wainwright, 1991, 1995). Feeding performance may be affected by morphology, which in turn may, in part, determine diet (Wainwright, 1995). Consequently, feeding performance is highly associated with fitness, since survival will be constrained by food acquisition (Huber et al., 2005). Feeding capabilities of a predator have been shown to affect the pattern of resource use in fishes as well as other vertebrates (Wainwright, 1988; Hernandez and Motta, 1997; Huber et a l., 2005 ; Mcbrayer, 2004; Herre l et al., 2006 ). Dietary shifts during ontogeny are common in fishes (Wainwright 1995; Hernandez and Motta, 1997; Ebert, 2002) and other vertebrates (Price and Grant, 1984; uch dietary shifts may be explained by changes in habitat, whereas others can be attributed to modification in the feeding abilities of the predators (Wainwright, 1995). Additionally, ontogenetic changes in morphology have been shown to be associated with changes in organismal performance For example, bite force performance in catfish ( Clarias gariepenus ) have been shown to vary through ontogeny, where small individuals (below
11 30 cm TL) that feed on hard prey show a disproportio nal increase in bite force and larger individuals that feed on elusive prey show disproportional smaller values. Some of these patterns were highly associated with cranial morphology and scaling of feeding elements (Herrel et al., 2005). When our interests are focused on measurement of feeding performance through ontogeny, top predators may offer insight into the upper limits of performance. Apex predators such as large bony fishes or sharks characteristically eat large prey, consequently they are expected to exert high values of absolute bite force (Huber et al., 2009). Moreover, fishes as ectotherms, can attain extreme body lengths making them a good model to quantify intraspecific scaling patterns (e.g. feeding performance) through performance during ontogeny has been the focus of several studies (Wainwright, 1988; predators (Erickson et al., 2003; Huber et al., 2006, 2009; Grubich et al., 2007). Sphyraena barracuda the great barracuda (Figure 1) is a top predator that inhabits reefs and sea grass beds in most of the tropical seas around the world. Male S. barracuda reach maturity at 55 cm TL (2 years) and females at 66 cm TL (4 years) (deSylva, 1963). The great barracuda can reach sizes up to 180 cm TL and weight more than 45 Kg (deSylva, 1963). Sphyraena barracuda is a lie in wait predator with a body morphology suited for rapid acceler ation (deSylva, 1963). Possessing an elongated jaw (Figure 2a, 2b), a common characteristic of ram feeding predators (Ferry Graham 2001; Porter and Motta, 2004) S. barracuda uses ram feeding to capture elusive prey Juveniles of S. barracuda
12 strike their p rey reaching velocities of 144.5 cm/s (Porter and Motta, 2004). Predatory behavior in this species starts with visual recognition of the prey followed by a rapid acceleration s fast starts (Porter, 2002; pers. obs ). A two dimensional analysis of bite forc e in an ontogenetic series of seven individuals of S. barracuda (20 8200 g) showed values of bite force ranging from 0.90 73 N at the most posterior bite point. Bite force scaled with positive allom et ry, however the mass of all the adductive muscles sc aled with isometry (Grubich et al., 2007). Dietary analyses of the great barracuda indicate a preference for elusive fishes (deSylva, 1963). Small individuals (from 0 45 cm TL) from tropical western Atlantic waters have a diet that includes atherinids, g obiids and clupeids while larger individuals (from 45 140 cm TL) switch to beloniforms, tetraodontiforms, hemiramphids and carangids (deSylva, 1963). DeSylva (1963) attributes the ontogenetic change in diet to a change in habitat since the great barracud a migrates from shallow waters surrounded by mangroves or sea grass beds to a coral reef environment. The goal of this study is to analyze the feeding performance of this top predator by investigating theoretical calculations of bite force during ontogeny. This study also employs electromyography to resolve the adductive musculature involved in the biting process. Additionally, this study uses a three dimensional static equilibrium model for calculations of bite force performance in contrast to the two dime nsional model used by Grubich et al., (2007). This study provides the most thorough analysis of the bite force performance in S. barracuda through ontogeny.
13 Materials and M ethods Twenty seven specimens of Sphyraena barracuda (23 130 cm TL) were collect ed from local fisher s and kept frozen until dissection. Unilateral dissections were performed on each individual and subdivisions of the adductor mandibulae complex identified following Winterbottom (1973). After identification, each subdivision was remove d and bisected through its center of mass perpendicular to the main fiber direction. Center of mass was found from the intersection point of two lines obtained from suspending each muscle from a weighted line. Anatomical cross sectional area (CSA) was trac ed from digital pictures (Canon Power shot A710is) using Sigma Scan Pro 4 (SYSTAT Software Inc., Point Richmond, CA, USA). Theoretical maximum tetanic force (Po) for each subdivision was determined by multiplying the CSA by the specific tension of fish mus cle (TS) (20 N/cm2, Altringham and Johnston, 1982) following Powell et al., (1984). Po = CSA x TS Three dimensional coordinates of origin and insertion of each adductor subdivision, in lever and out lever distances, jaw joint and five bite points along t he lower jaw (0%, 25%, 50 %, 75% and 100%) were obtained for each individual using a three dimensional digitizer (PATRIOT TM digitizer, Polhemus, Colchester, VT, USA). Mechanical advantage was then calculated from the ratio of the weighted in lever (based on the amount of force produce by each muscle) and the out lever for all five bite points. Force vectors were made for each subdivision using Po and 3D position for each muscle. Theoretical maximum bite forces produced along the lower jaw were calculated v ia
14 summation of bending moments about the lower jaw with a 3 D static equilibrium model in Mathcad 13 (Mathsoft, Inc., Cambridge, MA, USA), following Huber et al., (2005): FLJ= FJR+FA2+FA3+FA3 +FB = 0 Where FLJ are the forces acting on the lower jaw, FJ R is the jaw joint reaction, FA2, FA3 and FA3 are the forces generated by each adductive muscle and FB is the bite reaction force from the prey item (Huber, 2005). All variables (anterior and posterior bite force, anterior and posterior mechanical advanta ge, weighted in lever, anterior and posterior out lever, CSA y component of three dimensional coordinate for each force vector) were log transformed and linearly regressed against total length. To determine scaling patterns, slopes for each regression w ere compared with expected isometric slopes (bite force = 2, mechanical advantage = 0, lever distances = 1, y component of 3 D coordinates of force vector = 2) using a two tailed student t test (Sokal and Rohlf, 1995). Electromyography Two individuals of Sphyraena barracuda (20 30 cm TL) were housed in a 380 Animals were conditioned to feed with live goldfish ( Carassius auratus ) under bright light conditions (tungsten b ulbs, 500 W) for 2 3 weeks. Prey items (3 7 cm TL) were provided at least twice a week (~ 25 individuals each time) placing them in the corner of the tank with a small fish net. Electromyographic procedures were performed following
15 Motta et al., (1991) Bipolar electrodes were prepared from strands of teflon coated stainless steel alloy wire (0.06 mm diameter), with the end of the wires (1 mm) exposed and bent into an arrow shape to facilitate retention within the muscles. Before surgery, individuals we re anesthetized with 0.1 g/l of tricaine methanesulfonate (MS 222) in a recirculating flow tank. Bipolar electrodes were implanted in three adductor mandibulae muscle divisions (A1, A2 and A3) by 26 gauge hypodermic needle. During the surgery the gills wer e perfused with water and anesthetic. After implantation, wires were collectively glued and anchored to a surgical suture loop attached to the dorsal surface of the body, anterior to the dorsal fin. After completion, individuals recovered in the filming ta nk, (original tank subdivided in two portions were filming space was of 70 x 40 cm) for at least 24 hours, during this time food was not provided. Before the experiment started wires were connected to a 16 channel AC differential amplifier (Model 3500, A M systems, Inc, Carlsborg, WA, USA) (Gain: 10000x, Band pass filter: 100 5000 Hz). The experiment started when prey items (one for each feeding event) were introduced in the filming tank, once the feeding event occurred, amplified signals of muscular act ivity were notch filtered and captured with a data acquisition system (NI DAQ, National Instruments, TX, USA) connected to a computer. For each feeding event, data and high speed video images (Fastcam 512 PCI, Photron Inc, CA, USA) were gathered simultaneo usly at a rate of 250 frames/second. Obtained data was recorded and analyzed with Photron motions tools system (Software version1.2.0, Photron Inc, CA, USA). The total number of implanted muscles varied in each procedure due to technical difficulties, but electromyograhic data was obtained in most cases from the combination of two adductor muscles divisions (A1 and A2, or A1 and A3).
16 First capture bites (when prey was initially grasped within the jaws) were only included in the analysis. In several opportun ities, S. barracuda held or repositioned the prey between the jaws before eating it. In this case, where the capture event took longer than expected, data subsequent to the initial capture was not included in the analysis. The variables analyzed in this ex periment were onset and offset time of muscular activity for the adductor mandibulae divisions (A1, A2, A3), total duration of muscular activity for each division, jaw opening and jaw closing time. Values for each variable were averaged and the results wer e plotted in a horizontal floating bar chart. Onset, offset, jaw closure and bite duration were determined relative to the time of jaw opening. All animal experimentation was performed in accordance to with Institutional Animal Care and Use Committee of th e University of South Florida, protocol # 3022 and 3241. Results Anatomy The adductor mandibulae of Sphyraena barracuda is composed by four subdivisions A1, A2, A3 and A3 (Figure 2a and 2b). The most anterior subdivision, A1, originates on the lachryma l and part of the infraorbitals and inserts by a tendonous insertion on the maxilla posterior to its articulation with the palatine. The ventrolateral subdivision in the cheek, A2, originates along the posterolateral edge of the preoperculum and inserts on the dorsal terminal point of the triangular shaped articular. The largest and more robust subdivision A3 occupies the majority of the middorsal region of the cheek. This originates on the hyomandibula, preoperculum and sphenotic bones and inserts on the d entary over a notch in the Meckelian fossa by a thick tendonous insertion. Deeper to A2 and A3 lies the more
17 medial subdivision, A3 that originates in the anteromedial portion of the preoperculum and inserts also by a tendonous insertion in the Meckelian fossa of the dentary dorsal to the insertion of A3. Prey C apture and Muscle Activity In all feeding events once the prey was offered Sphyraena barracuda oriented toward it. Approach to the prey was by rapid acceleration after an s fast start. Based on th e size of the prey offered the duration and number of bites varied in each feeding event. When prey were small (3 cm), S. barracuda ram captured the goldfish with an open mouth, engulfing it whole and swallowing in with one bite. However when prey were lar ger (5 7 cm), multiple bites were required to reposition the prey between the jaws prior to swallowing. Large prey was processed by S. barracuda by positioning the prey between the most posterior teeth. Occasionally, lateral head shakes where used to cut the prey into smaller pieces. All the implanted subdivisions of the adductor mandibulae (A1, A2, A3) were activated during jaw adduction. However the onset and offset during activation differed for each subdivision. Subdivision A1 was the first to activat e 33.5 4 ms after the onset of mandible depression with a mean burst duration of 89 ms (21 bites, n = 2). The dorsal subdivision A3 was the second to activate, almost immediately after the onset of A1, with an onset of 36.7 3.4 ms after mandibular depres sion. This muscle showed the shortest duration time of 44.7 ms (7 bites, n = 2). The subdivision A2 was the last recorded division to become active with an onset of 45.4 6.6 ms after jaw opening with a mean
18 burst duration of 99 ms (12 bites, n = 2). Jaw closing occurred at 63.3 ms after jaw the initiation of jaw opening (31 bites, n = 2) (Figure 3). Theoretical Calculations of B ite force Theoretical values of bite force ranged from 1 93 N at the most anterior tooth and from 3 258 N at the most posterio r tooth in an ontogenetic series of individuals ranging from 18 130 cm TL (Table 1). Values of bite force increased towards the most posterior bite point reaching values almost three times higher than the most anterior ones (Appendix 1). The percentage of the force contributed from each subdivision of the addu ctor mandibulae complex was 63.7 % (A3), 20. 6 % (A2) and 15.7 % (A3 ). Bite force scaled with isometry for both, the anterior and the posterior bite points of the lower jaw (Table 2, Figure 4) The C SA of the majority of the adductive musculature (A2 and A3 ) scaled with isometry (Table 2, Figure 5), although the larger muscle subdivision A3 responsible for the largest amount of force, showed negative allometry (Table 2, Figure 5). However, scaling pa force vector (the axis that contributes the most to bite force) scaled with isometry. Mechanical advantage scaled with isometry for the anterior and posterior tooth position on the lower jaw (Table 2, Figure 6). Values of mechanical advantage ranged from 0.18 0.25 at the anterior bite point and from 0.37 0.62 at the posterior bite point (Table 2). Weighted in lever (Figure 7) and posterior out lever (Figure 8) scaled with isometry. However the anterior out lever sho wed negative allometry indicating that the lower jaw decreases disproportionally in length during ontogeny (Table 2, Figure 8)
19 Discussion Bite force in Sphyraena barracuda (18 130 cm TL) increases proportionally with total length during ontogeny. These r esults are supported by the isometric patterns found in almost all the variables that strongly influence bite force (mechanical advantage, CSA of adductive muscles, lever arms). Although, the A3 division of the adductor mandibulae showed a negative allomet ry in cross sectional area during ontogeny, this had no apparent effect on the overall scaling pattern of bite force. When the three dimensional coordinates of the force vectors were analyzed, the axis that contributes the most to bite force (y axis) showe d isometry for all the adductive muscles including A3 ( Table 2). Although mechanical advantage (anterior and posterior) scaled isometrically during ontogeny, negative allometry was found for the anterior out lever indicating a disproportional decrease in length of the lower jaw through ontogeny (measure to the most anterior lower tooth in S. barracuda which lies anterior to the upper jaw marginal teeth) This allometric pattern implies that larger individuals of S. barracuda have relatively shorter biting surfaces making their jaws more force efficient at the anterior lower tooth. Conversely the anterior bite point of the jaws of smaller individuals will close disproportionally faster. Sphyraena barracuda is a piscivore that primarily preys on elusive fishe s (deSylva, 1963). Young individuals consume mostly atherinids, gobiids and clupeids, while adults switch to tetraodontiforms, hemiramphids and carangids (deSylva, 1963). Difference in jaw lengths through ontogeny may be associated with the prey capture me thods of S. barracuda where smaller individuals may rely more on a rapid closure of the jaw to capture small elusive prey while larger individuals eat larger and consequently harder prey requiring more force efficient jaws to grasp or process their
20 prey. Similar patter n s have been reported in red snapper Lutjanus campechanus w h ere changes in the lever system (from a speed efficient to a force efficient) were associated with an ontogenetic switch in diet (from soft evasive prey to a more varied diet includ ing hard prey) (Case et al., 2008) In contrast, different patterns have been reported in other fishes, for example largemouth bass Micropetrus salmoides have proportional change in jaw length over ontogeny (Richard and Wainwright, 1995) whereas an African catfish Clarias gariepenus has a disproportional decrease in jaw length and consequently a more force efficient jaw over ontogeny, however this pattern was not expected since C. gariepenus switches to a more evasive prey type through ontogeny (Herrel et a l., 2005). The scaling pattern and the theoretically calculated values of bite force reported in this study differ from those described by Grubich et al., (2007). Values of bite force for S. barracuda (18 130 cm, TL/ 25 11900 g) in this current study r anged from 3 258 N at the most posterior bite point whereas Grubich et al., (2007) reported calculated values of bite force for S. barracuda (20 8200 g, this study was performed with mass, no TL was reported) of 0.9 73 N at the same bite position s The scaling patterns also differed, this study reported isometry in bite force and all the biomechanical variables that affect it (mechanical advantage, cross sectional area for the adductive musculature and lever arms). However, Grubich et al., (2007) found positive allometry in bite force, but the masses of all the adductive muscles scaled with isometry. The scaling pattern of the mechanical advantage was not reported but an increase through ontogeny was suggested by the authors. Differences in these result s may be related to variability in the described anatomy, the use of different theoretical models, the preservation of the specimens, or the
21 sample size. The anatomy described by Grubich et al, (2007) differed in that only two subdivisions of the adductor mandibulae were included in their calculations of bite force (A2 and A3) whereas the current study describes and includes a third subdivision (A3 ), resulting in an increase in the output forces. The theoretical models used by the two studies also differ, while Grubich et al., (2007) estimates values of bite force with a 2 D model (MandibLever 3.2) (Westneat, 2003) this study used a 3 D approach ( Mathcad 13 ) (Huber et al, 2005). Additionally, in the current study a larger sample size was analyzed, 27 fresh dead specimens of S. barracuda were used, in comparison to the 7 individuals (a compilation of preserved and fresh dead specimens) used by Grubich et al., (2007). The use of preserved specimens may alter the calculations of bite force since formalin prese rvatives can decrease the original mass by 8.4 13.4 % in some teleost fishes (Buchheister and Wilson, 2005) which may in turn affect cross sectional area, and consequently output values of bite force. The isometric scaling pattern of bite force in S. ba rracuda found in this study differs from that of other studies where positive allometry in bite force is often found: sheephead Archosargus probatocephalus (Hernandez and Motta, 1997); the blacktip shark Carcharhinus limbatus (Huber et al., 2006); spotted ratfish Hydrolagus colliei (Huber et al., 2008); lizards Anolis equestris and Anolis garmani (Herrel and Gibb, 2005); American alligator Alligator mississippiensis (Erickson et al., 2003). In most of the cases these results are attributed to a hyperallomet ric pattern of the adductive muscle, to a disproportional increase in the dimensions of the skeletal components, or to both variables. However in some other cases the scaling relationships are not straight forward. In the spotted ratfish positive allometry in bite force was not supported by positive
22 allometry in either the adductive musculature or head dimensions but to an allometric increase of mechanical advantage (Huber et al., 2008) The isometric pattern of bite force in S. barracuda suggests that a d isproportional increase in bite force may not be required by this species to overcome the challenges offered by its changing prey base. Positive allometry in bite force has been frequently associated with changes in diet (Wainwright, 1988, Meyers et al, 20 02), and in lizards hyperallometry in bite force was not only correlated with an increase in prey size but also have shown that during ontogeny, fishes that exert dis proportionally larger values of bite force increased their consumption of harder items in their diet, or become specialized towards durophagy (Wainwright, 1988; Hernandez and Motta, 1997). Sphyraena barracuda does not undergo a remarkable dietary shift, as individuals prey upon fishes throughout life (DeSylva, 1963). Consequently the lack of change in the diet of S. barracuda other than from more to less elusive prey, may contribute to the lack of selective pressure that would result in hyperallometric pat terns of bite force during ontogeny. Additionally, when comparing the absolute bite force of S. barracuda to other fishes, the values are not particularly large (Table 3). Absolute values of bite force represent the capabilities of a predator to consume an d process a specific prey or suite of prey items (Huber et al., 2006). Absolute values of bite force in this species are similar to those of a 61 cm TL lemon shark ( Negaprion brevirostris ) and a 71 cm TL whitespotted bamboo shark ( Chiloscyllium plagiosum ) ( Huber, 2006), both of which primarily consume teleost fishes and crustaceans. Perhaps more instructive, is size removed bite
23 force which reveals a remarkably low residual force for S. barracuda (Table 3, Figure 9). The question remains why the great barrac uda has such a low relative bite force. The skull of S. barracuda is suited for ram feeding, having a lack of premaxillary protrusion, and a long mandible equipped with numerous sharp teeth (Liem, 1993). Prey capture is characterized by s fast starts and rapid acceleration (characteristic of all sphyraenids, Webb, 1984; Porter and Motta, 2004). However, results from this study show that the size removed biting performance in this species is among the lowest recorded for fishes (Figure 9). Consequently, som e other morphological features may contribute to its predatory success. Sphyraena barracuda has extremely sharp teeth on the premaxilla, maxilla and palatine. Sharp teeth are known to facilitate better penetration into soft prey by concentrating force into a small surface area (Frazzetta, 1988). Performance tests with similarly sharp and pointed teeth from the mako shark Isurus oxyrinchus reveal a maximum penetration force of only 5 Newtons in teleost prey (Whitenack, 2009). Consequently the presence of sha rp, blade like teeth coupled with its ram feeding behavior may contribute to a successful prey capture of this species. The anteriorly recurved shape of the distal surface of the anterior teeth may facilitate holding the prey inside the gape once penetrate d (Whitenack; pers. com .). Piscivor es fishes generally rely in speed efficient jaws to capture evasive prey (DeSchepper et al, 2008). In S. barracuda out lever distance to the most anterior bite point showed a disproportional decrease in length through ont ogeny, implying a relatively more force efficient jaw closing mechanism as the animal grows. However, values of MA and jaw closing duration times characterize S. barracuda as having an overall speed efficient jaw closing mechanism which may contribute the capture and
24 retention of its prey (MA= .27) (Westneat, 2004, DeSchepper et al, 2008). Jaw closing duration in juvenile S. barracuda was reported as 8.1 ms (Porter and Motta, 2002), a value similar to other long jawed species such as the Florida gar Lepiso steus platyrhincus with a rapid closing of 7.3 ms but significantly less than the redfin needlefish Strongylura notate which has a longer jaw closing duration of 18.5 ms (Porter and Motta, 2004) Another morphological feature that may contribute to succes sful prey capture in S. barracuda is a rotational motion of the premaxillary teeth. This mechanism, previously mentioned by Gudger (1918) and Grubich et al (2007), is revealed by the electromyographic results. Jaw opening starts with the depression of the dent ary that pull the maxilla anteroventrally by the maxillomandibular ligament. When feeding on large prey the maxilla of S. barracuda pivots around its articulation with the anterior process of the palatine (Figure 10). This movement orients the anterior margin of the premaxilla into a vertical position. This orientation may confer two advantages. When the premaxilla swings more vertically it results in an increase in the overall gape size. Larger gape size may allow predators to exploit prey sizes that a re not utilized by other predators. This may delimit the hierarchical level occupied by a specific predator in the trophic chain (Lucifora et al., 2009). In S. barracuda prey size seems not be a constraint since this species is able to predate on individua ls a third of its size ( personal communication ). The second advantage may be associated with a change in the angle of attack of the premaxilary teeth. The rotational motion of the premaxilla results in a more orthogonal position of the premaxillary teeth t owards the prey, a position that seems to be more beneficial for prey penetration. Similar strategies are found in snakes were reorientation
25 of the fangs facilitates prey penetration, consequently reducing bite duration (Cundall, 2008) Electromyographic r esults indicate that the adductor mandibulae subdivision 1 is an active participant in this mechanism. This muscle inserts posterior on the maxilla to the pivot point of the palatine with the maxilla, consequently after the prey is grasped by the fang like teeth, A1 activates (before any other adductive musculature) rotating the premaxilla back to its more horizontal position consequently, adducting the maxilla while at the same time closing the anterior premaxillary teeth on the p rey preventing prey to esc ape. Conclusions Bite force in the great barracuda of total length 18 to 130 cm ranged from 3 258 N at the posterior bite point. The scaling pattern of this performance measure did not deviate from isometry. These results were supported by isometric patt erns of growth found in most of the variables that contribute to the bite force. Out lever distance to the most anterior bite point showed a disproportional decrease in length through ontogeny, resulting in adults having relatively more force efficient bi tes than younger individuals. However, values of MA and jaw closing duration times characterize S. barracuda as having an overall speed efficient jaw closing mechanism which may contribute the capture and retention of its prey. Sphyraena barracuda has one of the lowest relative values of bite force found among all described fishes, suggesting that other strategies may contribute to the feeding success of this predator.
26 Figure 1. Drawing of Sphyraena barracuda by H.L. Todd. (deSylva, 1963)
27 Figure 2a. Left lateral view of the adductive muscles of S. barracuda A1, adductor mandibulae subdivision 1, A2 adductor mandibulae subdivision 2 and A3 adductor mandibulae subdivision 3.
28 Figure 2b. Left lateral view of the adductive muscles of S. barracuda where A2, adductor mandibulae subdivision 2, has been removed to reveal the deepest subdivision A3 of this adductive muscle. A3 and A3 inserts by a tendonous insertion on the dentary in the Meckelian fossa.
2 9 Figure 3 Composite diagram for the muscular activity of S. barracuda Different bars refer to the different subdi visions of the adductor muscle, left error bars indicate one standard error of the onset of the motor activity relative to JO, right error bars indicate one standard error of the duration of the motor activity for each muscle. From bellow to above the first thick bar show duration time for A1 subd ivision, the second thick bar show duration time for A2 and the third thick bar show duration time A3 Onset time for each subdivision is relative to the jaw opening time (JO) in ms. Jaw closing time (JC) is also relative to JO time.
30 Table 1. Absolute values of bite force in 27 individuals of S. barracuda Total length (TL) expressed in cm, anterior values of bite force (ABF) and posterior values of bite force (PBF) expressed in Newtons (N). TL (cm) PBF (N) ABF (N) 18 3 1 20 3.5 1.7 22.5 4 .4 2.1 37.3 9 3.5 46.2 31.1 14.6 46.3 34.8 13.6 68.2 50 22.4 69.5 62.9 22.5 70.5 31 13 73.5 49 25.5 73.9 39 16.9 75.5 56.2 26 76.5 50.4 21.6 78.9 75.7 32.3 82 80.9 32.5 83 63.8 27 92 77.4 37.5 97 80.7 32.5 98.1 100 45.3 104 111.5 51.4 108 166.8 66.5 113.2 172.3 67.2 117 199 83.6 119.2 188.4 80.2 122 209.1 83.4 126.5 173.7 67 130 258.5 93.4
31 Figure 4. Bite force (N) vs. total length (cm). Log transformed values for S. barracuda of bite force linearly regressed against log tran sformed values of total length. Diamond shapes represent posterior bite force values (PBF) and squares shapes represent values of bite force at the most anterior position of the jaw (ABF). Log PBF= 2.18 log TL 2.30 Log ABF= 2.16 log TL 2.65
32 Figure 5. Log transformed values of cross sectional area (cm 2 ) of the adductor mandibulae muscle divisions of for S. barracuda linearly regressed against log transformed values of total length (cm). Diamond represent values of CSA of A2 muscle, squares represent values of CSA of A3 muscle and triangles represent values of CSA of A3 muscle. Log CSA A3 = 1.88 log TL 3.14 Log CSA A3B = 1.94 log TL 2.16 Log CSA A2 = 2.17 log TL 4.19
33 Figure 6. Log transformed values of mechanical advantage of for S. barracuda linearly regressed against log transformed values of total length (cm). Diamonds represent values of mechanical advantage at the most posterio r bite point (PMA) and squares represent values of mechanical advantage at the most anterior bite point (AMA). Log AMF= 0.07 log TL 0.79 Log PMF= 0.08 log TL 0.46
34 Figure 7. Log transformed values of weighted in lever (cm) of for S. barracuda linearly regressed against log transformed values of total length (cm). Log IL= 1.02 log TL 1.52
35 Figure 8. Log transformed values of out lever distances (cm) for S. barracuda linearly regressed against log transformed values of total length (cm). Diamonds represent values of out lever distances at the most posterior bite point (PO L) and squares represent values of out lever distances at the most anterior bite point (AOL). Log POL= 0.94 log TL 1.07 Log AOL = 0.95 log TL 0.73
36 Table 2. Scaling patterns of bite force in S.barracuda Results obtained from linear regressions of bite force and all variables involved with bite force in S barracuda Slopes from the regression equation were compared to isometric slopes by using a two tail student t test. Significant deviation from geometrical similarity is denoted in bold
37 Table 3. Values of anterior bite forces (ABF), mass remov ed bite force for 20 species of fishes obtained from the literature. ABF values were regressed against body mass; and residuals of the regression were obtained to eliminate the effect of mass among individuals. Species name Common name ABF (N) M ass (g) Residuals Etmopterus spinax a V elvet belly lanternshark 1.6 349.1 2.576 Etmopterus Lucifer a Black belly lanternshark 3.1 48.0 1.243 Carcharhinus limbatus b Blacktip shark 423.0 9833.0 0.354 Carcharhinus leucas c Bull shark 1023.0 140341 .0 0.0942 Sphyrna mokarran c Great hammerhead shark 2432.0 580598.0 0.0137 Heterodontus francisci d Horn shark 206.0 2948.0 0.297 Negaprion brevirostis a Lemon shark 79.0 1219.0 0.075 Heptranchis perlo a Sharpnose sevengill shark 245.0 1614.0 0.682 Sq ualus acanthias e Spiny dogfish 19.6 1065.0 1.094 Chiloscyllium plagiosum a Whitespotted bamboo shark 93.0 1219.0 0.051 Hydrolagus colliei f Whitespotted chimaera 106.0 870.0 0.293 Halichoeres bivittatus g Slippery dick 5.0 19.0 0.48 Halichoeres garnoti g Yellowhead wrasse 10.0 21.0 0.0312 Halichoeres maculipinna g Clown wrasse 11.0 18.0 0.175 Lachnolaimus maximus g Hogfish 290.0 209.0 1.671 Thalassoma bifasciatum g Bluehead wrasse 5.0 7.0 0.0477 Chilomycterus schoepfi h Striped burrfish 380.0 180.0 1.94 5 Sphyraena barracuda Great barracuda 83.0 11900.0 1.017 Archosargus probatocephalus i Sheepshead 309 998 1.061 Halichoeres garnoti g Yellowhead wrasse 10 21 0.0312 a Huber, 2006 b Huber et al., 2006 c Huber and Mara unpublished manuscrip t d Hu ber et al., 2005 e Huber et al., 2004 f Huber et al., 2008 g Clifton and Motta, 1998 h Korff and Wainwright, 2004 i Hernandez and Motta, 1997
38 Figure 9. Residual values of anterior bite force plotted against body mass for 20 species of fishes. R ed arrow indicates the position of S. barracuda anterior bite force for an individual of 11,900 g (122 cm TL).
39 Figure 10. Premaxillary rotation in S. barracuda Abduction of the dentary promotes the elevation of the upper jaw (maxilla and premaxilla) by the maxillomandibular ligament (Lmma). Rotation of the premaxilla occurs when the maxilla pivots with the palatine process when maximum gape size is reached.
40 Cha pter 3: Feeding Biomechanics of the B ull S harks ( Carcharh inus leucas ) During O ntogeny Introduction Vertebrates such as fishes that have indeterminate growth may undergo profound changes in their morphology, physiology and behavior. Morphological changes during ontogeny may influence function and consequently org anismal ecology (Wainwright, 1999; Herrel and Gibb, 2006) O ntogenetic variation in a form function complex is often easy to recognize and quantify, however its consequences to organismal ecology are not always straight forward. Consequently, measurements of performance may be a useful instrument to determine these associations (Wainwright, 1994). Measurements of performance have been frequently used to understand the consequences of ontogenetic change among form, function and ecology in different vertebra tes (Richard and Wainwright, 1995; Herrel et al., 1999, 2001, 2005; Hernandez studied in this regard include locomotion and feeding (Herrel and Gibb, 2006, Deban and 2005). One measure of feeding performance that has been broadly addressed is bite force (Herrel and Gibb, 2006). Bite fore has been frequently investigated in ontogenetic studies of feeding performance as it is assumed to influence organismal fitness sin ce it is related Huber et al., 2006; Kolmann and Huber, in prep ). Scaling patterns of bite force may be
41 allometric or isometric nevertheless, since force is proportio nal to length to the second power, juveniles are expected to perform differently than adults. Consequently deviations from geometrical similarity have been commonly reported in bite force (Herrel and Allometric chang es in bite force performance through ontogeny are often found and in some cases have been shown to be associated with ontogenetic changes in diet 2006; Huber et al., 2006; Huber et al, 2008) including dietary shifts in fishes (Wainwright, 1988; Hernandez and Motta, 1997; Herrel et al., 2005). Positive allometry in bite force may be beneficial to exploit new food sources inaccessible to other individuals. Additionally, positive allometry in b ite force may confer an advantage by making functionally difficult prey able to be captured earlier in life (Kolmann and Huber, in prep ). For example, in contrast to small or medium size sharks that prey mostly upon teleost fishes (Wetherbee and Cortes, 20 02), large sharks such as white sharks Carcharodon carcharias tiger sharks Galeocerdo cuvier, and bull sharks Carcharhinus leucas have been reported to prey on a wide variety of larger and harder prey including marine mammals and sea turtles (Cliff et al. 1989; Cliff and Dudley 1991; Lowe et al. 1996; Ebert, 2002; Cortes, 1999; Lucifora et al., 2009). Interestingly, isometry of bite force was predicted for large sharks because selective pressure for positive allometry was suggested to be relaxed since lar ge absolute values of bite force are associated with large body size (Huber et al, 2009). Scaling patterns for bite force have been described primarily in species that attain small lengths such as s triped burrfish Chilomycterus schoepfi the spiny dogfish Squalus acanthias or the yellowhead wrasse Halichoeres garnoti (Korff and Wainwrig h t 2004;
42 Huber and Motta, 2004; Clifton and Motta, 1998) However, information on this topic in large predators is scarce (see Erickson et al., 2003 and Huber et al., 2006, 2009 for exceptions). The study of bite force in large organisms such as marine top predators (e.g. large bony fishes or sharks) may greatly contribute to the understanding of the scaling patterns of feeding performance among vertebrates by helping to reve al if bite force in large predators is a product of selective pressure or an artifact of growth. Furthermore, large sharks as poikilotherms can achieve extreme ranges in body size, providing a good model to study intraspecific scaling patterns of bite forc e (Huber et al., 2009). The bull shark Carcharhinus leucas is a coastal shark characterized by a robust body and a rounded head (Figure 11). Reaching sizes up to 340 cm TL and 230 Kg in weight (Compagno, 1984), males of this species reach maturity at 210 220 cm TL (14 15 years) and females at lengths over 225 cm TL (18 years) (Neer et al., 2005) Individuals of C. leucas are found in all tropical and subtropical seas as well as in some freshwater ecosystems around the world (Compagno, 1984). Carcharhinu s leucas is an aggressive predator that swims near the bottom searching for prey (Compagno, 2005) and is commonly found in shallow waters (less than 30 m) (Compagno, 1984). Exhibiting an ontogenetic switch in diet, individuals smaller than 140 cm TL preda te mostly on bony fishes, whereas sharks above this length change their dietary preferences towards elasmobranchs and marine mammals (Compagno 1984, Cockcroft et al. 1989, Cliff and Dudley 1991, Last and Stevens 1994, Heithaus, 2001). The highest incidence of marine mammals in their diet occurs in larger individuals (above 180 cm TL) (Cliff and Dudley, 1991)
43 As a recognized top predator bull sharks offer an excellent model to study bite performance during ontogeny. With a clear dietary switch towards large r and harder prey through ontogeny, bite performance will in part determine its success as a top predator. The goal of this study is to investigate the feeding performance of C. leucas through ontogeny by the calculation of theoretical values of bite forc e with a 3 D static equilibrium model, and relate changes in bite performance to diet. Consequently, this study contributes to a better understanding of the link among form, function and ecology in C. leucas. Materials and M ethods A total of sixteen C. l eucas (73 285 cm TL, sexes not always determined) were obtained from commercial and recreational fishers off the Gulf coast of Florida. The largest individual of this study (285 cm TL, sex undetermined) was obtained from Cronulla, NSW, Australia. Animals were kept frozen until dissection. Unilateral dissections of the adductor mandibulae complex (formed by the preorbitalis and quadratomandibularis muscles (Motta and Wilga, 1995)) were performed and the muscles involved in jaw adduction removed and bisecte d through their center of mass: preorbitalis dorsal (POD), preorbitalis ventral (POV), quadratomandibularis dorsal division 1 and 2 (QD 1+2), quadratomandibularis dorsal division 3 (QD 3), quadratomandibularis dorsal division 4 (QD 4) and quadratomandibula ris ventral (QV) Anatomical cross sectional areas (a CSA) were obtained from the bisected muscles. Two areas (one from each portion of the bisected muscle) were traced from digital pictures (Canon Power shot A710is) with Sigma Scan Pro 4 (SYSTAT Software Inc., Point Richmond, CA, USA)
44 (except for QD 1+2) and the average area taken. Since QD 1+2 showed a pinnate architecture, physiological cross sectional area (p CSA) were obtained in all the individuals (except for the largest individual TL = 285, where th e QD complex was treated as a whole) by bisecting each muscle through its center of mass parallel to the main fiber direction. Physiological cross sectional areas were calculated according to Powell et al., (1984). CSA = muscle mass/muscle density x cos x 1/ fiber length Where density of fish muscle is 1.05 g cm 3 (Powell et al., 1984; Wainwright, 1988); is the angle of fiber pinnation obtained from the average angle of 5 pinnate fibers evenly distributed across the muscle. Fiber length and angle were estimated from digital pictures (Canon Power shot A710is) using Sigma Scan Pro 4 (SYSTAT Software Inc., Point Richmond, CA, USA). Theoretical maximum tetanic force (Po) was then calculated for each subdivision following Powell et al., (1984). Po = CSA x T S Where CSA was replaced for the anatomical or physiological cross sectional area of each muscle depending on their architecture, and TS refers to the elasmobranch muscle specific tension 28.9 N/cm 2 (Lou et al., 2002). Three dimensional positions of the o rigin and insertion of each muscle, jaw joint and five bite points along the lower jaw relative to a reference point (0% the most
45 posterior, 25%, 50%, 75% and 100 % for the most anterior) as well as the in lever (distance from each muscular insertion to ja w joint) and out lever (distance from each bite point to the jaw joint) distances were determined for each individual by using a three dimensional digitizer (PATRIOT TM digitizer, Polhemus, Colchester, VT, USA). Mechanical advantage was then calculated from the ratio of the weighted in lever (relative to the amount of force produced by each muscle) to out lever for the five bite points. Theoretical maximum bite forces were calculated creating three dimensional force vectors for each adductor muscle by using a 3 D static equilibrium model with Mathcad 13 (Mathsoft, Inc., Cambridge, MA, USA) following Huber et al., (2005). Theoretical maximum bite force was determined by the summation of the forces acting on the lower jaw in static equilibrium conditions (Hube r et al., 2005). F LJ = F JR + F POD + F POV + F QD12 + F QD3 + F QD4 + F QV + F B = 0 Where F LJ are the forces acting on the lower jaw, F JR is the jaw joint reaction, F POD F POV F QD12 F QD3 F QD4 F QV are the forces generated by each muscle of the adductor man dibulae complex and F B is the bite reaction force from the prey item (Huber et al., 2005). Logarithmic transformations were applied to all the data and linear regressions were performed for each variable (anterior and posterior bite force, CSA of all the a dductive musculature, anterior and posterior MA, anterior in lever and posterior out lever) against total length. Scaling relationships were determined comparing expected
46 slopes for geometric similarity (bite force = 2, mechanical advantage = 0 lever arms = 1) to obtained slopes by using a two tailed student t test (Sokal and Rohlf, 1995). Results Values of bite force ranged from 36 2,128 N at the most anterior bite point and from 170 5,914 N at the most posterior bite point in an ontogenetic series of 16 individuals ranging from 73 2 8 5 cm TL (Table 3). Values increased almost three times from the most anterior bite point to the most posterior bite point of the lower jaw (Appendix 4 ). Bite force scaled with positive allometry at the most anterior bite point (ABF) and with isometry at the most posterior bite point (PBF) (Table 4, Figure 13). The majority of the cross sectional area (CSA) of the subdivisions of adductor mandibulae complex (POV, POD, QD 4 and QV) scaled with positive allometry except for two subdivisions of the QD muscle (QD 1 + 2 and QD3) that scaled with isometry (Table 4, Figure 13 17). The adductor muscle that generates the largest value of force is Q V (41.1 %), followed by QD 1+2 (3 1.1 %), PV (13.5 %), QD 3 (6.8 %), POD (3.8 %) and QD 4 (3.6 %). Values of mechanical advantage varied from 0.24 0.37 (mean = 0.31) at the most anterior bite point (AMA) and from 0.87 1.6 (mean = 1.1) at the most posterior bite point (PMA). Mechanical advantage scaled with isometry for these two bite poi nts (Table 4, Figure 18). The weighted in lever and posterior out lever scaled with isometry (Table 4, Figure 19 20). However the anterior out lever scaled with positive allometry (Table 4, Figure 20).
47 Discussion Bull sharks have a disproportional increa se in bite force during ontogeny at the most anterior bite point attributable to the disproportional increase in the cross sectional area of the majority of the adductive muscles However, two subdivisions of quadratomandibularis muscle (QD1+2 and QD 3) s caled with isometry. These results may be associated with the difficulty of anatomically separating the subdivisions QD 3 and QD 4 of the quadratomandibularis muscle, contributing to a significant source of variability, especially in smaller individuals. D ifferences in the scaling patterns of bite force between the two bite positions were found. Although the slopes for both regressions (ABF = 2.6 and PBF = 2.4) appear higher than the expected slope of 2 for geometrical similarity, only the slope for anterio r bite force was significantly greater than expected T he reason for unexpected result is not clear, but may be associated with intrinsic variability in the number of functional teeth among the sampled individuals. Bite positions were identified and digiti zed at the base of each t oot h W hile the anterior bite points were always easy to identify, posterior bite point positions varied depending on the location of the last functional tooth. Consequently, variability in the posterior bite force may result in gr eater variability and lack of significance (see out lever below) Positive allometry in bite force is a recurrent scaling pattern found among vertebrates, including the horn shark Heteredontus francisci (Kolmann and Huber, in prep ), spotted ratfish Hydrola gus colliei (Huber et al., 2008), lizards Sceloporus undulates, Sceloporus magister and Cnemidophorus tigris (Meyers et al, 2002), and the American alligator Alligator mississippiensis (Erickson et al., 2003). Additionally, the
48 scaling pattern of bite forc e in bull sharks share some similarities to the one found for the closely related blacktip shark Carcharhinus limbatus where positive allometry was reported in bite force an d all the biomechanical variables that affect this measurement of performance (MA, CSA of the adductor muscles, lever arms) (Huber et al., 2006). The similarity in the scaling pattern of bite force found in these two predators suggests that positive allometry in bite force may be under selective pressure T hese results appear to contrad ict the assumption that large shark s would not undergo selective pressure for allometric bite force because of their attainable large size (Huber et al., 2009). However further studies are needed to reach a better understanding on the scaling patterns of b ite force in these large predators. Positive allometry in bite force may have important implications for organismal ecology as disproportional increase in bite force may facilitate dietary switches during ontogeny allowing organisms to exploit resources th at were not available before and Huber, in prep ). Carcharhinus leucas exhibits an ontogenetic switch in diet. In South African populations individuals below 140 cm TL prey upon teleost fishes, switching to large elasmobranchs and mammals above that size (Cliff and Dudley, 1991). Moreover C. leucas have been found to be one of the principal shark species (with C. carcharias and G. cuvier ) that predate on bottlenose dolphins o ff Natal, South Africa (Cockcroft et al., 1989). Consequently, a disproportional increase in bite force during ontogeny may be advantageous to capture and process larger or harder prey and to increase prey handling efficiently, increasing the rate of net e nergy intake (Herrel et al, 2001; Meyers et al, 2002;
49 Huber, in prep ). Additionally, different prey types such as teleost fishes, elasmobranchs and invertebrates that are frequently found as part of the diet of several sharks species ( Isurus oxyrinchus Galeocerdo cuvier Negaprion brevirostris, Prionace glauca ) have been shown to require different amount s of force (2 90 N) to penetrate skin and muscle by different shark teeth (Whitenack, 2009). Although these relatively low values do not reflect the force necessary to sever cartilage and bone or whole prey, t he y suggest that different prey types impose different functional constraints to each predator. Overall values of m echanical advantage (anterior and posterior mean values were 0.31 and 1.1 respectively) characterized bull sharks with a force efficient jaw (Westneat, 2004) Mean values of mechanical advantage obtained in this study were similar to the ones obtained for the b l a cktip shark C. limbatus (anterior and posterior mean values were 0.34 and 1.1 respectively) but lower than the durophagous spotted ratfish Hydrolagus colliei (anterior and posterior mean values were 0.49 and 1.5 respectively) (Huber et al., 2006; 20 08). Although MA scaled with isometry, positive allometry of the anterior out lever suggested that the jaw is getting disproportionally longer during ontogeny. Additionally, the isometric pattern found on the posterior out lever may be a product of the var iability on the position of the last functional tooth. T he out lever regressions reveal less variability in the anterior out lever than the posterior out lever (R 2 = 0.97 vs. R 2 = 0.81 respectively). Absolute values of bite force are crucial to understand the maximal capabilities of a predator (Huber et al., 2006). Bull sharks can generate bite forces of almost 6000 N at the back of the jaw and 2128 N at the most anterior tooth (TL = 285 cm). These values
50 are one of the highest values ever reported among ve rtebrates and are comparable to the absolute bite force produced by large r sharks such as the great hammerhead Sphyrna mokarran (anterior bite force 2432 N, 434 cm, TL) (Huber and Mara, unpublished data) (Table 5). W hen removing the effect of size, compari sons with nine chondrichthyan species and one bony fish reveal that Carcharhinus leucas ha s the highest relative value of bite force among all the compared fishes followed by t hree other chondrichthyan species: the sharpnose sevengill shark Heptranchis pe rlo (Huber, 2006) the horn shark Heterodontus francisci (Huber et al., 2005) and the whitespotted chimaera Hydrolagus colliei (Table 5) (Huber et al., 2008) However, t he majority of the species that also have relatively high values of bite force are duro p h a gous species D urophagous species are frequently associated with high values of bite force (Wainwright, 1988; Hernandez and Motta, 1997; Huber et al., 2005, 2008; Kolmann and Huber, in prep ). Durophagous species not only rely in higher bite force perfor mance but also o n morphological specialization including molariform teeth, hypertrophy of the adductor muscles and greater calcification of the jaws that help them to overcome the constraints associated to hard prey consumption (Huber et al., 2008). Value s of bite force in bull sharks and other shark species have been found to be extremely high in comparison to the values that are required to penetrate common prey items. This high pattern of bite force has been found among lizards, where prey items showed smaller force requirements than th at exerted by the predators (Herrel et al., 1999, McBrayer, 2002). Values of force required to penetrate different prey types such as fish skin and muscle, or crabs ranged from 2 to 100 N with teeth from different shark sp ecies (Whitenack, 2009). However, in several cases sharks may be capable of produc ing bite
51 forces in excess of thousands of Newtons ( C. limbatus S. mokarran C. carcharias and C. leucas ) (Huber et al., 2006, 2009; Wroe et al., 2008). Th e question remains why bull sharks, among other sharks, are cap able of produc ing such extreme values of bite force? Despite force to failure values obtained by Whitenack, (2009) there is no available data to suggest how much force is required to penetrate or cut whole prey i tem s such as fish or larger prey such as mammals or turtles, or the force necessary to penetrate bone or to slice through a vertebral column. Consequently additional information on the mechanical constraints that larger prey items may offer are required in order to provide some insights to understand why some sharks can exert extreme values of absolute bite force. Effective predation can be achieved by several other mechanisms besides force generation (Huber and Motta, 2004). Sharks have different ways to c apture prey (ram, biting, suction and filter feeding) (Frazzetta, 1994; Motta and Wilga, 2001; Motta, 2004) and several morphological features have been shown to contribute to their capture methods and predatory success, such as tooth morphology (Frazzeta, 1988, 1994; Capetta, 1987; Motta, 2004; Whitenack, 2009), tooth angle (Lucifora et al., 2001) and feeding behavior such as head shaking (Frazzeta, 1994; Motta, 2004) or jaw protrusion (Motta and Wilga, 2001; Motta, 2004) Bull sharks have triangular shape d teeth, and like the majority of the carcharhinid sharks they present dignathic heterodonty with serrated triangular upper teeth and narrow smooth lower teeth (Cappetta, 1986; Compagno, 1984). Different tooth morphologies have been proposed to play differ ent roles in the cutting mechanics in sharks, where pointed teeth facilitate rapid penetration into the prey, and serrations may penetrate
52 deeper by concentrating stress at their tips (Frazzeta, 1994; Motta, 2004). Additionally teeth inclination has been f ound to affect prey puncture and to enhance prey holding in Carcharhinus taurus (Lucifora et al., 2001) Head m orpholog y in C. leucas may affect its feeding performance. The jaw of sharks can be modeled as a semicircular saw where cuts are performed by the rotation of the jaw around a semicircular center during head shaking (Frazzetta, 1994). A broader head in C. leucas may be associated with broader jaws where the radius of the tooth circumference may coincide with the centroid of the body around which the shark rotates during head shaking, leading the teeth to cut through the same path on each head shake and therefore exerting a deeper cut on the prey with each lateral swing of the head. Conclusions Bull sharks have an allometric increase in bite force du ring ontogeny reaching values of almost 6000 N (TL 285 cm) at the posterior bite points. Positive allometry of the majority of the adductive muscles is responsible for the scaling pattern of bite force found in this species. Overall values of mechanical ad vantage indicate this species has a force efficient jaw. Absolute values of bite force in the bull shark are among the largest values reported among fishes and after removing the effect of size C. leucas ha s the highest value of bite force among all the compared fishes. High values of bite force in large bull sharks may be associated with their dietary shift to larger prey.
53 Figure11. Illustration of Carcharhinus leucas (from Compagno, 1984).
54 Table 4. Absolute values of bite force in 16 individuals of C. leucas Total length (TL) expressed in cm, anterior values of bite force (ABF) and posterior values of bite force (PBF) expressed in Newtons (N). TL (cm) ABF (N) PBF (N) 73 36.4 169.8 75 59 214 76 53.8 123 77.5 50.7 166.5 78.5 6 7.8 218 81.8 58.2 270.2 82.2 95.1 338 85.1 78.6 241 101 147 854 108 232.7 931.1 117 316.7 1144 187 644 1168 201 795 2451 240 1168.1 2761 258 1023 3721 285 2128 5914
55 Figure 12. Bite force (N) vs. total length (cm). Log transfo rmed values for C. leucas of bite force linearly regressed against log transformed values of total length. Diamond shapes represent posterior bite force values (PBF) and squares shapes represent values of bite force at the most anterior position of the jaw (ABF). Log PBF= 2.39 log TL 2.15 Log ABF= 2.61 log TL 3.17
56 Figure 13. Log transformed values of cross sectional area (cm 2 ) of the adductor mandibulae muscle divisions of C. leucas linearly regressed against log transformed values of total length (cm). Diamond represent values of CSA of PV muscle squares represent values of CSA of PD muscle and triangles represent values of CSA of QV muscle. Log CSA QV = 2.43 log TL 4.15 Log CSA PD = 2.70 log TL 5.71 Log CSA PV = 2.31 log TL 4.41
57 Figure 14. Log transformed values of cross sectional area (cm 2 ) of the adductor mandibulae muscle divisions of for C. leucas linearly regressed aga inst log transformed values of total length (cm). Diamonds represent values of CSA of QD 1+2 muscle, squares represent values of CSA of QD 3 muscle and triangles represent values of CSA of QD 4 muscle. Log CSA QD 1+2 = 2.45 log TL 4.35 Log CSA QD 4 = 2.89 log TL 6.12 Log CSA QD 3 = 2.49 log TL 5.14
58 Figure 15. Log transformed values of mechanic al advantage of for C. leucas linearly regressed against log transformed values of total length (cm). Diamonds represent values of mechanical advantage at the most posterior bite point (PMA) and squares represent values of mechanical advantage at the most anterior bite point (AMA). Log PMF= 0.00 log TL 0.03 Log AMF= 0.12 log TL 0.76
59 Figure 16. Log transformed values of weighted in lever (cm) of for C. leucas linearly regressed against log transformed values of total length (cm). Log IL= 1.19 log TL 1.97
60 Figure 17. Log transformed values of out lever distances (cm) for C. leucas linearly regressed against log transformed values of total length (cm). Diamonds represent values of out lever distances at the most posterior bite point (POL) and squares represent values of out lever distances at the most anterior bite poin t (AOL). Log AOL= 1.11 log TL 1.29 Log POL= 1.19 log TL 2.00
61 Table 5. Scaling patterns of bite force in C. l eucas Results obtained from linear regressions of bite force and all variables involved with bite force in C. leucas Slopes from the regression equation were compared to isometric slopes by using a two tail student t test. Significant deviation from geometrical similarity is denoted in bold
62 Table 6. Values of anterior bite forces (ABF) for nine c hondrichthy an species and one bony fish gathered from the literature. ABF values were regresse d against body mass and residuals from the regression were obtained to eliminate the effect of mass when comparing among all the individuals Species name Common name TL ABF Residuals Sphyrna mokarran a great hammerhead shark 434.0 2432.0 1.46 2 Carcharhinus leucas bull shark 285.0 2128.0 2.349 Sphyraena barracuda b barracuda 122.0 83.0 1.483 Carcharhinus limbatus c blacktip shark 152.0 423.0 0.96 Heterodontus francisci d horn shark 79.0 206.0 0.222 Heptranchis perlo e sharpnose sevengill shark 85.2 245.0 0.203 Chiloscyllium plagiosum e whitespotted bamboo shark 71.0 93.0 0.0102 Squalus acanthias f spiny dogfish 64.2 19.6 0.116 Negaprion brevirostis e lemon shark 61.0 79.0 0.22 Hydrolagus colliei g whitespotted chimaera 51.0 106.0 0. 623 a Huber and Mara, unpublished manuscript. b Habegger et al, unpublished manuscript. c Huber et al., 2006 d Huber et al., 2005. e Huber, 2006. f Huber and Motta, 2004. g Huber et al., 2008
63 Figure 18. Residual values of anterior bite force plotted against body mass for 10 fish species. A rrow indicates the position of C. leucas anterior bite force for an individual of 285 cm TL. Carcharhinus leucas has the largest value of bite force followed by t hree other chondrichthyan species, the sharpnose sevengill shark Heptranchis perlo, and two durophagous species the horn shark Heterodontus francisci and the whitespotted chimaera Hydrolagus colliei
64 Chapter Four: General D iscussion The morphology of an organism can determine its perfor mance having a profound impact o n its fitness, consequently organismal performance is the most important link between organismal phenotype and its ecological success (Koehl, 1996). Several measurements of performance have been thoroughly investigated in di fferent aspects of the life history of vertebrates (e.g. locomotion and feeding performance). Additionally, one measure of feeding performance that has been broadly addressed is bite force (Herrel and Gibb, 2006). Bite force plays a crucial role as a proxy for feeding performance because it is related to food acquisition and therefore organismal survival (Binder and Van Valkenburgh, 2000; Herrel et al., 2001; Huber et al., 2004, 2005, 2006). Consequently bite force has been investigated in numerous extant a nd some exti nct vertebrate groups (Wainwrig h t 1988; Herrel et al., 2001; 2002; 2005; McBrayer and White, 2002; Aguirre, et al., 2003; Dumont and Herrel, 2003; Anderson and Westneat, 2004; Wroe et al., 2005; 2008; Huber and Motta, 2004; Huber et al., 2005; 2009). Bite force performance can be greatly influenced by organismal size and age since morphology in most cases, change s during ontogeny (Meyers et al., 2002; Herrel and Gibb, 2005). Changes in bite force performance may shape organismal ecology by cha nging their use of resources: reducing dietary breadth, leading to earlier in life (Wainwright, 1988; Hernandez and Motta, 1997; Kollman and Huber, in
65 prep ) Bite force may scale during ontogeny with different patterns, allometry or isometry. However, most of the studies investigating bite force report patterns of positive allometry through ontogeny (Wainwright, 1988; Wainwright and Richards, 1995; Hernandez and Motta, 19 Huber et al., 2006; Aguirre et al., 2002; Erickson et al., 2003; Grubich et al, 2007; Meyers et al., 2002) The ultimate goal of the majority of the se studies investigating bite force through ontogeny is to associate this measurement of performance to the feeding ecology of the species of interest (Wainwright and Richard, 1995; Wainwright, 1988). Consequently, high values of bite force have been commonly linked to species that prey upon hard prey whereas l ow values of bite force are frequently associated with species that prey upon soft and elusive prey (Wainwright, 1988; Mittelbach et al., 1992 Hernandez and Motta, 1997; Grubich et al., 2007; Huber et al, 2008; DeSchepper et al., 2008; Kolmann and Huber, i n prep ). Additionally, changes in feeding performance through ontogeny have been associated with the major changes in the dietary preference of organisms (Wainwright, 1988; Wainwright and Richard, 1995; Hernandez and Motta, 1997; Vincent et al, 2007). Alth ough bite force performance has been extensively investigated, the majority of the studies on this topic have been confined to relatively small species or those from lower trophic levels. Large predators, such as large bony fishes or elasmobranchs, may pro vide an important contribution to a better understanding of how bite performance scale s with size among vertebrates. Having larger ranges of sizes and occupying top positions in the marine ecosystem, the study of bite force in
66 large predators may provide i nsights on their maximal feeding capabilities and its consequences in the food web dynamics in marine ecosystems. In this current stud y I analyzed bite force performance, the feeding biomechanics and the association with diet during ontogeny of two marine top predators the great barracuda ( Sphyraena barracuda ) and the bull shark ( Carcharhinus leucas ) These species s har e some similarities including large size and similar trophic levels (at least in part of their life history). These two s pecies provide an interesting model to analyze bite force performance through ontogeny. Additionally, occupying two different marine ecosystems, S. barracuda and C. leucas share high predatory success, and provide a good opportunity to evaluate if predatory success can be attained with similar bite force performance or by a combination of force generation and/or o ther feeding strategies The main result from this study is that Sphyraena barracuda and Carcharhinus leucas present important differences in their bite force perf ormance as well as in the scaling pattern of the associated biomechanical variables through ontogeny. Relative values of bite force in S. barracuda are among the lowest values found among fishes reported to date. These values are similar to th at of the bla ck belly lantern shark Etmopterus lucifer and spiny dogfish Squalus acanthias (Huber, 2006) (Table 3). Conversely, values of relative bite force in C. leucas are among the highest reported for all shark species being larger than th ose reported for the gre at hammerhead shark Sphyrna mokarran or the b la cktip shark Carcharhinus limbatus (Table 4) (Huber and Mara unpublished data; Huber et al., 2006). The scaling pattern
67 of all the biomechanical variables also differed between the two species Bite force and t he majority of the biomechanical variables that influence bite force showed isometry for S. barracuda In C leucas anterior bite force scaled with positive allometry and this result was supported by the same scaling pattern in the majority of t he adductiv e musculature. Although it is important to recognize that th ese two species can occupy two different ecosystems where prey items are not always the same c omparisons between juvenile S. barracuda and C. leucas when their diets are similar, have shown impo rtant differences between their bite force performance. For example, when comparing two individuals of similar sizes (73 cm) of S. barracuda and C. leucas, the differences in bite force performance are still high (50 N and 170 N respectively) where bull sh arks can exert more than three times the maximum bite force than the great barracuda even though the two species predate on bony fishes. Even though th ese two top predators play a similar successful role in the rely on the same feeding strategies or similar biting performance to occupy high er level trophic positions. In other words S. barracuda and C. leucas perform similar biological role with different biomechanical a pproaches. Similar biological roles in natur e can be performed in a variety of ways (Koehl, 1996). Patterns of functional equivalence or functional redundancy have been reported in several systems. For example, hind limb dimensions or diversity in muscular architecture may lead to the same success i n the locomotion patterns in lizards, or in snakes, different behavioral strategies to predate upon whiptail lizards
68 lead to similar predatory success (Toro et al., 2004; Green, 1986). Additionally, functional equivalence has been suggested to affect the d iversity and distribution of the j aws in labrid fishes, promoting morphological diversity (Alfaro et al., 2005). In this study S. barracuda and C. leucas have been shown to have different feeding strategies to attain similar success as marine top predators Alternative morphological and behavioral mechanisms that may enhance the feeding behavior in this two species were also suggested Sphyraena barracuda have been shown to have low values of bite force performance T his finding has been also described in sh arks such as S. acanthias where several other variables, other than force generation, were used to justify its success as a predator (e.g. teeth morphology and head shaking) (Huber and Motta, 2004). Similar strategies have been recognized and suggested to enhance S. barracuda predatory success F or example, the combination of extremely sharp teeth, speed efficient jaws and a mechanism of premaxillary rotation that may improve prey penetration by the premaxillary teeth may compensate the relatively lower bi te force found in S. barracuda Additionally S. barracuda does not undergo a remarkable switch in diet through ontogeny, negating the necessity for a proportional increase in bite force through ontogeny Carcharhinus leucas however, present a different sc enario, having one of the largest values of bite force among most other large shark species studied to date. B ull sharks have a disproportional increase in anterior bite force through ontogeny which may be associated with a switch in the diet over ontogeny since individuals above 140 cm TL change their dietary preference towards large elasmobranchs and
69 mammals (Cliff and Dudley, 1991). Positive allometry may allow organisms to exploit resources that were not available before and have important implications in organismal ecology (Hernandez and Motta, 1997; Meyers et al., 2002; Herrel and in prep ). The lack of information on the force required to process common prey items of C. leucas constraint our understanding of the sele ctive pressures associated with the high values of bite force found in this species. Several morphological features besides force production have been suggested to be associated with effective predation in bull sharks. Bull sharks attained large body size (maximum length 350 cm TL) and are characterized by wide rounded heads T his morphological characteristic in addition to tooth morphology and head shaking behavior may enhance bull sharks cutting mechanics and favor processing of large prey
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90 Appendix 1 Values of bite force of five tooth positions in S. barracuda Where 100 % is the most anterior bite point and 0 % i s the most posterior bite point in an ontogenetic series of 27 individuals of S. barracuda ranging from 18 130 cm, total length (TL)
91 Appendix 2 Lever distances in S. Barracuda Values of in lever, anterior out lever (100 %) and posterior out lever (0%) for 27 individuals of S. barracuda ranging from 18 130 cm, total length (TL)
92 Appendix 3. In lever distances (cm) for each subdivision of the adductor mandibulae complex Values are shown for 27 individuals of S. barracuda ranging from 18 130 cm, total length (TL).
93 Appendix 4 Values of bite fo rce of five tooth positions in C. leucas Where 100 % is the most anterior bite point and 0 % i s the most posterior bite point for 16 individuals of C. leucas ranging from 73 285 cm, total length (TL) TL (cm) 100% (FRONT) 75% 50% 25% 0 % (BACK) 73 36.40 38.20 49.30 69.40 169.80 75.00 59.00 53.60 65.50 98.30 214.00 76.00 53.80 58.70 68.00 83.30 123.00 77.50 50.70 58.30 70.70 101.40 166.50 78.50 67.80 76.70 92.50 132.00 218.00 81.80 58.20 63.70 81.10 121.60 270.20 82.20 95.10 ~ ~ ~ 338.00 85.10 78.60 86.10 97.70 133.00 241.00 101.00 147.00 160.10 187.00 248.00 854.00 108.00 232.70 236.10 284.10 419.00 931.10 117.00 316.70 ~ ~ ~ 1144.00 187.00 644.00 ~ ~ ~ 1168.00 201.00 795.00 886.00 1097.00 1582.00 2451.00 240.00 1168.10 ~ ~ ~ 2761.00 258.00 1023.00 ~ ~ ~ 3721.00 285.00 2128 ~ ~ ~ 5914.00
94 Appendix 5 Lever distances in C. leucas A nterior out lever (100 %) and posterior out lever (0%) for 16 individuals of C. leucas ranging from 73 285 cm, total leng th (TL)
95 Appendix 6 In lever distances (cm) for each adductor muscle Values for 16 individuals of C. leucas ranging from 73 285 cm, total length (TL)