Oxygen consumption in midwater fishes and crustaceans from the eastern Gulf of Mexico

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Oxygen consumption in midwater fishes and crustaceans from the eastern Gulf of Mexico

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Title:
Oxygen consumption in midwater fishes and crustaceans from the eastern Gulf of Mexico
Creator:
Donnelly, Joseph
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Tampa, Florida
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University of South Florida
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English
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vii, 48 leaves : ill. ; 29 cm.

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Marine fishes ( lcsh )
Crustacea ( lcsh )
Dissertations, Academic -- Marine Science -- Masters -- USF ( FTS )

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General Note:
Thesis (M.S.)--University of South Florida, 1986. Bibliography: leaves 44-48.

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University of South Florida
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Universtity of South Florida
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All applicable rights reserved by the source institution and holding location.
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020391782 ( ALEPH )
15219873 ( OCLC )
F51-00021 ( USFLDC DOI )
f51.21 ( USFLDC Handle )

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OXYGEN CONSUMPTION IN MIDWATER FISHES AND CRUSTACEANS FROM THE EASTERN GULF OF MEXICO by Joseph Donnelly A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in the department of Marine Science in the University of South Florida August, 1986 Major Professor: Joseph J. Torres, Ph.D.

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Graduate Council University of South Florida St. Petersburg, Florida CERTIFICATE OF APPROVAL MASTER'S THESIS This is to certify that the Master's Thesis of Joseph Donnelly with a major in Harine Science has been approved by the Examining Committee on 2 February 1986 as satisfactory for the Thesis requirement for the of Science degree, Thesis Committee: Major Professor: Joseph J. Torres, Ph.D. Hember: Thomas L. Hopkins, Ph.D. Member: John C. Briggs, Ph.D.

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ACKNOWLEDGEl1ENTS As with all scientific endeavors, this has been far from a solitary effort. Very special thanks to Jose' Torres without whose help, guidance and support this work would have not been possible. Also, thanks to Jack Gartner and Tom Lancraft for their much needed assistance in identifying animals as well as their critical reviews of this manuscript; to Brad Weigle for all the computer help and the companionship over the past years; to all ._my other "standard station" comrades, especially those fortunate few who were kind enough to share the countless hours spent over respirometer racks; and to "Professor" Mullins for the total shop experience and for making sure the past four years were never dull. Lastly, for two very special people, one who will unfortunately probably never see this and the other who knows every aspect of this work. JMB, your impact on the past four years of my life has been more than you will ever realize, thanks. And to the Big Guy, thanks for the patience and motivation to see this through but more importantly for the peace of mind. ii

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TABLE OF CONTENTS LIST OF TABLES iv LIST OF FIGURES v ABSTRACT vii INTRODUCTION 1 HYDROGRAPHY OF THE EASTERN GULF OF MEXICO 4 MATERIALS AND METHODS 7 RESULTS 10 Regulation of oxygen consumption 10 Oxygen consumption as a function of temperature 24 Oxygen consumption as a function of depth of occurrence 32 DISCUSSION 33 CONCLUSIONS 43 LITERATURE CITED 44 iii

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LIST OF TABLES 1. List of species, number tested, wet weights, and depth distributions. 2. Weight-specific respiration rates, Q10's, and Pc values for fish and crustaceans. 3. Changes in the P in relation to changes in temperature and respiration fate for species with multiple runs. 4. Comparison of respiration rates with data from other investigators on con-specific fish and crustaceans. 5. Comparison of respiration rates with data from other investigators on con-generic fish and crustaceans. iv 11 14 23 34 36

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LIST OF FIGURES 1. Temperature vs. depth in the eastern Gulf of Mexico 5 at 27N, 86W, 2. P vs. oxygen consumption rate for species with runs 19 af multiple temperatures. Letters are reference symbols for species listed in Table 1. 3. Oxygen consumption rate vs. temperature: crustaceans, 21 (a) all crustaceans (b) Carids (c) Penaeids (d) Sergestids (e) Euphausiids. 4. Oxygen consumption rate vs. temperature: fishes. 22 (a) all fishes (b) Myctophidae (c) non-Myctophidae. 5. Oxygen consumption vs. minimum depth of occurrence: 27 crustaceans. (a) at 7C (b) at l4C (c) at 20C. 6. Oxygen consumption vs. minimum depth of occurrence: 2 8 fishes. (a) at 7C (b) at l4C (c) at 20C. 7. Oxygen consumption rate vs. m1n1mum depth of occurrence: 29 crustaceans -temperature considered. 8. Oxygen consumption vs. minimum depth of occurrence: 3 0 fishes -temperature considered. 9. Percent water content vs. m i n imum dept h o f occurrence. 31 (a) fishes (b) crustaceans. v

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OXYGEN CONSUMPTION IN MIDWATER FISHES AND CRUSTACEANS FROM THE EASTERN GULF OF MEXICO by Joseph Donnelly An Abstract Of a thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in the Department of Marine Science in the University of South Florida August, 1986 Major Professor: Joseph J. Torres, Ph.D. vi

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Oxygen consumption was measured as a function of temperature, oxygen partial pressure and species depth of occurrence for thirty-nine species of midwater fish and crustaceans collected from the eastern Gulf of Mexico. Overall Q10's of 2.19, 2.28, 2.09 and 3.16 were recorded for Sergestid, Penaeid, Carid and Euphausiid crustacean groups, respectively, while values of 5.59 and 1.64 were recorded for Myctophid and non-Myctophid fish groups, respectively. Individual Q10's were generally consistent within each group. All of the species tested were capable of regulating their oxygen consumption at P0 2 levels normally encountered within the eastern Gulf. P values ranged from 20-40 mmHg c and were found to increase slightly with increasing temperature and respiration rate. Decline in respiration with increasing minimum depth of occurrence was found to be primarily a function of temperature alone. Changes in size, dry weight and water content contributed only a small fraction of the observed decrease. This finding contrasts considerably with studies from the eastern Pacific where temperature contributes only minor input to changes in respiration rate with depth. Environ-mental and biological reasons and implications are discussed. Abstract approved: _______________________ Major Professor: Joseph J. Torres, .Ph.D. Associate Professor I Marine Science

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1 INTRODUCTION Despite the obvious difficulties associated with studying mesopelagic species, most notably collecting specimens alive and in good condition, there is a substantial literature describing their metabolism (Belman & Gordon, 1979; Childress, 197la, b, 1975, 1977a; Childress & Price, 1983; Childress & Somera, 1979; Childress et al., 1980; Gordon, 1972a, b, 1975; Gordon et al., 1976; Hiller-Adams & Childress, 1983; Meek & Childress, 1973; Mickel & Childress, 1982; Napora, 1964; Pearcy & Smail, 1968; Quetin & Childress, 1976; Smith & Hessler, 1974; Smith & Laver, 1981; Teal, 1971; Teal & Carey, 1967; Torres et al., 1979; Torres & Childress, 1985). However, with the exception of three limited studies in the Atlantic (Napora, 1964; Teal, 1971; Teal & Carey, 1967) all of the available data deal with eastern Pacific species from temperate systems. Thus, there is little information concerning respiration on species from tropical-subtropical areas and as y e t no studies at all from the Gulf of Mexico. Hydrographic conditions among these regions, most notably the temperature and dissolved oxygen profiles, are quite different. Temperature in the upper lOOOm range s from 4 to l7C in the eastern Pacific (GEOSECS, 1981; Scripps, 1965) compared to 5-29C in the eastern Gulf of Mexico (this study). Dissolved oxygen l evels within the lOOm mixed layer are near saturation for both howe ver below tha t t h e profiles differ significantly. In the eastern Pacific, o2 levels drop sharply to as low as 0.2 ml/1,

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2 forming a severe and extensive minimum layer (Childress, 1975; GEOSECS, 1981), while levels in the eastern Gulf remain fairly high throughout the water column. Considering the significant physical differences among these regions, individuals could very well show a varied, if not dissimilar, metabolic adaptation to the midwater environment. Studies from the eastern Pacific encompassing a variety of organisms (Childress, 197la, b, 1975; Gordon et al., 1974; Meek & Childress, 1973; Smith & Hessler, 1974; Torres et al., 1979) have shown oxygen consumption to decrease with increasing minimum depth of occur-renee (MDO: That depth below which 90% of the population lives, Childress, 197lb). The reasons for the decline are not completely understood and whether or not the same causative factors are present in other oceanic systems is also unknown. Childress (1977b) noted that distinctive temperature regimes probably result in dissimilar depths of occurrence among congeners from different locations. It follows that in addition to directly affecting metabolic rates, a varied temperature profile could indirectly influence the oxygen consumption-MOO relation-ship. Other factors such as the depth and severity of the oxygen minimum layer and the degree of vertical migration within the two communities could also have considerable effect upon the adapted char-acteristics of individual species: For example, the very low oxygen partial pressure (P0 2 ) in the minimum layer off southern California has resulted in a number of physiological adaptations including lower respiration rates, enhanced regulatory capabilities, and increased blood oxygen affinities (Arp, unpub.; Childress, 197la, 1975, 1977a; Douglas et al., 1975; Gordon et al., 1976; Meek & Childress, 1973). In contrast, the oxygen minimum layer in the eastern Gulf of Mexico is

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considerably less severe (Hopkins, 1985; Nowlin, 1971; Schmidt, 1925) and thus presents a different set of selective pressures to the biological community. 3 It seems clear then that to fully understand the open-ocean environment and the animals that reside there, information from geographically separate and hydrographically distinctive areas is needed. This study is a contribution to that end. In addition to broadening the data base on the biology of deep ocean systems, the more specific objectives of this research are threefold: (1) to examine the metabolic expenditures of a variety of species of epi-and mesopelagic micronekton from a tropical-subtropical system through the direct measurement of oxygen consumption rates; (2) to elucidate the effects of temperature and external PC2 on oxygen consumption; (3) to investigate the phenomenon of decreased oxygen consumption with increased MDO in species from a tropical-subtropical system.

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4 OF THE EASTERN GULF OF MEXICO The hydrography of the eastern Gulf around the standard sampling station at 27N, 86W is dictated by the Gulf Loop Current. The current is a core of warm, saline water that pulses in a clockwise rotation, entering through the Yucatan channel and exiting through the Florida Straits with maximum current velocity and subsequent Loop penetration into the northern Gulf usually occuring in the summer (Nowlin, 1971). The water often encountered at 27N, 86W is not Loop Current itself but rather Loop Transition Water (Jones, 1973). This water has slightly lower surface temperatures and salinities and can be differentiated from the Loop Current by the depth of the 22C isotherm. In the Loop Current the 22C isotherm occurs between !50-200m while in the Loop Transition Water it occurs shallower between 50-lOOm (Jones, 1973). Figure 1 shows the average temperature vs. depth profile for the sampling area during the collection periods; from the depth of the 22C isotherm the profile indicates that Loop Transition Water was present at the site for all eight cruises. While vertical temperature profiles fluctuate, biological characteristics are similar with relatively little variation in zooplankton and micronekton species composition (Hopkins, 1982). Dissolved oxygen reaches a minimum of roughly 2.8 ml/1 at a depth of 450-500m, decreasing from a surface level of approximately 4.7 ml/1. Below the minimum layer o2 concentration increases with depth to a level of 3.9 ml/1 at lOOOm. There is a slight secondary o2 minimum of around

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25 22 """" u 20 0 .._, w c: 1-< c: 15 w a. :E w 110 5 0 200 400 600 800 DEPTH (m) Figure 1. Temperature vs. depth in the eastern Gulf of Mexico at 27N, 86W. 1000 5

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. 3.2-3.9 ml/1 occurring at 250m presumably as a consequence of increased biological activity (Hopkins, unpublished data; Nowlin, 1971). 6

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MATERIALS AND METHODS Animals were collected over eight cruises from June 1981 to July 1985 from a fifteen mile radius around 27N, 86W. Sampling was conducted from either the R/V Bellows or R/V Suncoaster, both operated by the Florida Institute of Oceanography. Nets used in sampling were 1.8 x 1.8m or 1.8 x 3.6m Tucker-type trawls fitted with a thermally protecting cod-end bucket (Childress et al., 1978) towed at a speed of approximately two knots. Tows were conducted during both day and nighttime hours over various depths within the upper 1000 m. 7 Upon net retrieval the entire catch was transferred immediately to a bucket of chilled seawater. To help prevent thermal shock, specimens from deep trawls were put in 5-10C water while shallower catches were placed in warmer, 15-20C water. Whenever possible animals were tested immediately (ie. within 30 minutes). If not, they were kept in aerated seawater maintained in the dark at 5-l0C for two to seventy two hours. In all instances only the hardiest specimens were selected for use. Oxygen consumption rates were determined by allowing an individual to deplete the oxygen in a sealed, water-jacketed chamber filled with filtered seawater (0.45 urn pore size). The time required varied from four to twenty hours depending on the size of the animal, its physical condition, and its level of activity throughout the run. Experimental temperatures were maintained within 0.1C by means of a refrigerated water bath. The oxygen partial pressure in the chambers was monitored

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8 continuously by Clark-type polarographic oxygen electrodes (Clark, 1956). To insure proper functioning of the electrodes, stirring bars were placed in each chamber, turned at the lowest possible speed, and separated from the main portion of the respirometer using a perforated false bottom to minimize disturbance of the animal. Electrodes were calibrated at each experimental temperature before each run using airand nitrogen-saturated seawater. Streptomycin and Neomycin (SO mg/1 each) were added to the water to minimize bacterial growth. To measure the possible contribution of bacterial respiration, controls were conducted after selected runs. At the completion of selected runs the animal was removed, its volume replaced with fresh filtered water, the chamber resealed, and o2 consumption monitored for 4-6 more hours. In all cases any microbial respiration was negligible in comparison to the total rate of the animal (<5%). The respirometers used were of three basic designs: (i) a long glass tube jacketed by a lucite cylinder permanently sealed at one end with a lucite plate with a removable, a-ring sealed, lucite lid at the other end; (ii) a cylindrical pyrex dish jacketed by a piece of PVC pipe sealed at the bottom with a PVC plate and with a removable, a-ring sealed, lucite lid; (iii) a rectangular all-lucite chamber and jacket sealed at the bottom and with a removable, a-ring sealed, lid. All three types incorporated a lucite sieve plate placed inside the chamber to separate the stirring bar from the rest of the chamber. The electrodes were introduced into the chambers and held in place via air-tight PVC pipe fittings screwed into the lucite lids. Chamber sizes ranged from 59-3365ml to accommodate a wide size range of animals. Data were recorded using either a potentiometric strip chart

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recorder or a digital data-logger system. Oxygen consumption rates for a particular P02 were determined by measuring the decline in oxygen concentration over a 30 minute interval around that P02 For data acquired with the chart recorder, rates were read directly as the slope of the curve. For data acquired with the data-logger, rates were calculated as the means of values measured over the 30 minute period 9 around a given P02 The average respiration rate was calculated as the mean of individual rates recorded at P02's over a flattened portion of the rate vs P02 curve (usually occurring between 30 and 90 mmHg). Average wet weight-specific respiration rate (V02 ) values are considered to be estimates of routine respiration, as the rates were obtained over an intermediate P02 range subsequent_ to the initial excitory period but still above the compensation point (Pc: that P02 below which 0 2 consumption is no longer independent of external P02 ; Prosser, 1973) for the animal (analogous to the 30-70 mmHg rates of Childress, 1975). The maximum rate reflects the highest measured 0 2 consumption and was commonly recorded within the first two hours of the run. Similarly, the minimum rate is the lowest measured 0? consumption at a P02 above the P c All of the animals in the study withstood some degree of physical trauma during collection and testing, therefore the respiration rates reported here should be regarded only as estimates of natural .metabolism. Nonetheless, on species where multiple runs at the same temperature were available agreement among the measured rates was generally good.

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10 RESULTS Thirty nine species of fish and crustaceans representing eighteen families were examined (Table 1) with a total of one-hundred-sixty-five respiration runs conducted at five experimental temperatures (Table 2). For purposes of analysis and discussion, all the animals are categorized into six groups: Myctophids, non-Myctophids, Euphausiids, Sergestids, Penaeids, and Carideans. For several species only single runs were obtained. This is due to the high diversity in the Gulf of Mexico, which precludes predictable repetitive capture for most species. Although some important species are represented here, the data set is characteristic of the indigenous micronekton community in the eastern Gulf of Mexico (Gartner et al., MS in prep.; Gartner & Hopkins, unpublished data). Regulation of oxygen consumption Values for the P. were recorded for three fish and fifteen c crustacean species (Table 2). No values were available when (i) the run was terminated before a Pc was reached or (ii) no clear abrupt change in respiratory rate was readily apparent. All of the values recorded are sufficiently lower than the lowest P02's present in the eastern Gulf of Mexico (65 mmHg at a depth of approximately SOOm; Hopkins, unpublished data) to allow for completely aerobic metabolism throughout

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11 Table 1. List of species, number tested, wet weights, and depth distributions. Ref. Wet Weight(g) % AFDW as Depth Distribution (m) Species Sym. # (range) H 2 0 % of DW day night MOO MYCfOPHIDAE Myctophum A 2 1.462 72.1 73.9 500-600 0-600 0 affine (.315-2.609) Diaphus B 2 0.168 47.6 78.6 400-500 80-200 90 moll is (.112-.223) Lampanyctus c 1 5.923 87.8 72.5 400-1000 80-1000 130 lineatus Larnpanyctus D 1 1.186 76.2 80.5 550-600 100-600 120 nobilis Lampanyctus E 1 0.361 77.5 80.6 550-700 80-200 110 alatus Lepidophanes F 3 1.292 74.5 76.9 500-800 80-1000 105 guentheri ( .937-1.696) Bolinichthys G 1 0.658 74.4 82.5 600-700 80-130 105 photo thorax NON-MYCfOPHIDAE GONOSTOHATIDAE Gonostoma H 7 10.192 88. 2 72.6 450800 80-900 140 elongatum (4.906-19.72) STERNOPTYCHIDAE Argyropelecus I 1 3.478 79.2 78.5 160-900 130-900 165 aculeatus MELAMPHAEIDAE Scopelogadus J 1 12.220 85.7 82.0 800-1000 400-600 500 mizolepis mizolepis MELANOSTOMIIDAE Echiostoma K 1 1.637 89.3 70.0 70-450 70-450 100 barbatum

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12 Table 1 (cont'd). ANOPLOGASTERIDAE Anoplogaster L 2 17.368 90.1 72.7 * 600 cornuta (7.817-26.92) CHIASMOOONTIDAE Pseudoscopelus M 2 2.060 62.3 89.8 * 800 altipinnis ( 1. 58-2. 54) MORIDAE Physiculus N 1 0.704 65.9 90.4 * 200 fulvus CERATIIDAE Cryptopsaras 0 1 3.517 88.0 74.0 * couesi CETOMIMIDAE Cetomimus sp. p 1 1.168 87.4 85.3 1000 1000 1000 ANGUILLIFORMES Leptocephalus Q 1 6.473 94.9 95.2 * 0 scalar is EUPHAUSIIDAE Thysanopoda R 7 0.174 73.5 85.7 400->800 100-700 200 monacantha (.137-.222) Thysanopoda s 1 0.173 71.7 69.9 > 500 >500 500 orientalis Thysanopoda T 1 0.173 75.6 74.6 200->400 200->400 200 obtusifrons Bentheuphausia u 1 0.134 68.2 77.3 >1000 >1000 1000 amblyops SERGESTIDAE Sergestes v 3 0.284 69.4 89.3 40-850 0-500 125 corniculum (.047-.419) Sergestes w 3 0.209 72.3 84.9 300-600 0-600 150 armatus (.054-.318) Sergia grandis X 3 0. 713 76.2 81.9 600-800 300-600 400 (.629-.764)

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13 Table 1 (cont'd) Sergia filictum y 3 1.764 76.4 84.4 300-1000 0-900 150 (.488-2.447) Sergia z 4 0.202 74.7 84.6 500-1000 0-200 125 talismani (.114-.332) Sergia robust us a 9 1.635 75.0 82.1 300-1000 100-900 225 (.519-2.937) Sergia b 12 0.286 70.6 85.4 120-1000 0-1000 195 splendens ( .116-. 412) PENAEIDAE Gennadas d 2 0.512 75.2 83.8 275-700 75-925 135 scutatus (.123-.180) Gennadas e 2 0.265 67.5 300-950 275-950 410 capensis (.194-.336) Gennadas valens f 20 0.534 75.5 85.7 250-800 100-425 290 (.238-.819) Hymenopenaeus g 1 0.802 81.6 77.3 >800 >800 800 cf.aphoticus Funchalia h 11 2.086 72.9 86.6 150-1000 0-900 70 villosa (.478-2.634) CAR IDEA Acanthephyra i 12 1.391 73.6 79.6 400-1000 200-550 325 purpurea (.481-2.584) Acanthephyra j 1 2.183 74.1 83.4 800-1000 500->1000 500 acanthitelsonis Oplophorus m 9 2.010 69.5 81.0 350-600 90-450 100 gracilirostris ( .112-3.130) Systellaspis n 25 1.118 71.6 84.8 100-1000 100-900 150 debilis (.158-1.888) Pasaphaea r 1 3.221 76.2 86.2 400-600 400 merriami Parapandalus t 5 0.284 70.8 82.7 100-600 80-350 150 richardi (.054-.380) Depth distributions not known or uncertain; MOO given is best estimate of minimum depth for adult specimens.

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14 Table 2. Weight-specific respiration rates, Qla's, and P values for fish and crustaceans. c Exp. Minimum Maximum OW AFDW Ref. Temp. # of VOJ rate rate conversion conversion (T i_!.!l-2) Pc Sym. deg.C runs (cr (cr) (q) (range) (range) mmHg A 7 1 0.038 0.010 0.136 2.97 3.41 ( .027) 14 1 0.194 0.194 0.468 4.20 6.30 10.27 ( .000) (7-14) B 20 2 0.292 0.292 0.292 1.91 2.43 (.125) (.125) (.125) ( 1. 79-2 .03) (2.27-2.59) c 7 1 0.017 0.013 0.021 8.19 11.30 (.004) D 7 1 O.O.:.:i 0.027 0.103 4.21 5.23 (.018) E 14 1 0.060 0.032 0.086 4.44 5.51 (.019) F 7 1 0.049 0.011 0.116 4.05 4.94 ( .028) 14 1 0.084 0.081 0.087 3.98 5.09 2.16 (.003) (7-14) 20 1 0.229 0.228 0.231 4.24 5.26 5.32 ( .001) ( 14-20) 3.27 (7-20) G 17 1 0.311 0.266 0.355 3.91 4.74 ( .045) H 7 4 0.039 0.029 0.050 8.45 11.66 ( .015) (.008) (.023) (7.66-9.18) (10 .66-12.46) 14 3 0.071 0.060 0.077 8.46 11.64 2.35 ( .023) (.019) (.026) (7.73-8.98) (10.44-12.94) (7-14) I 20 1 0.151 0.128 0.173 4.81 6.13 ( .023) J 14 1 0.024 0.014 0.036 6.97 8.50 (.005)

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15 Table 2 (cont'd). K 14 1 0.039 0.032 0.126 9.36 13.37 (.005) L 7 1 0.032 0.028 0.071 8.85 12.88 (.003) 20 1 0.155 0.056 0.262 11.69 15.26 3.37 45-50 ( .084) (7-20) M 7 1 0.021 0.020 0.021 2.54 2.82 (.001) 14 1 0.252 0.160 0.305 2.76 3.07 34.81 (.056) (7-14) N 10 1 0.310 0.286 0.334 2.93 3.24 ( .024) 0 20 1 0.122 0.107 0.174 8.30 11.22 35-4(} (.011) p 7 1 0.082 0.072 0.094 7.92 9.29 45-50 (.008) Q 14 1 0.021 0.020 0.043 19.56 20.55 (.001) R 7 5 0.086 0.064 0.151 4.06 4.73 ( .019) (.019) (.063) (3.72-4.26) (4.19-5.05) 14 2 0.196 0.122 0.296 3.06 3.57 3.24 (.008) (.053) (.062) (3.02-3.09) (3.53-3.61) (7-14) s 7 1 0.083 0.076 0.097 3.53 5.05 (.010) T 20 1 0.358 0.239 0.517 4.09 5.48 (.119) u 7 1 0.074 0.068 0.080 3.14 4.06 ( .006) v 7 2 0.090 0.081 0.146 3.24 3.58 ( .067) (.070) ( .078) ( 2. 35-4 13) (2.47-4.68) 20 1 0.170 0.129 0.300 3.18 3.84 1.63 35 (.043) (7-20) w 7 2 0.085 0.034 0.466 3.53 4.09 (.005) (.014) (.033) (3.38-3.68) (3.86-4.31) 14 1 0.217 0.168 0.379 3. 77 4.59 3.81 25-30 (.'036) (7-14)

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16 Table 2 (cont'd). X 14 1 0.159 0.108 0.317 3.57 4.49 30-35 (.036) 17 1 0.165 0.139 0.217 5.18 6.31 1.13 (.022) (14-17) 20 1 0.251 0.206 0.289 3.87 4.61 4.05 45-50 (.030) ( 17-20) 2.14 (14-20) y 14 1 0.061 0.061 0.081 3.87 4.58 (.001) 20 2 0.182 0.146 0.271 4.41 5.23 6.18 (.015) (. 036) (.059) (4.13-4.68) (4.80-5.65) (14-20) z 7 1 0.141 0.046 0.574 4.05 4.81 (.093) 20 3 0.263 0.185 0.654 3.93 4.63 1.62 35 (. 028) ( .040) (.206) (3.35-4.24) (3.93-5.03) (7-20) a 7 1 0.094 0.081 0.103 4.65 5.62 ( .007) 12 1 0.094 0.082 0.187 3.94 4.65 (.014) 14 5 0.174 0.111 0.292 3.70 4.57 2.41 30 (.032) (.006) ( .037) (3.02-4.30) (3.51-5.57) (7-14) 17 1 0.209 0.176 0.234 4.20 5.03 1.84 25-30 (.022) ( 14-17) 20 1 0.351 0.308 0.533 4. 72 5.70 5.63 ( .047) (17-20) 2.22 (7-17) b 7 6 0.086 0.071 0.111 3.52 4.15 ( .027) ( .028) (.020) (2.63-4.40) (3.08-5.34) 14 1 0.187 0.165 0.301 3.27 3.99 3.03 (.022) (7-14) 20 5 0.300 0.194 0.541 3.94 4.56 2.20 35 (.056) ( .057) (.111) (3.65-4.42) (4.11-5.14) (14-20) 2.61 (7-20) d 14 1 0.146 0.083 0.232 3.67 4.69 ( .063)

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17 Table 2 (cont'd). 20 1 0.215 0.199 0.763 4.39 4.92 1.91 30-35 ( .017) (14-20) e 7 2 0.098 0.071 0.218 3.08 ( .001) (.022) ( .096) (2.99-3.16) f 7 7 0.092 0.067 0.190 4.31 5.11 (.018) (.023) (.092) (3.49-4.85) (4.07-5.81) 14 5 0.137 0.100 0.205 3.76 4.46 1.77 25-30 (.026) (.026) (.084) (2.75-4.56) (3.10-5.81) (7-14) 20 8 0.244 0.190 0.464 4.08 4.63 2.62 30-35 (.054) (.059) (.147) (3.67-4.76) (4.18-5.75) (14-20) 2.12 (7-20) g 7 1 0.063 0.100 0.055 5.45 7.05 (.007) h 7 5 0.061 0.040 0.106 3.56 4.08 2o-2s (. 024) ( .023) (.057) (3.32-3.79) (3.77-4.41) 20 6 0.217 0.177 0.362 3.80 4.37 2.65 30-35 (.051) ( .060) (.118) (3.71-3.92) (4.29-4.44) (7-20) i 7 10 0.085 0.067 0.152 3.89 4.76 25-30 (.014) ( .016) (.117) (3.23-4.76) (3.80-5.80) 14 1 0.199 0.120 0.359 2.79 3.37 30-35 (.061) (7-14) j 10 1 0.071 0.068 0.074 3.86 4.63 35-40 (.022) m 7 4 0.095 0.072 0.198 3.41 4.21 25 (. 041) (.049) (.108) (2.98-3.98) (3.61-4.98) 20 5 0.303 '1.238 0.498 3.18 3.92 2.44 30 (.083) (.068) (.132) (2.64-3.73) (3.06-4.87) (7-20) n 7 12 0.077 0.056 0.164 3.07 3.48 25 (.014) ( .020) ( .098) (2.50-3.35) (2.81-3.76) 14 3 0.156 0.100 0.250 3.22 3.97 2.74 30 (.035) ( .028) ( .079) (3.03-3.33) (3.79-4.26) (7-14) 20 10 0.164 0.128 0.297 3.83 4.61 1.09 35-40 (.037) (. 038) (.096) (3.02-4.64) (3.38-5.92) (14-20) 1. 79 (7-20) r 7 1 0.147 0.129 0.188 4.20 4.87 30 (.011)

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18 Table 2 (cont'd). t 7 3 0.094 0.064 0.139 3.56 4.29 (.062) ( .038) (.074) (3.18-3.80) (3.86-4.51) 14 1 0.136 0.089 0.177 2.56 3.10 1.69 (.033) (7-14) 17 1 0.245 0.234 0.257 4.01 4.98 7.11 25-30 (.010) (14-17) 2.61 (7-17) Note: V02 minimum and maximum rates expressed as ul 0 2/mg wt wgt/hr.

PAGE 27

u Q. 40 30 19 X h Figure 2 0.022 0.115 0.198 0.288 OXYGEN CONSUMPTION RATE (IJI 02mg WET WEIGHT-1-h-1) P vs. oxygen consumption rate for species \oJiti.1 runs a t temperatures. Letters are reference symbols f o r species listed in Table 1

PAGE 28

20

PAGE 29

Figure 3. Oxygen consumpt.ion rate vs. temperature: crustaceans. (a) all crustaceans (b) Carids (c) Penaeids (d) Sergestids (e) Euphausiids.

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I 0 u 0 N 0 N .. 0 0 I o.!:. 0 0 .. .. -' ci C,l .. 0 II -0 .. .. -.. O J2 ft 0 0 .. 0 0 N o!. .. .. 02 -. .II., C,l .. 0 I -0 ""' .. .. a: O 2 .. .. 0 .. 0 0 .. oJ. .. o2 0 0 C,l .. 0 I -0 I 01 .. c eo .. .. .. 0 0 0 0 0 0 c,_ 'f ,_.1HOI3M .13M !It) 3.1 Vl:t NOI.ldftn&NOO N30AXO 0 .. .. .. 2 .. oil 0 N .. N 2 .. ., .... (J .... w Ct ;:::) .... c Ct w CL. 2 w .... .., 0 .. 0 ci I : .. .. 0 ci 0 I 01 .. a: 0 .., 0 I.; 0 I ci 0 I 01 Q:.. -0 ., o .. -( .LH013M 13M tit) L-L31 Vij NOI..Ldftn&NOO N30AXO 0 .. 21 .... ., (J .... I 0 w Ct ;:::) .... c Ct w CL. :1 w ....

PAGE 31

I I I I o .aot w ... .... I t ,.O.Ott ..... Jt ,.o.o1a.-o.oee I a: ........ ... 110.741 ... z I .... a.o.ooe .... C>O.OOI .... <>0.011 0 1-a10C7-...._.I a10cr-...._.. Qao(7-... l.e4 % ... CJ A. w O.t 2 :: I / I II / II ., z 1-0 w O.t2L / 0 z a w t a.at / I I CJ > I t-, )( 0 I .II .17 .b II I I I I I I I I I I I I I I I I I I I I 1 10 12 14 10 20 I 7 10 II 14 t1 17 10 I 1 10 11 1411 17 TEMPERATURE (C) Figure 4. Oxygen consumption rate vs. temperature: fishes. (a) all fishes (b) Hyctophidae (c) non-Myctophidae. I c I 10 N N

PAGE 32

24 the vertical ranges of the individuals. In species that are able to regulate their oxygen consumption, the Pc is found to be directly related to temperature and respiration rate (Childress, 1971a; Quetin & Childess, 1976; Prosser, 1973). In this study, Pc's at two different temperatures were obtained for Sergia grandis, Gennadas valens, Funchalia villosa, Acanthephyra purpurea, Oplophorus gracilirostris and Systellaspis debilis. In all cases the Pc was found to increase with increasing temperature and vo2 (Fig.2). Assuming a direct one would expect that small increases in temperature or V02 would result in small increases in the Pc. This is case for valens, A purpurea and [. villosa but seemingly not so for grandis, Q. gracilirostris and debilis (Table 3). For S. grandis, a small increase in temperature and vo2 results in a large increase in the P For 0. gracilirostris, large increases cause only c a minor P change while for S. debilis, a large temperature increase c but a small vo2 increase result in a relatively large Pc change. Whether these individual variations reflect actual differences in physiological response or are simply artifacts of changing activity levels or the general condition of the animals is not clear. Oxygen consumption as a function of temperature Respiration rate versus temperature is plotted for each of the six groups (Figs.3 & 4). shown for each group. Q 's were calculated over a 7-20C range and are 10 Values for the crustacea ranged from 2.09 to 3.16 (Figs.3b-3e) with an overall Q10 of 2.27 (Fig.3a). The Q 10 for the euphausiids (3.16) was higher than for the three decapod groups

PAGE 33

25 (2.09-2.28) indicating a greater change in oxygen consumption rate between daytime and nighttime The wide range of individual rates among the non-myctophids (Fig.4c) results in a poor group correlation and a questionable Q10 value. However, the myctophid group had a much higher correlation and a Q10 of 5.59 indicating a substantial change in respiration over daytime and nighttime depths. This is not surprising as all of the species tested are strong daily migrators and present in shallow epipelagic waters at night. Individual Q10's were measured for four fish and fifteen crustacean species (Table 2). With some exceptions the values are all within the commonly observed range of 1.5 to 3.0 (as noted by Childress, 1977). Anoplogaster cornuta, Thysanopoda monacantha, Sergestes armatus and Acanthephyra purpurea showed Q10's slightly higher than this range. For A. cornuta (3.37) the Q10 is likelya consequence of (i) a large difference in weights between the two specimens and (ii) the fact that 20C is outside the normal temperature range experienced under natural conditions. Both factors would serve to widen the difference in measured rates, thereby elevating the Q10. For I monacantha CQ10= 3.24), armatus and A. purpurea (Q10=3.37) the higher Q101s appear to be purely a consequence of individual variations among runs as weights were comparable and experimental temperatures well within the species natural range. Myctophum affine, Pseudoscopelus altipinnis and Sergia filictum showed greatly elevated Q10's. This was due primarily to large disparities in activity levels among individual runs. The problem was further compounded in tl. affine by a large weight difference between specimens and in E altipinnis by the poor physical condition of the specimen tested at 7oC.

PAGE 34

Q10 values over multiple temperature ranges were obtained for one myctophid and six decapod species (Table 2). Sergia splendens and Gennadas valens show small changes in Q at higher temperatures. S. 10 splendens, an epipelagic resident at night, shows a slight decrease in Q10 (3.03-2.20) possibly implying some thermal compensation at higher 26 temperatures. G. valens is upper-mesopelagic at night so the increased Q10 at the higher temperature (1.77-2.62) may reflect temperature limitations at the upper depth of its vertical range. The remaining five species show large changes in Q Systellaspis debilis, 10 epipelagic at night, shows a considerable drop in Q (2.74-1.09) 10 implying a substantial increase in compensation. Sergia robustus shows a combination of effects. the upper temperatures of its range there is a small compensatory reaction and a concomitant drop in Q10 (2.41-1.84). At temperatures outside the normal range of the animal, the thermal effect becomes exaggerated and the Q10 increases sharply to 5.63. An inability to cope with temperatures above its natural range is also shown by Sergia grandis, with a Q increase at 10 higher temperatures of 1.13-4.05. For Lepidophanes guentheri, the large increase in Q10 (2.16-5.32) may reflect natural changes in activity between daytime and nighttime depths. The elevated Q is consistent 10 with the observed response of the myctophid group in general (Fig.4b) as well as with the findings by Torres et al.(1979) of higher than normal Q values for vertically migrating myctophid 10 species.

PAGE 35

..... 3: en .... Zw 03: 0 z a w E (!) "' >0 X 0 :::L ""' 0.90 0.60 0.30 .... 0.20 y.106x -0.030 2 R .009 95% CI.169 1 e I I W 0.07 ... 0.060 1 250 500 I 750 ... ... ... 1 al 1000 0 y=0.147x 0 025 R 2=0.003 95% CI=0.360 I 250 b 500 0 MINIMUM DEPTH OF OCCURRENCE (m) y=0.105x 0 166 R 2=0.087 95% CI=0.379 250 Figure 5. Oxygen consumption vs. minimum depth of occurrence: crustaceans. (a) at 7C (b) at 14(; (c) at 20C. c 500 N .......

PAGE 36

w-.... s: a:z 0 ... l: ... CJ G. :1 w ::) (I) ... z w 0 0 z Ot w I? CJ c-.. > 0 )( :a. 0 0.20 ... .J r.03aa o.ooa R 1.001 16 .. CI.226 0.070 t : 0.030 t 0.010! I I I 250 600 760 I t- a 1000 0 =0.2 '6. -0.212 R 2.770 86 .. Cl.222 250 500 b 750 0 MINIMUM DEPTH OF OCCURRENCE (m) 250 r.818x -0.288 R2.727 86 .. CI.334 500 750 Figure 6. Oxygen consumption vs. depth of occurrence: fishes. (a) at 7C (b) at l4C (c) at 20C. c 1000 I ...:I (I;)

PAGE 37

w t-.,.. -c: .c a:.,... z t-C) O.w 0t Zw 0 z a w I? c:J N >0 )( 0 0.90 0.80 0.70 o.eo 0.50 0.40 0.30 0.20 0 .10 -0.582 y=3.733x R 2 ::a0.779 95'4 CI=O. 159 250 500 750 100 200 300 800 850 29 1000 20 11 14 10-12 1 5 TEMPERATURE IN D E PTH IN T ERVA LS(ec) MINIMUM DEPTH OF OCCURRENCE (m) Figure 7 Oxygen consumption rate vs. minimum depth of occurrence: crusta ceans -temperature considered.

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30 o .ao 0.70 o .eo o .so 0.-'0 Ye.oe1x -o.aoo w A 2.779 1-I -c .c 0 30 CI=O.S52 a: z I 0 1J: 0.20 1-a. 2 w 3: (/) 1z w 0 3: 0.10 (J a 0.09 z e 0.08 w C'l 0 07 > 0 o.oe >< 0 o.os o.o. 0.03 2SO 500 750 1000 100 200 300 600 850 J"---2oe 11 1 10-12 1 s TEMPERATURE IN DEPTH INTERVAL (C) MINIMUM DEPTH OF OCCURRENCE (m) Figure 8. Oxygen consumption vs. minimum depth of occurrence: fishes -temperature considered.

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90. eo # 70 ... 60- J: Y=0.009x +74.953 R 2=0.06S w 95% CI.021 ... so-w a 0 T u. 250 500 750 1000 0 ?/! ... z w ... 90 z 0 y=0.001x+72.878 (.) R 2.009 a: CI.008 w 80-... < .. I 70 b eo 0 250 500 750 1000 MINIMUM DEPTH OF OCCURRENCE (m) Figure 9. Percent water content vs. minimum depth of occurrence. (a) fishes (b) crustaceans. 31

PAGE 40

Oxygen consumption as a function of depth of occurrence Minimum depth of occurrence values as well as day-night depth distributions are listed in Table 1. With the exception of a few crustacean and non-myctophid fish species all of the depth information 32 was generated at the same station in the eastern Gulf of Mexico as this study (Gartner & Hopkins, unpublished data, Hopkins et al., 1981). To examine the effects of increasing depth of occurrence on respiration rate, plots of vo2 versus MDO at 7, 14, and 20C were constructed for both the fishes and crustacea (Figs.Sa-c & 6a-c). b Regression curves of the form y:ax are shown in each graph. No effect ; is apparent at any temperature for the crustaceans, and the fish show a moderate decreasing effect only at higher temperatures. When V02 is plotted against MDO taking into account the changes in environmental temperature with depth (Figs.7 & 8) both groups show a significant drop in the slope of the curves. The drop strongly indicates that any decreases in respiration rate with depth are governed predominantly by temperature alone. This finding differs significantly from studies in the eastern Pacific where temperature w a s f ound to be only a minor factor (Childress, 1975; Torres et al., 1979: 17% & 2 % respectively). Percent water content with stayed essentially the same for the crustacea (Fig.9b) and only slightly increased for the fishes (F. 9 ) It ;s therefore presumed not to have much effect on any a depth-related changes in wet weight-speciiic respiration rate.

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33 DISCUSSION The data from this study provide new and important information on midwater animals from a tropical/subtropical system. Indications are that temperature is a major factor controlling the ecology of the midwater community in the eastern Gulf of Mexico. Dissolved oxygen never becomes physiologically limiting, as all the species were able to regulate their oxygen consumption at the ambient POz levels found throughout their vertical range. The fact that individual Pc values were considerably lower than the lowest P0 2 present in the Gulf may reflect regulatory extremes based upon different 0 2 concentrations within the species geographic distribution. None of the species are restricted to the Gulf of Mexico. The important fact to note concerning dissolved 0 2 in the Gulf is not the exact level to which the animals can regulate their respiration but that the concentration of oxygen itself puts no direct constraints on the species' activity levels. This is quite different than the situation in the eastern Pacific. Species inhabiting the California Current system, although able to regulate their o2 consumption rates at the extreme P0 2's present there, can only do so at the expense of decreased activity. Comparison with data from other investigators on identical species is (Table 4). The rates for Acanthephyra purpurea and Systellaspis debilis agree well, while my rates for Parapandalus richardi are somewhat lower than those of Teal (1971). Rates for

PAGE 42

34 Table 4. Comparison of respiration rates with data from other investigators on con-specific fish and crustaceans. Data this study Data other studies Temp. VOJ Temp. VOz Species deg.C (std. ev) deg.C (std.dev.) Reference Anoplogaster cornuta 7 0.032 5.0 0.024 Torres et al., 1979 (0.003) (0.009) II II 5.5 0.019 Childress, 1975 II 7.0 0.032 Gordon et al., 1976 (0.003) Thysanopoda 14 0.196 10.0 0.190 Teal & Carey, 1967 monacantha (0.008) Thysanopoda 20 0.358 20.0 0.980 II II obtusifrons (0.119) Bentheuphausia 7 0.074 7.5 0.031 Torres & Childress,1985 amblyops (0.006) (0.004) Parapandalus 7 0.094 5.0 0.125 Teal, 1971 richardi (0.062) II 17 0.245 15.0 0.340 " (0.010) Acanthephyra 7 0.085 5.0 0.075 II purpurea (0.014) 14 0.199 15.0 0.160 " (0,061) 7 0.077 5.0 0.060 " Systellaspis debilis (0.014) II 14 0.156 15.0 0.140 II (0.035) 0.164 20.0 0.220 II " II 20 (0.037) " 15.0 0.650 Napora, 1964 20.0 1.000 " II II Values available from graphs only so are not exact.

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S. debilis from Napora (1964) are considerably higher than mine. As noted by Teal (1971) the difference is most likely the result of different experimental methods. Rates for euphausiid species range 35 from slightly lower for Thysanopoda monacantha to considerably lower forT. obtusifrons (Teal & Carey, 1967), and roughly twice that of Torres & Childress (1985) for Bentheuphausia amblyops. For both B. amblyops and l obtusifrons I only have single runs so the differences probably reflect individual variation. My rate for Anoplogaster cornuta is slightly higher than that of Childress (1975) but agrees well with rates from Torres et al. (1979) and Gordon et al. (1976) even though all three of the previous rates are from individuals collected in the eastern Pacific. Comparison with eastern Pacific congeners (Table 5) shows my data to range from similar to 2-3 times higher. Some of the differences may be attributed to individual variability. The remainder can be explained by differences in temperature regimes, species depth of occurrence between the two areas (Childress, 1975) and as disparities in lifestyles among species. The effects of temperature on metabolism show varying trends among fish and crustaceans. All of the 1nyctophids examined are strong vertical migrators to epipelagic waters at night. Their elevated Q10 (5.59) may reflect significant changes in respiration between daytime and nighttime depths. This seems to be supported by the feeding strategies exhibited by the group as a whole. Studies dealing with feeding chronology of vertically migrating myctophids show strong evidence o f diel feeding patterns w ith nighttime feeding predominating for the majority of the species (Baird et al., 1975; Clarke, 1978;

PAGE 44

36 Table 5. Comparison of respiration rates with data from other investigators on con-generic fish and crustaceans. Data this study Data other studies Temp. vo2 Temp. VOl Species deg.C (std.dev) Species deg.C (std. ev.) Reference D.mollis 20 0.292 Diaphus theta 5.0 0.107 Torres et al., 1979 (0.125) (0.020) " 7.0 0.208 " (0.020) L.nobilis 7 0.045 Lampanyctus 5.0 0.041 " ,, (0.018) ritteri (0.004) 10.0 0.059 " II S. m. 14 0.024 Scopelogadus 5.0 0.014 " mizolepis (0.005) mizolepis (-----) bispinosus Sergestes 7 0.085 Sergia phorcus 5.5 0.024 Childress, 1975 armatus (0.005) (0.003) 14 0.217 Sergestes 10.0 0.288 Pearcy & Small,1968 (0.036) similis (-----) G.valens 7 0.092 Gennadas 5.5 0.027 Childress, 1975 (0.018) propinquus (0.003) 0.085 Acanthephyra 5.5 0.036 II A.purpurea 7 (0.014) curtirostris (0.005) Systellaspis 5.5 0.033 " S.debilis 7 0.077 (0.014) cristata (0.002) 0.147 Pasaphaea 5.5 0.021 II P.merriami 7 (0.011) emarginata (0.003)

PAGE 45

Holton, 1969; Hopkins & Baird, 1977, 1981, 1985; Merrett & Roe, 1974; Tyler & Pearcy, 1975). This type of life strategy, ie. definite diel 37 changes in activity and feeding periodicity, agrees with the suggestions of Marshall (1954) who proposed that nighttime migrations into epipelagic layers is primarily feeding related, and with the in situ observations of Barham (1971) who found vertically migrating species to be considerably less active at their daytime depths relative to their nighttime depths. Data for Gonostoma elongatum, a vertically migrating gonostomatid, seem to imply a somewhat different strategy. Although present in epipelagic waters at night, elonaatum shows a more typical temperature effect (Q10 of 2.35) possibly indicating a lesser degree of activity change than that of migrating myctophids. Interestingly, Clarke (1978) found Q. elongatum t6 exhibit an acyclic feeding pattern. This is not to say that feeding patterns are controlled by temperature but rather that metabolic effects due to temperature changes with depth may be useful in delineating the evolved feeding strategies of certain species. Q 's for the crustacea showed good consistency and very strong 10 similarity among the three decapod groups in particular. Values of 2.09, 2.28 and 2.19 for the caridea, penaeidae and sergestidae, respectively, imply comparable metabolic responses to changing temperatures even though a variety of depth distributions and vertical migrations are exhibited by the species involved. Each group contains both epi-and mesopelagic representatives with migration patterns k n ex;stent Ind;v;dual <)_ 's measured ranging from strong to wea or no 0 in the more frequently tested species (Sergia robustus, S. splendens,

PAGE 46

Gennadas valens, Funchalia villosa, Qplophorus gracilorostris and Systellaspis debilis) also showed good similarity, but perhaps more 38 expectedly so. All of these are epipelagic or upper-mesopelagic (MDO from 70-290) and undergo significant diel migrations. Varying q10s for S. debilis and S. robustus support a case for thermal compensation at the higher temperatures of their depth ranges, but data are insufficient at this time to make any definite conclusions. Sharp changes in Q lQ values in general, however, coincide well with the depth distributions for the majority of species, indicating a close association between thermal effect and the extremes of a species' vertical range. Available information concerning feeding chronology in midwater decapods seems to support the Q10 data presented here. Although nighttime appears to be the primary feeding period for the majority of species, data from other investigators provide strong evidence of feeding in various degrees throughout the diel period with certain species feeding equally both day and night (Donaldson, 1975; Foxton & Roe, 1974; Heffernan & Hopkins, 1981; Walters, 1976). This implies a more uniform activity level over a given temperature and depth range resulting in a depression of the observed Q In support of this, lQ several studies dealing with the effects of pressure on metabolism (Childress, 1977a; Mickel & Childress, 1982; Napora, 1964, Teal, 1971) indicate that moderate pressures (20-60 atm) tend to increase metabolism in mesopelagic species (MD0<150m). Furthermore, George (1979) noted that these increasing effects will only be exhibited in regions where the 0 minimum layer is either absent or insignificant. Such is the 2 case in the eastern Gulf of Mexico. Temperature data for the euphausiidae (Q10 of 3.16) suggests a

PAGE 47

39 somewhat higher diel activity change than that of the decapods, however the observation may be anomolous. The four species tested include a strongly migrating epipelagic species, a non-migrating mesopelagic species, a non-migrating bathypelagic species, and a weakly migrating upper-mesopelagic species. Of these, only the epipelagic species (Thysanopoda monacantha) involved multiple runs. Any conclusions concerning temperature effects for this group could be biased towards the single species. Feeding strategies among euphausiids show wide variability both in diet and feeding periodicity (Mauchline & Fisher, 1969; Mauchline, 1980). Specific data on T. monacantha from Hu (1978) indicate a strong diel pattern with nighttime feeding predominant, which seems to correlate with the high Q10 presented here (3.24). Data for this species are too insufficient for any definite conclusions to be drawn. Perhaps the most significant result concerning the importance of temperature in the ecology of the Gulf is the role it plays in the oxygen consumption/minimum depth of occurrence relationship. Data indicate that temperature alone is responsible for the majority of the observed decrease in respiration with depth. As noted earlier, this is considerably different than the situation in the eastern Pacific. The reasons can be examined in two ways: as an artifact of experimentation or data manipulation, or as a direct result of the selective forces of the physical parameters within each system. Comparing this study with that of Torres et al. (1979) and Childress (1975), the primary studies addressing respiration and MDO in the eastern Pacific, I find that my data are biased towards shallow living species while theirs are biased towards deeper living species. For fishes, 69% of my species show an

PAGE 48

40 MDO less than 200m while 60% of the species studied by Torres et al. (1979) had MDO's greater than 300m and of those 73% were greater than 500m. For crustaceans, 63% of my species have a MDO less than 300m with 80% of those being less than 200m. Of crustaceans studied in Childress (1975), 63% lived deeper than 300m and 71% of those lived deeper than 500m. This disparity in depth distributions is unavoidable when you consider the migration patterns exhibited within the two communities. In the eastern Gulf, 62% of the fish and 63% of the decapods are strong vertical migrators and come up into shallow waters at night (Hopkins et al., 1981) while in the eastern Pacific only 32% of the fish and 13% of the crustaceans undertake significant migrations (Pearcy et al., 1977). Furthermore, Youngbluth (1976) noted that many species can alter their depth distibution and vertical migration over changing periods of upwelling and downwelling. It seems clear then that the dissimilarities between the species studied, although making direct comparison difficult, cannot be regarded as experimental artifact but rather as inherent characteristics of the biological communities of the two systems. A more likely reason for the dissimilarities is that the unique physical parameters of the two regions have resulted in evolved differences in the relative importance of temperature. Temperature extremes in the upper lOOOm range from 5 to 29C for the eastern Gulf. but between 4 and l7C for the eastern Pacific. The very nature of the wider thermal range would exert greater selective pressures in the Gulf. Conversely, the importance of dissolved oxygen is much higher in the eastern Pacific due to the expansive and severely low o2 minimum layer there, a condition that places considerably greater constraints on

PAGE 49

41 respiration and levels. Factors such as increasing size or increased water content with depth only contribute slightly to the decrease in respiration, the importance of both being slightly higher in the fish than the crustaceans. Mean dry weights for species living shallower than 200m compared to those below 200m are 0.401g and 0.963g, respectively, for the fish and 0.216g and 0.258g, respectively, for the crustaceans. Percent increase in water content from the surface to lOOOm is 10.7 for the fish and only 1.4 for the crustaceans (from figs.9a & b). Changes in protein content with depth have been shown for both fish and crustaceans (Childress & Nygaard, 1973, 1974). More important than quantity, however, are changes in protein quality and levels of enzyme activity. Childress (1975) suggested that deeper living animals, as a consequence of a less variable habitat, "would require much less enzymatic machinery to deal with environmental fluctuations than would shallower living ones." The hypothesis is substantiated in fishes by the findings of Torres et al.(l979) and Childress & Somera (1979). Decreases in protein-specific respiration with depth measured by Torres et al.were considerably greater than decreases in protein content, implying a change in the nature of muscle metabolism over and above changes in mere muscle mass. In conjunction, Childress and Somera found disproportionate declines in both glycolytic and citric acid cycle enzyme activity with depth relative to changes in contractile and structural protein concentrations. It should be noted however that the studies dealing with changes in enzyme activity and muscle mass involve species from the temperate eastern Pacific. Not only does a much greater percentage of the Gulf of Mexico community migrate but they tend

PAGE 50

42 to migrate further when they do migrate. Thus, vertical migration requires that Gulf of Mexico species retain a more developed musculature to perform these migrations. It's possible then that marked declines in protein content with increasing depth may not be present in the Gulf of Mexico community and that the traditional arguments for absence of need for muscle in midwater animals do not apply here. Chemical composition of Gulf of Mexico species is presently under investigation but as yet no data are available. Such information should prove very useful in further understanding the respiration data presented in this study. It appears that while a large percentage of the observed decline in respiration with depth remains unexplained for eastern Pacific species, rate decreases in Gulf of Mexico species are attributable almost completely to temperature.

PAGE 51

43 CONCLUSIONS 1) Dissolved oxygen is not physiologically limiting in the eastern Gulf of Mexico. All of the species examined were capable of regulating their respiration rate at the lowest ambient oxygen partial pressure. 2) Pc values ranged from 20-40 mmHg and increased slightly with increased temperature and respiration rate. 3) As groups, Sergestids, Penaeids and Carids showed no thermal compensation, ie. Q10 values were around two. Myctophids showed under compensation, ie. Q10 values were considerably greater than two. These observed responses correlate with activity and feeding patterns exhibited within each group. 4) Changes in temperature were responsible for the majority of the observed decline in respiration with increased depth of occurrence. This is a consequence of the wide thermal range present in the eastern Gulf and the fact that the bulk of the mesopelagic community migrates extensively.

PAGE 52

LITERATURE CITED Arp A.J. (unpublished) Oxygen minimum zone fishes with high oxygen affinity hemoglobins. 44 Baird R.C., T.L.Hopkins and D.F.Wilson (1975) Diet and feeding chronology of taaningi (Myctophidae) in the Cariaco Trench. Copeia,2, 356-36 Barham E.G. (1971) Deep-sea fishes: Lethargy and vertical orientation. In: Proceedings of the international symposium on biological sound scattering in the ocean. G.B. Farquhar, editor. Scientific Reports of the Maury Center for Oceanography. Vol.5, pp.100-118. Belman B.W. and M.S.Gordon (1979) Comparative studies on the metabolism of shallow-water and deep-sea marine fishes. V. Effects of temperature and hydrostatic pressure on oxygen consumption in the mesopelagic Zoarcid Melanostigma pammelas. Marine Biology,50,275-281. Childress J.J. (1971a) Respiratory rate and depth of occurrence of midwater animals. Limnology and Oceanography,16,104-106. Childress J.J. (1971b) Respiratory adaptations to the oxygen minimum layer in the bathypelagic mysid Gnathophausia ingens. Biological Bulletin,141,109-121. Childress J.J. (1975) The respiratory rates of midwater crustaceans as a function of depth of occurrence and relation to the oxygen minimum layer off southern California. Comparative Biochemistry and Physiology,50A,787-799. Childress J.J. (1977a) Effects of pressure,temperature and oxygen on the oxygen consumption rate of the midwater copepod Gausia princeps. Marine Biology,39,19-24. Childress J.J. (1977b) Physiological approaches to the biology of mid water organisms. In:Oceanic Sound Scattering Prediction. Neil R. Anderson and Bernard J. Zahuranac, editors. Plenum Press, 1977. pp.301-324. Childress J.J., A.T.Barnes, L.B.Quetin and B.H.Robison (1978) Thermally protecting cod ends for the recovery of living deep-sea animals. Deep-Sea Research,25,419-422. Childress J.J. and M.H.Nygaard (1973) The chemical composition of mid-

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46 Gordon M.S. (1975) Effects of temperature and pressure on the oxidative metabolism of fish muscle. In:Comparative Physiology-Functional Aspects of Structural Materials. L.Bolis, H.P.Maddrell and K. Schmidt-Nielsen, editors. North Holland Publishing Company, 1975. pp.211-223. Gordon M.S., B.W.Belman and P.H.Chow (1976) Comparative studies on the metabolism of shallow water and deep-sea marine fishes. IV. Patterns of aerobic metabolism in the mesopelagic deep-sea fangtooth fish Anoplogaster cornuta. Marine Biology,35,287-293. Heffernan J.J. and T.L.Hopkins (1981) Vertical distribution and feeding of the shrimp genera Gennadas and Bentheogennema (Decapoda: Penaeidea) in the eastern Gulf of Mexico. Journal of Crustacean Biology,1,461-473. Hiller-Adams P. and J.J.Childress (1983) Effects of prolonged starvation on 0 2 consumption, NH4 excretion, and chemical composition of the bathypelagic mysid Gnathophausia_ Marine Biology, 77,119-127. Holton A.A. (1969) Feeding behavior of a vertically migrating lanternfish. Pacific Science,23,325-331. Hopkins T.L. (1982) The vertical distribution of zooplankton in the eastern Gulf of Mexico. Deep Sea Research,29,1069-1083. Hopkins T.L. and R.C.Baird (1977) Aspects of the feeding ecology of oceanic midwater fishes. In:Oceanic Sound Scattering Prediction. Neil R. Anderson and Bernard J. Zahuranac, editors. Plenum Press, 1977. pp.325-360. Hopkins T.L. and R.C.Baird (1981) Trophodynamics of the fish Valenciennellus tripunctulatus. I. Vertical distribution, diet and feeding chronology. Marine Ecology-Progress Series,5,1-10. Hopkins T.L. and R.C.Baird (1985) Aspects of the trophic ecology of the mesopelagic fish Lampanyctus alatus (Family Myctophidae) in the eastern Gulf of Mexico. Biological Oceanography,3,285-313. Hopkins T.L., D.M.Milliken, L.M.Bell, E.J.McMichael, J.J.Heffernan and R.C.Cano (1981) The landward distribution of oceanic plankton and micronekton over the west Florida shelf as related to their vertical distribution. Journal of Plankton Research,3(4),645-658. Hu V.J.H. (1978) Relationships between vertical migrations and diet in four species of Euphausiids. Limnology and Oceanography,23,296-306. Jones J.I. (1973) Physical oceanography of the northeast Gulf of Mexico and Florida continental shelf area. Section IIB. In: State University System of Florida (Coord.) pp.1-11. A summary

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of knowledge of the eastern Gulf of Mexico. Marshall N.B. (1954) Aspects of Deep-Sea Biology. Hutchinson, London. 380 p. Mauchline J. (1980) The biology of mysids and euphausiids. Advances in Marine Biology,18,1-681. Mauchline J. and K.R.Fisher (1969) The biology of euphausiids. Advances in Marine Biology,7,1-454. Meek R.P. and J.J.Childress (1973) Respiration and the effect of pressure in the mesopelagic fish Anoplogaster cornuta (Berciformes). Deep Sea Research,20,1111-1118. Merrett N.R. and H.S.J.Roe (1974) Patterns and selectivity in the feeding of certain mesopelagic fishes. Marine Biology,28,115-126. 47 Mickel T.J. and J.J.Childress (1982) Effects of pressure and pressure acclimation on activity and oxygen consumption in the bathypelagic mysid Gnathophausia Deep Sea Research,29,1293-1301: Napora T.A. (1964) The effect of hydrostatic pressure on the prawn Systellaspis debilis. Narragansett Marine Laboratory Occasional Publication,2,92-94. Nowlin W.D.Jr. (1971) Water masses and general circulation of the Gulf of Mexico. Oceanology International,6,29-33. Pearcy W.G. and L.F.Small (1968) Effects of pressure on the respiration of vertically migrating crustaceans. Journal of the Fisheries Research Board of Canada,25,1311-1316. Pearcy W.G., E.E.Krygier, R.Mesecar and F.Ramsey (1977) Vertical distribution and migration of oceanic m icronekton off Oregon. Deep Sea Research,24,223-245. Prosser C.L. (1973) Oxygen: respiration and metabolism. In: Comparative Animal Physiology, C.L.Prosser, editor. Saunders,pp.165-206. Quetin L.B. and J.J.Childress (1976) Respiratory adaptations of Pleuroncodes planipes to its environment off Baja California. Mar1ne B1ology,38,327-334. Schmidt J. (1925) On the contents of oxygen in the ocean on both sides of Panama. Science,61,592-593. Scripps Institution of Oceanography of the University of California (1965) Oceanic observations o f the Pacific, 1959. Berkeley and Los Angeles: Univers ity of Californi a Press 1965. Smith K.L. and R.R.Hessler (1974) Respiration of benthopelagic fishes: in situ measurements at 1230 meters. Science,184,72-73.

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Smith K.L. and M.B.Laver (1981) Respiration of the bathypelagic fish Cyclothone acclinidens. Marine Biology,61,261-266. Teal J.M. (1971) Pressure effects on the respiration of vertically migrating decapod crustacea. American Zoologist,11,571-576. Teal J.M. and F.G.Carey (1967) Effects of pressure and temperature on the respiration of euphausiids. Deep Sea Research,14,725-733. Torres J.J. and J.J.Childress (1985) Respiration and chemical composition of the bathypelagic euphausiid Bentheuphausia amblyops. Marine Biology,87,267-272. Torres J.J., B.W.Belman and J.J.Childress (1979) Oxygen consumption rates of midwater fishes as a function of depth of occurrence. Deep Sea Research,26A,185-197. Tyler H.R.Jr. and W.G.Pearcy (1975) The feeding habits of three species of lanternfishes (family Myctophidae) off Oregon, USA. Marine Biology,32,7-11. Walters J.F. (1976) Ecology of Hawaiian sergestid shrimps (Penaeidae, Sergestidae). Fishery Bulletin, United States,74,799-836. Youngbluth M.J. (1976) Vertical distribution and diel migration of Euphausiids in the central region of the California Current. Fishery Bulletin,United States,74,925-936. 48


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