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The vertical distribution and feeding ecology of Euchaeta marina in the eastern Gulf of Mexico during summer

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Title:
The vertical distribution and feeding ecology of Euchaeta marina in the eastern Gulf of Mexico during summer
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viii, 37 leaves : ill. ; 29 cm.
Language:
English
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Shuert, Paul G
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University of South Florida
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Tampa, Florida
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Copepoda -- Feeding and feeds   ( lcsh )
Euchaeta marina   ( lcsh )
Dissertations, Academic -- Marine Science -- Masters -- USF   ( fts )

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General Note:
Thesis (M.S.)--University of South Florida, 1983. Bibliography: leaves 28-31.

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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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aleph - 029850493
oclc - 10729697
usfldc doi - F51-00019
usfldc handle - f51.19
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SFS0040021:00001


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THE VERTICAL DISTRIBUTION AND FEEDING ECOLOGY OF EUCHAETA MARINA IN THE EASTERN GULF OF MEXICO DURING SUMMER by Paul G. Shuert 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 December, 1983 Major Professor: Thomas L. Hopkins

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Graduate Council University of South Florida Tampa, Florida CERTIFICATE OF APPROVAL MASTER'S THESIS This is to certify that the Master's Thesis of Paul G. Shuert with a major in Marine Science has been approved by the Examining Committee on September 7, 1983 as satisfactory for the thesis requirement for the Master of Science degree. TI1esis Committee: Major Professor: Thomas L. Hopkins Member. Joseph J. Torres Member: Norman J. Blake

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ACKNOWLEDGEMENTS I would like to thank the following persons who helped in making this study possible: T.L. Hopkins for his guidance during the planning and execution of this project, for the critical review of this work, and for providing valuable shiptime for the collection of samples; N.J. Blake for providing materials and advice for the histological aspect of this study; J.J. Torres for providing valuable shiptime for some of these samples; and V.E. McGough for her help and patience in typing of this manuscript. ii

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LIST OF TABLES LIST OF FIGURES ABSTRACT INTRODUCTION METHODS RESULTS TABLE OF CONTENTS "Vertical Distribution and Population Data Feeding DISCUSSION CONCLUSIONS LIST OF REFERENCES APPENDIX 1 iii iv v vii 1 3 7 7 13 20 26 28 32

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LIST OF TABLES Table 1. Number of samples used and tow duration information from each cruise. Average tow duration is in parenthesis. Table 2. Numerical distribution of sexes and copepodid stages of Euchaeta marina. Data is from FS 4 samples only. . 12 Table 3. Stomach contents of 40 adult female Euchaeta marina. . 19 iv

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LIST OF FIGURES Fl.gure 1 Total b f E h t i 100 m 3 num er o uc ae a mar na per versus depth (meters), grouped in 2-hour intervals. Numbers in parenthesis are the number of samples taken at that depth. No data was taken between 1600 and 1800 hrs. Both CI and FS samples were used. 8 Figure 2. Depth of population means over 24 hours: (A) total number of individuals; (B) adult females; (D) gravid females; (E) copepodid stage V males and females; (F) copepodid stages IV and earlier both males and females. A is FS and CI samples combined at 2 hour intervals. B-F is FS samples only, at 3 hour intervals. . . . . . . . . . 10 Figure 3. Feeding chronology of adult females. (A) percent of individuals examined with 25% or greater of fore-and mid-gut fullness. Number of individuals examined for each time interval is in parenthesis. (B) percent of individuals examined with 50% or greater of fore-and mid-gut fullness (solid line) and 50% or greater v

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of hind-gut filled (dashed line). Number of individuals examined is the same as in (A). . . . . . . Figure 4. Feeding chronology of copepodid stage IV and earlier stages of both males and females combined. (A) the percentage of individuals examined with 25% or greater fore-and mid-gut fullness. Number of indiv-iduals examined is in parenthesis. (B) the percentage of individuals examined with 50% or greater fore-and mid-gut fullness (solid line), and the percentage of individuals examined with 50% or greater of hind-gut fullness (dashed line). The number of individuals examined is the same as in (A). vi 14 17

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THE VERTICAL DISTRIBUTION AND FEEDING ECOLOGY OF EUCHAETA MARINA IN THE EASTERN GULF OF MEXICO DURING SUNMER by Paul G. Shuert An Abstract A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in the Department of Science in the University of South Florida December, 1983 Major Professor: Thomas L. Hopkins vii

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ABSTRACT Euchaeta marina is an abundant copepod predator in the epipelagic zone of temperate to tropical latitudes. In the eastern Gulf of Mexico, most of the population occurs above 100 meters. The population maintains a daytime center near 50 meters, ascending to 30-40 meters around dusk. Most of the population shows midnight descent from near the surface. A pre-dawn ascent to around 15 meters preceeds the return to the daytime center near 50 meters. Copepodid stages IV and smaller gave no evidence of midnight descent, but remained at 30 meters throughout the night. Cyclic feeding is apparent with feeding rates in adult females showing higher activities at night than during the day. The diet of adult females consisted, exclusively, of small crustaceans less than 1 mm body length. Adult males do not feed. Abstract approved: Major Professor Professor, Department of Marine Science Date of Approval viii

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1 INTRODUCTION In the eastern Gulf of Mexico, as in most oceanic regions, copepods are the principal macrozooplankton group. Copepods, on the basis of net collections, numerically constitute as much as 96% of the total zooplankton population in the eastern Gulf of Mexico (Hopkins, 1982; Hopkins, Bennett and Shuert, 1979) and even greater percentages of the zooplankton biomass. While much of the work on copepods has been focused on particle-feeding grazers, both in the laboratory (Frost, 1977, 1974; Boyd, 1976; Cannon, 1928; and others) and in nature (Harding, 1974; Arashkevich, 1968; Beklemishev, 1954; and others), relatively little information is available concerning carnivorous copepods. Euchaeta marina is an abundant carnivorous copepod typical of the epipelagic zooplankton community from temperate to tropical latitudes (VerVoort, 1963). This species is 3.7-7.7% of the total copepod population by numbers in the upper 1000 meters in the eastern Gulf of Mexico (Hopkins, Bennet and Shuert, 1979) making it one of the major copepod predators and the most abundant members of the genus. The other members of the genus which have been the subject of diet composition studies are Euchaeta media, marina, hebes, E. acuta, concinna, norvegica, scotti, and timidula (Harding, 1974; Mullin, 1966; Wickstead, 1962; Geinrikh, 1958). Also, prey selectivity has been examined experimentally for elongata (Yen, 1982) and norvegica (Bamstedt and Holt, 1978). No information is available, however, on

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feeding chronology in this genus, and gut content studies for all species examined have been based on relatively few individuals. 2 This paper reports on the trophic ecology of Euchaeta marina in the eastern Gulf of Mexico. Information will be presented on the vertical distribution of this species in the upper 150 meters and on the feeding chronology and diet of adult females. Data for this study was taken in summer months at 27N, 86W in the eastern central Gulf of Mexico. This station, while essentially in oligotrophic boundary waters, has many tropical-subtropical gyre characteristics (Hopkins, 1982; Longhurst, 1976; McGowan, 1974; and Vinogradov, 1970). The location has been well studied as to species composition, diurnal distribution and standing crops of both zooplankton and micronekton (Morris and Hopkins, 1983; Hopkins, 1982), and investigations have been made on the trophodynamics of a number of the micronekton species (Hopkins and Baird, in press; Baird and Hopkins, 1981; Hopkins and Baird, 1981; Heffernan and Hopkins, 1981; Hopkins and Baird, 1977).

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3 METHODS Plankton samples used for this study were taken on two consecutive summer cruises in 1975 and 1976 on the R/V Columbus Iselin, and two summer cruises in 1980 and 1982 on the R/V Bellows. R/V Columbus Iselin samples (designated CI) were taken with 162 mesh collapsible square 2 mouth plankton nets, either 0.19 m or 0.44 m4 in mouth area, which were mounted in the mouth of a closing Tucker Trawl (see Hopkins and Baird, 1975; Hopkins, Baird and Milliken, 1973). Depth was controlled with conducting cable-depth transducer system. Tows were horizontal and depth variation was limited to -15 meters. R/V Bellows samples (designated FS) were collected using 202 2 mesh, circular mouth opening/closing plankton nets, either 0.44 m or 2 0.28 m (Bongo nets) in mouth area. Depth was monitored using wire angle and a meter wheel and depth was recorded with a BenthosR time-depth recorder. In most cases combinations of nets and depths were fished simultaneously. A trawling speed of two knots was maintained for all samples. Tow duration was variable, ranging from 13 to 136 minutes (Table 1). The volume of water sampled was estimated with General OceanicsR digital flowmeters nounted in the mouth of each net, the flowmeters recorded only when nets were open.

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Table 1. Number of samples used and tow duration information from each cruise. Average tow duration is in parentheses. Cruise Date R/V Columbus Iselin June 1975 R/V Columbus Iselin June 1976 R/V Bellows June 1980 R/V Bellows July 1982 II Samples Used 46 14 26 24 Range of Tow Duration in }linutes 45 93 (57) 42 136 (63) 13 34 (34) 18 72 (34) 4

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5 Euchaeta marina were sorted and counted using either entire collections or an aliquot. Sample splitting was done with a Metoda Box plankton splitter (Metoda, 1969). Total number of E. marina were counted in all FS and CI samples or subsamples examined. Individuals from FS samples were also counted and measured by sex and developmental stage. Total body length, from the tip of the rostrum to the end of the caudal rami, was measured to the nearest 0.1 mm. Total number of E.marina from both FS and CI samples were grouped into twelve 2-hour time intervals to show the diurnal distribution pattern of the total population. Additionally, FS data were grouped into eight 3-hour time intervals to show the diurnal vertical pattern of both sexes and the later developmental stages. Mean depth of the population was calculated for each time interval. Twenty nine FS samples were used to investigate feeding chronology in adult females. Copepodid IV and younger stages were also included in the chronology investigation because they demonstrated different vertical distribution patterns from older individuals. Seventeen additional night-time samples were used in the latter study. Histological preparations were made on 2 to 10 undamaged individuals from each sample and each developmental group. A total of ten adult males ,vere also prepared histologically for gut analysis. Histological preparation included running sorted specimens through six solutions of Autotechnicon S-29 dehydrant, three solutions of Autotechnicon UC-670 clearing agent, and two solutions of hot paraplast at ten minute intervals. Immediately following the last paraplast bath, the samples were placed under vacuum in molten paraplast for 30 minutes and then embedded in blocks. Consecutive 7 um thick sections were mounted on

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slides for staining. The slides were stained with hematoxyline and eosin then coverslipped. 6 Longitudinal sections through the gut were examined for the amount of food in the fore-and mid-gut areas combined and in the hind-gut area. An average of 10 sections through each copepod were examined, thus enabling a composite picture of gut fullness to be developed from individual sections. Based on the composite picture, an arbitrary scale of 0 to 4 was used to indicate the degree of fullness where 0 = empty, 1 = 25% full, 2 = 50% full, 3 = 75% full, and 4 = 100% full. Categories 1 and 2 were used to examine feeding chronology. Thus, category 1 would represent the lowest level of feeding, and category 2 a more active level. Diet composition was analyzed through examination of all sections of 40 adult females with full guts. In most cases food was fragmented, consisting primarily of crustacean body parts.

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RESULTS Vertical Distribution and Population Data The vertical distribution pattern in Figures 1 and 2A indicates that Euchaeta marina maintained a population center near 50 meters during the day. The population moved upward at dusk and centered at 20 and 30 meters by 2000 hrs. There was a period of sinking near midnight (0000-0200 hrs) followed by a second apparent migration toward the surface before dawn. Shortly after dawn the population center returned to the daytime depth of 50 meters where it remained until dusk. This diel pattern of vertical distribution was apparent in each data set (i.e., both the FS and CI series) which were combined in preparing Figures 1 and 2A. Based on FS samples, adults of both sexes and copepodid stage V individuals (Figures 2B-E) showed little variation in vertical distribution from that described for the total population. However, copepodid IV and smaller stages (Figure 2F) maintained a night-time population center at 30-40 meters and did not show the "midnight sinking" pattern. Shortly after dawn, and throughout the day, the distribution of copepodid IV and younger stages coincided with that of the remainder of the population. The percentage of fenmles (65%) in the population was nearly twice that of males (35%) based on FS samples (Table 2). The percentages of combined adults (35%), copepodid stage V (30%), and remaining composite 7

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8 Figure 1. 3 Total number of Euchaeta marina per 100 m versus depth (meters), grouped in 2-hour intervals. Numbers in parenthesis are the number of samples taken at that depth. No data was taken between 1600 and 1800 hrs. Both CI and FS samples were used.

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0 1 2 3 4 0 -----(3) 20 (1)--(1) ,-... (f) a: w 1w -40 6 0 80 1 I 100 (1) I-I (2) D... 120 1(1) w l (1) o 140 1 ( 1) 160 1800-2000 0 1 2 3 0 20 ,-... Cf) ( 1) a: 40 / w I-60 r) w 80 - ( 1) I 100 ID... 120 w 0 140 160 0600-0800 4 =#= OF INDIVIDUALS X 1000 PER 100m3 0 1 2 3 4 -.........__(3) (1) ......._____(2) \ (1) ,---.11) c 2000-2200 0 1 2 3 \ ;(2) r ( 1) 0800-1000 4 0 1 2 3 4 (1/(1) ( 1) L ,. ( 1) I (3) ( 1) 2200-0000 0 1 2 3 4 I < 2 > ( 2 ) (2) 1000-1200 0 1 2 3 4 (5)1 I I I j <2) ( 1) ---( 1 ) ( 1) (1) (1 ) /11> (1) ( 1 ) 0000-0200 0 1 2 3 \( 1 ) ( 1) ( 1) I (') ( 1 ) I ( 1) 1200-1400 4 0 1 2 3 4 (2)j(1) / /(2) r / (1) f' \(1) ( 1) 0200-0400 0 1 2 3 I <2> 4 (2J--____ ---(1) /"
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10 Figure 2. Depth of population means over 24 hours: (A) total number of individuals; (B) adult males; (C) adult females; (D) gravid females; (E) copepodid stage V males and females; (F) copepodid stages IV and earlier both males and females. A is FS and CI samples combined at 2 hour intervals. B-F is FS samples only, at 3 hour intervals.

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TIME 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 CXl ,.... 0 (') co 0) C\1 lO CXl CXl ,.... 0 (') co 0) C\1 lO CXl CXl 0 (') co 0) C\1 lO CXl C\1 0 0 0 0 C\1 0 0 0 0 C\1 0 0 0 0 0 0 0 20 20 20 40 40 40 60 60 60 en 80 80 80 0: A 8 c w 1w z 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 CXl ,.... 0 (') co 0) C\1 lO CXl CXl 0 (') co 0) C\1 lO CXl CXl 0 (') co 0) C\1 lO CXl I: C\1 0 0 0 0 C\1 0 0 0 0 C\1 0 0 0 0 1-0 0 0 c.. w 0 20 20 20 40 40 40 60 60 60 80 80 80 D E F

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Table 2. Numerical distribution of sexes and copepodid stages of Euchaeta marina. Data is from FS samples only. Class Average % Range males 35 6-57 females 65 43-94 adults 35 0-75 adult males (8) (0-24) adult females (20) (0-25) (non-gravid) gravid females (6) (0-26) stage V 30 2-86 stage V males (19) (0-57) stage V females (11) (0-33) stage IV and smaller 35 0-98 12

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13 earlier stages (35%) in these collections were roughly equal. Gravid females, those with attached egg sacs, were 6% of the population or approximately one-third of the adult females numbers. The actual abundance of gravid females was probably underestimated since some of the egg sacs may have been dislodged during sample processing. Egg sacs contained an average of 10 blue eggs (range 5-13) which averaged 0.28 rnm in diameter. Feeding Results of gut fullness, averaged for hourly intervals (Figure 3A) indicated that adult females fed to some extent throughout the die! cycle although a higher percentage of individuals showed evidence of feeding at night. Figure 3B, which presents data for more "successful" feeders, (i.e., gut fullness greater than or equal to 50%) reveals that a rapid increase in fore-and mid-gut fullness occurred by early evening (2000 hrs) which was followed by a three hour period of decline. Gut fullness was highest at 0100 hrs, with 80% of the individuals examined showing 50% or greater gut fullness. During the morning (0400-1200 hrs), occurrances of 50% or greater gut fullness in the population decreased to an average of 20% of the individuals examined followed by an increase to a 45% incidence at 1400 hrs. In mid-afternoon (1500 hrs) guts were least full, with none of the 11 individuals examined containing food. The p ercentage o f hind-guts with 50 % or greate r fullness tracked combined fore-and mid-gut fullness (Figure 3B), though displaced one hour later. Depth related trends in feeding activity were not seen in the data for adult f e males analysed from different d epth horizons.

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14 Figure 3. Feeding chronology of adult females. (A) percent of individuals examined with 25% or greater of fore-and mid-gut fullness. Number of individuals examined for each time interval is in parenthesis. (B) percent of individuals examined with 50% or greater of fore-and mid-gut fullness (solid line) and 50% or greater of hind-gut filled (dashed line). Number of individuals examined is the same as in (A).

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Q 100 (5) A 100 8 w z <( 80 80 I \ X I I I \ w I I I I I I (/') 60 60 I \ ( 15) , I ...J ' /I I I <( , \ I \ I I I I :::::> I I I I Q (26) I I 40 40 I I > I I 1 I Q ,, z ( 17) 20 20 u.. 0 ( 11) ?fl. 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 CX) -0 (f) <0 0> C\1 t() CX) CX) -0 (f) <0 0> C\1 t() CX) -C\1 0 0 0 0 --C\1 0 0 0 0 --TIME TIME

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16 Figure 4 shows the results of feeding chronology analysis for night samples (1800-0600 hrs) of copepodid IV and younger composite stages of both males and females grouped together. Generally, feeding levels for these groups were low throughout the diel cycle, although some peak periods were evident. Based on those individuals with 507. or greater gut fullness, feeding intensity increased at 2200 hrs, which was followed by a decline to a zero level. A second apparent increase in feeding intensity occurred between 0400-0500 hrs. Fore-and mid-gut contents indicated that adult female E. marina ingest small crustaceans, primarily copepods. Identifiable copepods were mostly small harpacticoid and cyclopoid crustaceans (Table 3). Some fragments of early copepodid stages of calanoids were found as well. Among the recognizable food were fragments of Oncaea, Microsetella, and Centropages. Identification of specific prey types was difficult, and very few guts contained sufficient recognizable items to reconstruct whole prey items. Adult males did not appear to feed. Histological examination revealed that their guts were thin and thread-like and contained no food or fragments of any kind. Feeding appendages were also noticably reduced in size compared to females and earlier male stages.

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17 Figure 4. Feeding chronology of copepodid stage IV and earlier stages of both males and females combined. (A) percentage of individuals examined with 25% or greater fore-and mid-gut fullness. Number of individuals examined is in parenthesis. (B) percentage of individuals examined with 50% or greater fore-and mid-gut fullness (solid line), and the percentage of individuals examined with 50% or greater of hind-gut fullness (dashed line). The number of individuals examined is the same as in (A).

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Q 100 A 100 w z <( 80 80 X w (f) 60 60 ..J <( (2) (34) :::> Q 40 40 -> Q z 20 20 u. 0 ?fl. 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 CX) ..... 0 ('I) co < C\1 10 CX) CX) ..... C\1 0 0 0 0 ..... ..... ..... ..... TIME 1\ I I I \ I I I I I J 0 0 0 0 ..... 0 C\1 0 I' I \ I \ \ I \ I \ I \ I \ \ 0 0 ('I) 0 8 0 0 0 0 co < 0 0 TIME 0 0 0 0 C\1 10 ..... ..... 0 0 CX) ..... ...... 00

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19 Table 3. Stomach contents of 40 adult female Euchaeta marina. Copepod # 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 Stomach Contents copepod mandible and 2nd maxilliped crustacean fragment Oncaea fragments; other copepod parts copepod fragments copepod fragments crustacean fragments Microsetella fragments Pleuromamma pigment organ; other copepod parts harpacticoid-like thoracic segments crustacean fragments; copepod 2nd maxilliped Centropages-like fragments crustacean spines; fragments 2 copepod manidbles; Corycaeus-like fragments copepod fragments; piece of feeding appendage copepod fragments amphipod-like gnathopod harpacticoid-like spines crustacean fragments copepod fragments Oncaea periopod segment Oncaea-like periopod segment; crustacean spines crustacean spines copepod fragments mandible; crustacean spines copepod periopod segment copepod-like fragments copepod-like feeding appendage crustacean spines and fragments copepod fragments Oncaea-like periopod segment; crustacean fragments crustacean spines crustacean fragments; spines copepod fragments, crustacean spines crustacean fragments harpactacoid-like periopod segment copepod fragments copepod-like fragments crustacean spines copepod-like fragments copepod mandible; crustacean fragments

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20 DISCUSSION Earlier work on the vertical and spatial distribution of copepods in tropical and subtropical latitudes has established the importance of Euchaeta marina in the plankton in terms of numbers and biomass. Abundance estimates range from 0.54% of the population in the subtropical northeastern Atlantic (Roe, 1972) to as much as 7.7% of the zooplankton in the upper 1000 meters in the eastern Gulf of Mexico (Hopkins, unpublished data; Hopkins, Bennett and Shuert, 1979). The vertical depth range of marina, which has been reviewed by Owre and Foyo (1967) and Roe (1972), can be extensive, up to 2260 meters in the Caribbean, and 2400 meters in the Atlantic (Leavitt, 1928). However, the species is essentially epipelagic with as much as 96% of the population being found in the upper 100 meters. Data for the eastern Gulf of Mexico are similar, indicating that the vertical range of E. marina extends to at least 1000 meters (Hopkins, personal communication), with most of the population occurring in the upper 100 meters. Total numbers of individuals under a square meter of sea surface in the upper 100 meters at 27N, 86W, based on night samples, averaged 1355. Patterns of diurnal vertical distribution of Euchaeta marina vary regionally. Roehr and Moore (1965) showed the population to range from 1 54 meters during the d a y to 104 meters a t night in the F lorida Straits Roe (1972) on the other hand indicated a possible reverse migration, the

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21 population descending from 40 meters during the day to 50 meters at night. The present data indicates a daytime center near 50 meters with an ascent to near 30 meters occurring after dusk (1800 hrs). A period of sinking occurs around midnight which is followed by a second ascent to near the surface just prior to dawn. This species also occurs at the surface in dense patches, this being recorded during the day in the Caribbean (Owre and Foyo, 1976), as well as in the eastern Gulf of Mexico at night (present study). The "midnight sinking" observed here has not previously been reported for marina or for any other member of this genus. Not all of the population showed evidence of diurnal migration, however, and "midnight sinking" was more apparent in copepodid V and adult stages than in younger stages. Many theories have been proposed to explain the adaptive value of vertical migrations (see McLaren, 1963, for review). In the present case, synchronous migrations to the surface by marina adults possibly enhances contact among reproductively mature individuals, and the surface swarm evidence by one of our night samples points to this. In this collection the percentage of adults was higher than in most other shallow collections though the male:female ratio remained relatively the same. The apparent "midnight sinking" seen in the population has been shown to occur in Sagitta (Pearre, 1973). A model was proposed in which the diel vertical migration of Sagitta elegans was influenced by a positive phototaxis and a light-controlled geotaxis. It was suggested that feeding satiation also influenced this balance, causing "midnight sinking" of satiated individuals. The feeding data in the present study

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22 suggests that "midnight sinking" in this population may also be influenced by the degree of satiation in the population. Feeding activity in adult female Euchaeta marina does appear higher before sinking occurs, and the apparent movement away from the surface of the population center may be the result of some members of the population, either actively or passively descending to deeper waters after successful feeding. The pre-dawn ascent in the population would allow those individuals to feed again in food-rich waters before returning to daytime depths. These data show an increase in stomach fullness in adult females just prior to the dawn descent of the population. Copepodid IV and younger stages showed low levels of feeding throughout the night including the period adult females and other older stages were undertaking the midnight descent. This suggests that feeding to satiation may occur to a lesser degree in the juvenile stages and thus, offer a possible explanation for no apparent midnight descent in the younger stages (see Figure 4B). Feeding, in fact, was heaviest just prior to the dawn descent of these stages. The diet of Euchaeta marina is reflected in its feeding appendages. This species, as typical of the genus, has large, praying mantis-like maxillipeds with long, stout setae that appear poorly suited for filtration. These appendages indicate a feeding mode involving the capture of large particles, and carnivory. The data on diet composition support this in showing adult females fed exclusively on small copepods and other crustaceans. Among the small crustacean food, prey selection appeared primarily size oriented in that the recognizable prey items b elonged to a variety o f prey type s approximately 1 mm or l e s s in length. Prey size was shown to be a significant factor in Labidocera

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23 trispinosa feeding on early developmental stages of calanoid copepods (Landry, 1978), and experimental data on Euchaeta elongata (a 4.0 mm copepod) also indicate an apparent size selectivity on Pseudocalanus (0.95 mm body length) when offered a variety of sizes of prey (Yen, 1982). The number of small metazoan plankton in the size range ingested by E. marina (<1 mm) was estimated to be 30 to 35 times more abundant than the larger zooplankton (>1 mm) in the eastern Gulf of Mexico at 27N, 86W, with 94% of the numbers of the < 1 mm zooplankton concentrated in the upper 200 meters (Hopkins, 1982). The data also indicate that a maximum in numbers and biomass of this size fraction occurs at 25 meters. The initial ascent of E. marina at dusk to near 30 meters may be influenced by the vertical distribution of this small prey. This data shows that adult males of Euchaeta marina appear not to feed. This has been shown of other adult males of marine copepods including Euchata scotti, Calanus tenuicornis, Gaetanus (Gaidius) brevispinus, and Scaphocalanus magnus (Mullin, 1966). In experiments with! norvegica fed a mixture of natural prey, average ingestion rates were less than one individual per day (Bamstedt and Holt, 1978). Adult female E. elongata showed higher capture rates (0.67-3.18 prey/preditor/day) under experimental conditions (Yen, 1982). In the present study only one prey individual was recognizable, with the maximum (1 gut) being 3 prey items. Other unrecognized food material was also observed in guts. A preliminary check on estimated capture rates is possible through energetics modeling. Daily met abolic needs of adult female Euchaeta marina can be gauged from existing data on this species and data on

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24 congeners. Respiration rates for adult female Euchaeta marina run at experimental temperatures in the range of those at the study site range from 2.24-2.69 0 2/animal/hour (average 2.47 o2/animal/hour) (Ikeda, 1970). Assuming an oxycaloric equivalent of 4.7 Kcal/liter 0 2 (Gordon, 1977), the daily metabolic needs of adult females can be -4 estimated as 2.79 x 10 Kcal/day. Similarly, the energy yield of prey items can also be estimated from available data. Average dry weights for small (1 mm) calanoids at the study siue were estimated at 0.010 mg (Hopkins, personal communication). Assuming that crustaceans contain 60% protein, 38% fat, and 2% carbohydrates, and that the caloric content of protein, fat, and carbohydrate are 4.5, 9.5, and 4.0 Kcal/gm, respectively, (Gordon, 1977), then the caloric yield of an average small -6 calanoid is approximately 6.40 x 10 Kcal. Based on these calculations, an adult female would have to consume at least four prey items per day or 0.040 mg dry weight of crustacean prey to meet respiratory demands alone. These calculations suggest that marina may feed during each of the peak periods shown in Figure 3B, or that they capture more than one prey item during each feeding period. Evidence o f some multiple prey captures and of multiple feeding periods is seen in this data; however, the exact mode needs further investigation. Crustaceans less tha n 1 mm body length in the upper 100 meters average 852,000 under a squar e meter of sea surface in the eastern Gulf of Mexico. Assuming a uniform distribution of prey animals, Euchaeta marina would have to search approximately a volume of 120 cc of water for each encounter. Euchaet a marina m a y be a tactile preditor r elying on active movement of prey for detection, as has been proposed for E.

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25 elongata (Yen, 1982). Consequently, successful encounters are probably lower than would be expected from visual cue predation given the amount of prey available. Individuals may compensate for low encounter rates by consuming entire prey which are large relative to predator body size. Itoh (1970) classified copepods into feeding groups based on the shape of the cutting edge of the mandible and the second maxilla, and suggested that members of the family Euchaetidae were well suited for grasping, holding, and biting zooplankton prey. The present diet information confirms this.

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26 CONCLUSIONS (1) Euchaeta marina adults and copepod stage V individuals maintained a daytime depth center near 50 meters, moving upward in the water column at dusk to near 30 meters. This segment of the population exhibited an apparent midnight sinking between 0000 and 0200 hrs followed by a second migration to near the surface prior to the return at dusk to their daytime depth of 50 meters. (2) Copepodid stages IV and earlier composite individuals showed the same vertical pattern as that described for older individuals with the exception that a midnight descent was lacking. Instead, these individuals remained at 30-40 meters throughout the night, then descended to the daytime center near 50 meters after dawn. (3) Feeding chronology data for adult females indicated that they fed prior to their midnight descent and then again after the population returned to the surface after 0200 hrs. (4) Feeding chronology data for copepodid IV and earlier stages indicated low levels of feeding throughout the night. (5) Diet analysis for adult females showed them to prey exclusively on small copepods and crustaceans. (6) Evidence was presented to suggest that the vertical distribution of Euchaeta marina was influenced in part by their feeding chronology.

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27 (7) Energetics modeling for adult females suggest that these individuals may either capture several prey per feeding period or that multiple feeding periods are necessary to satisfy their daily metabolic needs. This hypothesis needs further testing.

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Arashkevich, Y.G., (1968). northeastern Pacific. LIST OF REFERENCES The food and feeding of copepods in the Oceanology, 695-709. Baird, R.C., and T.L. Hopkins, (1981). Trophodynamics of the fish Valencienellus tripunctulatus. II. Selectivity, grazing rates, and resource utilization. Marine Ecology Progress Series, 5: 11-19. Bamstedt, U. and M.R. Holt, (1978). Experimental studies on the deep-water pelagic community of Korsfjorden Norway. Prey size preference and feeding of Euchaeta norvegica (copepoda). Sarsia 63(4): 225-236. Beklemishev, C.W., (1954). The feeding of some of the common copepods in the seas of the Far East. Zoologicheskij Zhurnal, 11: 1210. Boyd, C.M., (1976). Selection of particle size by filter feeding copepods: a plea for reason. Limnology and Oceanography, 21: 175-180. 28 Cannon, H.G., (1928). On the feeding mechanism of the copepods Calanus finmarchicus and Diaptomus gracilis. British Journal of Experimental Biology, 131-144. Frost, B.W., (1977). Feeding behavior of Calanus pacificus in mixtures of food particles. Limnology and Oceanography, 472-491. Frost, B.W., (1974). Feeding processes at lower trophic levels in pelagic communities, p. 59-77. In: C.B. Miller, editor. The Biology of the Oceanic Pacific. Oregon State. Geinrikh, A.K., (1958). On the nutrition of marine copepods in the Tropical Region. Doklady Akademiya Nauk SSSR, 119: 229-233. Gordon, M.S., (1977). Animal Physiology: Principles and Adaptions, 3rd edition. MacMillan Publishing Co., Inc., New York. 699 pp. H di G C H (1974) The food and feeding of the deep-sea copepods. ar ng, Journal of the Marine Biological Association U.K., 54: 141-155.

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29 Heffernan, J.J., and T.L. Hopkins, (1981). Vertical distribution and feeding of the shrimp gerera Gennades and Bentheogennema (Decapod: Penaeidea) in the eastern Gulf of Mexico. Journal of Crustacean Biology, 1(4): 461-473. Hopkins, T.L., (1982). The vertical distribution of zooplankton in the eastern Gulf of Mexico. Deep-Sea Research, 29(9): 1069-1083. Hopkins, T.L., and R.C. Baird, (1981). Trophodynamics of the fish Valenciennellus tripunctulatus. I. Vertical distribution, diet and feeding chronology. t1arine Ecology -Progress Series, 1-10. Hopkins, T.L., J.L. Bennett, and P.G. Shuert, (1979). Zooplankton study at the Florida west coast O.T.E.C. Site. A final report to the Lawrence Berkeley Laboratory. 41 pp. Hopkins, T.L., and R.C. Baird, (1977). Aspects of the feeding ecology of oceanic midwater fishes. In: N. Anderson and B.J. Zahuranec, editors. Proceedings International Symposium on Predication of Sound Scattering in the Ocean. Plenum Press, New York, pp. 325-360. Hopkins, T.L., and R.C. Baird, (1975). Net feeding in mesopelagic fishes. Fishery Bulletin U.S., 21= 908-914. Hopkins, T.L., R.C. Baird, and D.M. Milliken, (1973). A messenger-operated closing trawl. Limnology and Oceanography, 18: 488-490. Hopkins, T.L., and R.C. Baird, (in press). hatchetfishes (Sternoptychidae) in the Bulletin of Marine Science. The feeding ecology of four eastern Gulf of Mexico. Ikeda, T., (1977). Relationship between respiration rate and body size in marine plankton animals as a function of the temperature of habitat. Bulletin of the Faculty of Fisheries, Hokkaido University, 21(2): 91-112. Itoh, K., (1970). A consideration on feeding habits of planktonic copepods in relation to the structure of their oral parts. Bulletin of the Plankton Society of Japan, 17(1): 1-10. Landry, M.R., (1978). Predatory feeding behavior of a marine copepod, Labidocera trispinosa. Limnology and Oceanography, 1103-1113. Leavitt, B.B., (1938). The quantitative vertical distribution of macrozooplankton in the Atlantic Ocean Basin. Biological Bulletin, Marine Biological Laboratory, Woods Hole, 2!= 376-394. Longhurst, A.R., (1976). Interactions between zooplankton and phytoplankton profiles in the eastern tropical Pacific Ocean. Deep-Sea Research, 23: 729-254.

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McGowan, J.A., (1974). The nature of oceanic ecosystems. Hiller, editor. The Biology of the Oceanic Pacific. Oregon State University Press, Corvallis. In: C.B. pp. 9-28. 30 McLaren, I.A., (1963). Effects of temperature on the growth of zooplankton, and the adaptive value of vertical migration. Journal the Fisheries Research Board of Canada, 20(3): 685-727. Morris, M.J., and T.L. Hopkins, (1981). Biochemical composition of the crustacean zooplankton from the eastern Gulf o f Mexico. Journal of Experimental Marine Biology and Ecology, 1-19. Metoda, S., (1959). Devices of simple plankton apparatus. Faculty Fisheries, Hokkaida University: 74-98. M.M., (1966). Selective feeding by calanoid copepods from the Indian Ocean. In: H. Barnes, editor. Some Contemporary Studies in Marine Science. George Allen and Unwin Ltd., London. pp. 545-554. Owre, H.B. and M. Fo yo, (1976). Caribbean Zooplankton. Part l Siphonophora, Heteropoda, Copepoda, Euphausiacea, Chaetognatha and Salpidae. Office of Naval Research, Department of the Navy. 712 pp. Owre, H.B., and M. Foyo, (1967). Copepods of the Florida Current. Fauna Caribaea, No. 1. Institute of Marine Science, University of Miami, Florida. -y37-pp. Pearre, S., (1973). Vertical migrati on a nd feeding in Sagitta e legans Verr ill. Ecology, 54(2): 300-314. Roe, H.J.S., (1972). The vertical distribution and diurnal migrations of calanoid copepods collected on the Sond Cruise, 1965. II. S ystematic account: fam ilies Euchaetidae up to and including the Metridiidae. Journal o f the M arine Biological 52(3): 525-552. Roehr, M.G. and H.B. Moore, (1965). The vertical distribution of some common copepods in the Straits of Florida. Bulletin of Marine Science, 15(3): 565-570. V erVoort, W., (1963). P e l a gic Copepod a I. Atlantidae Report, 7: 77-194. Vinogradov, M.E., (1970). Vertical distribution of the oceanic zooplankton. Translat i on of monograph TT 69-59015 3 4 6 pp. Reference In: United Sta t e s Government Research a nd D e v elopment R eports 70(15): 64. Wic k s tead, J.H., (196 2). F o o d a nd feed i n g in pelagic copep od s Proceedings the Zoological Society o f London, 139: 545-555.

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Yen, J., (1982). Sources of variability in attack rates of Euchaeta elongata Esterly, a carnivorous marine copepod. Journal Experimental Marine Biology and 105-117. 31

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32 APPENDIX 1

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.-\1'/'LNIJ I X I Summary of sample data for Euchaeta marina, arranged in 2 hour intervals. Cruise Cl-1 = Columbus Iselin 1975, CI-11 = Columbus Iselin 1976, FS-1 = Bellows 1980, FS-11 = Bellows 1982. Local time is based on local noon at the sample site. Feeding samples are those used in histological analysis. Timer Interval 1800-2000 2000-2200 Sample fl 13-S 2-S 25-S 32-S 5 6 57 252 27 53 55 221 228 14-S 3-S 0-16 15-05 26-S 57 30-06 59 33-S 60 Cruise If CI-1 CI-1 CI-1 CI-1 FS-1 FS-1 FS-II er-r FS-1 FS-II FS-II CI-I Cl-I CI-I CI-I CI-II CI-II CI-I FS-II CI-II FS-II FS-I FS-II Local Time 1835-1920 1837-1922 1845-1930 1845-1945 1914-1933 1911-1937 1917-1947 1825-1958 1932-2004 1740-1820 1917-1947 1839-19 39 1821-1920 2000-2045 2002-2050 2015-2215 1952-2035 2120-2225 2013-204 7 2113-2156 2111-214 7 2115-2215 2111-214 7 Depth 0 0 0 15 20 40 65 100 110 118 118 135 150 0 0 0 15 15 17 30 35 so 69 Number of E. marina Per 7550 2495 768 1523 2411 1611 62 60 13 4 62 103 163 294 184 158 14 72 2527 61 1701 1035 1588 1100 If i\dul t Females Examined for Feeding 10 8 2 7 5 9 w w

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Appendix 1 Number of II Adult Females Depth E. marina Per Examined for Timer Interval Sample II Cruise II Local Time (m) 100 m3 Feeding 241 FS-I 1940-2045 75 392 242 FS-I 2121-2232 75 313 229 er-r 2037-2134 120 43 2200-0000 15-S CI-r 2142-2227 0 49 16-S CI-r 2245-2330 0 96 4-S er-r 2247-2335 0 109 7 FS-r 2234-2251 5 1585 10 75 FS-II 2210-2241 15 29 61 FS-II 2215-2247 26 964 6 8 FS-r 2230-2254 30 1358 50-02 Cr-II 2115-2246 50 472 62 FS-II 2215-2247 86 194 30 FS-I 2149-2223 90 141 223 er-r 2313-0015 100 66 248 er-r 2325-0034 100 50 230 er-r 2328-0028 100 23 29 FS-r 2156-2229 125 90 10 0000-0200 0-17 er-n 2311-0127 0 136 17 S er-r 2345-0030 0 42 5-S er-r 0000-0045 0 142 18-S err 0056-0151 0 36 6-S er-r 0110-0155 0 223 76 Fs-rr 0017-0047 5 55 5 79 FS-II 0017-0047 6 27 6 77 FS-II 0135-0153 9 83 w .1::-

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Appendix 1 Number of If Adult Females Depth E. marina Per Examined for Timer Interval Sample If Cruise II Local Time (m) 100 m3 Feeding 27-S Cl-l 2345-0045 15 396 15-03 Cl-Il 0110-0156 15 731 30-07 Cl-II 0117-0203 30 428 66 FS-II 0126-0157 35 1570 5 34-S Cl-l 2355-0052 so 2648 65 FS-Il 0126-0157 69 4167 10 249 Cl I 0122-0222 75 453 63 FS-II 0022-0055 79 2491 32 FS-I 0035-0057 90 250 10 254 Cl-I 0114-0219 100 51 231 Cl-l 0125-0225 150 45 0200-0400 0-18 CI-Il 0145-034.5 0 173 7-S Cl-l 0217-0302 0 305 1 FS-l 0156-0214 2 986 9 28-S Cl-l 0215-0315 15 788 2 FS-l 0154-0216 16 740 78 FS-Il 0250-0310 26 29 69 FS-II 0330-0406 35 3045 10 35-S CI-I 0223-0323 so 729 11 FS-l 0337-0356 96 909 7 68 FS-II 0228-0303 108 17 7 224 CI-I 0136-0234 130 239 12 FS-I 0331-0402 14 7 511 9 0400-0600 13 FS-l 0419-0ll35 2 22162 10 72 FS-II 0431-0510 17 2288 w \ .. n

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Appendix 1 Number of It Adult Females Depth E. marina Per Examined for Timer Interval Sample tt Cruise It Local Time (m) 100 m3 Feeding 71 FS-II 0431-0510 35 414 73 FS-II 0532-0602 35 672 10 14 FS-I 0414-0440 38 554 0600-0800 15-04 CI-II 0721-0811 15 0 30-01 CI-II 0730-0815 30 1833 17 FS-I 0 72 7-0752 50 441 7 16 FS-I 0644-0801 90 98 10 0800-1000 22-S CI-I 0744-0845 0 0 10-S CI-I 0745-0850 0 0 37-S CI-I 0810-0910 50 1444 50-02 CI-II 0820-0905 50 1209 19 FS-I 0910-0938 90 62 20 FS-I 0910-0942 125 323 1000-1200 23-S CI-I 1045-1145 0 0 11-S CI-I 1045-1145 0 0 30-S CI-I 0946-1045 15 14 15-01 CI-II 1100-1145 15 29 21 FS-I 0950-1018 20 75 22 FS-I 1005-1022 50 545 10 38-S CI-I 1045-1145 50 2138 1200-1400 1-S CI-I 1325-1430 0 0 4 FS-I 1227-1250 21 529 23 FS-I 1214-1239 72 601 10 w a--

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Appendix 1 Number of If Adult Females Depth E. marina Per Examined for Time Interval Sample If Cruise If Local Time (m) 100 m3 Feeding 24 FS-I 1214-1242 96 383 234 CI-I 1315-1426 130 138 1400-1600 12-S CI-I 1345-1445 0 0 24-S CI-I 1350-1450 0 0 31-S CI I 1349-1447 15 0 15-02 CI-II 1429-1515 15 27 30-02 CI-II lLtJS-1510 30 4104 9 FS-I 1403-1427 38 2459 10 39-S CI-I 1341 -1441 so 1479 50-01 CI-II 1358-1445 so 3084 50-04 CI-II 1525-1607 so 692 25 FS-I 1533-1601 90 26 3 26 FS-I 1533-1601 125 53 8 49 FS-II 1423-1453 130 99 5