Feeding Ecology of Four Hatchetfishes (Sternoptychidae) in the Eastern Gulf of Mexico

Home , Argyropelecus, Argyropelecus aculeatus, Argyropelecus hemigymnus, Sternoptychidae

BULLETIN OF MARINE SCIENCE, 36(2): 260-277, 1985

FEEDING ECOLOGY OF FOUR HATCHETFISHES (STERNOPTYCHIDAE) IN THE EASTERN GULF OF MEXICO

Thomas L. Hopkins and Ronald C. Baird

ABSTRACT Vertical distribution and trophic ecology of hatchet fishes were investigated in the eastern Gulf of Mexico. The four principal species, Argyropelecus aculeatus. A. hemigymnus, Ster- noptyx diaphana and S. pseudobscura, ranged in abundance from 21-53 x 103 km-2 in the upper 1,000 m. There is strong evidence for time-space and food resource partitioning among these species. Depth of habitat and diet characteristics are reflected in cryptic adaptations and functional morphology of the two genera. A. aculeatus appeared to feed in the epipelagic zone « 200 m) early at night. Ostracods and copepods were the most important {biomass} food for smaller size classes, and pteropods, euphausiids and fish for larger individuals. A. hemigymnus apparently foraged in late afternoon in the 300-500-m zone. Ostracods and copepods were the principal food of all size classes. Cyclic feeding was not evident in either species of Sternoptyx. S. diaphana. which occurred primarily at 500-800 m, fed largely on copepods, ostracods and amphipods as juveniles and on amphipods and euphausiids at maturer sizes. S. pseudobscura which occurred mostly below 800 m, ingested primarily copepods, polychaetes and euphausiids as juveniles and took proportionately more amphipods and fish as adults. Non-random food choice was apparent, the Argyropelecus species selectively feeding on ostracods and S. diaphana on ostracods and amphipods. Much of the food of S. pseudobscura. inexplicably, was epipelagic in origin. Diet, depth and morphological characteristics of these hatchetfish species support the case for reduction of intraspecific competition through evolution.

Knowledge of the feeding strategies of organisms important to the trophic hierarchy is central to our understanding of oceanic ecosystems. Through accu- mulation of information of the trophodynamics of key species we can ultimately construct a picture of how pelagic ecosystems function. Among the dominant micronektonic zooplanktivores in the ocean are midwater fishes and their impact on zooplankton populations must be considered in any model of oceanic ecosys- tem dynamics. At tropical-subtropical latitudes the mesopelagic fish community is characterized by high diversity with a hundred or more species often found in the upper 1,500 m (Gibbs and Roper, 1970; Badcock, 1970; Rass, 1971). Most of these species are zooplanktivorous (Legand and Rivaton, 1969; Merrett and Roe, 1974; Hopkins and Baird, 1977; Clarke, 1978; 1980; Gorelova, 1980, and many others) and it is of considerable theoretical interest to elucidate the under- lying factors regulating the coexistence of species in these complex assemblages. In the eastern Gulf of Mexico there are four relatively common species of hatchetfishes (Sternoptychidae) which together constitute an important component ofthe mesopelagic fish fauna (Maynard et aI., 1975, and the authors' unpubl. data). The four species have pan-oceanic distributions between 40"N-40oS and all are zooplanktivores (Merrett and Roe, 1974; Repelin, 1972; Hopkins and Baird, 1975). It would be predicted that these related taxa would tend to evolve life strategies directed towards minimization of competitive interactions and opti- mization of partitioning of available food resources in time and space. The vertical distributions and diets of the four species, Argyropelecus aculeatus, A. hemigym-

260 HOPKlNSANDBAIRD:HATCHETFISHFEEDINGECOLOGY 261 nus, Sternoptyx diaphana and S. pseudobscura, are examined in light of this concept. Hydrographic Setting. - The sampling that constituted the basis of this research was conducted around 27°N, 86°W in the eastern Gulf of Mexico between 1970 and 1977 during the months of May through October. During these months the eastern Gulf is dominated by the flow of the Subtropical Undercurrent which is an anticyclonic gyre of Caribbean water (Leipper, 1970; Nowlin, 1971; Maul, 1977; Molinari and Mayer, 1980). Known as the Loop Current, it flows into the eastern Gulf through the Yucatan Channel and exits through the Florida Straits. In some years the edge of the loop nearly reaches the Mississippi delta. The principal sampling locale lies east of the Loop Current axis and the water typically found at 27°N, 86°W is boundary or Transition water. This water can be identified by the depth of the 22°C isotherm which lies between 50 and 150 m; in the Loop Current it occurs between 150 and 200 m (Nowlin, 1971; Jones, 1973). The two bodies of water are biologically similar in that they are oligotrophic and have similar species composition (EI-Sayed, 1972; Michel and Foyo, 1976; Hopkins, 1982). Transition water, however, appears to be somewhat more pro- ductive (Jones, 1973; Hopkins, 1982). Temperature profiles at 27°N, 86°W during the summer show surface water temperatures to be 28-30°C, with the mixed layer extending to 30-50 m. The portion of the thermocline with the greatest slope extends from the bottom of the mixed layer to approximately 150 m, at which depth the temperatures are 15- 18°C. At 500 m, and the deepest horizon sampled, 1,000 m, temperatures are 8- 9°C and 4-5°C, respectively. Our data (unpubl.) show Transition water to be well oxygenated at all depths, with the minimum, 2.7-2.9 ml O2 per liter, occurring between 400 and 500 m.

METHODS

Information on the vertical distribution of hatchetfishes presented here is restricted to collections from around 27°N, 86°W (Table 1). Individuals taken at other locations in the eastern Gulf, however, were included in diet analysis. Closing Tucker trawls having either 1.8 x 1.8-m or 1.8 x 3.6-m mouth dimensions, a 4-mm mesh (1.1-cm stretched) trawl body and either a 0.3-, 0.5- or l.O-mm mesh codend plankton net were used in sampling. The trawls were opened and closed with a messenger operated release system. The volume of water filtered was estimated using dial-type meters which were modified so that they recorded only when the trawls were open and fishing. Filtration efficiency was assumed to be 100%. Trawl depth was monitored through wire angle measurements or with a conducting cable-depth transducer system. Towing accuracy using the depth transducers was ± 10 m at depths shallower than 200 m and ± 20 m below this depth. Depth distributions of the four hatchetfish species were determined from horizontal discrete depth tows made with the transducer system on three RV COLUMBUSISEUNcruises between 1975 and 1977. The abundances of the four hatchetfish species were estimated from 19 oblique sweeps of the upper 1,000 m made on RV BELLOWScruise VIII in 1981. These collections were also made using a depth transducer. Trawling speed was approximately two knots and the angle of the trawl mouth, estimated from tows just beneath the surface, varied from 30-45° depending on weight loading of the trawl. Mouth angle data were used in adjusting calculations for determining volumes filtered. Zooplankton was sampled concurrently using collapsible I62-ltm mesh plankton nets suspended in the mouth of the trawl. These were either 44 x 44 cm or 66 x 66 cm in mouth dimensions. Row through the plankton nets was also estimated with mechanical dial-type flowmeters. Further details on the construction and operation of the trawl-plankton net system are reported in Hopkins et al. (1973) and Hopkins and Baird (1975; 1981). Zooplankton and micronekton collections were preserved in 5-10% v/v borax buffered formalin. Fishes were subsequently transferred to 50-70% isopropyl alcohol. All stomach analyses and morphometric measurements were made using alcohol preserved specimens. In the laboratory, hatchetfishes were sorted from collections, identified, then measured to the nearest I mm (standard length, SL). For diet analysis the entire digestive tract was removed. The stomach was defined as the muscular, darkly pigmented, anterior portion of the digestive tract extending from 262 BULLETIN OF MARINE SCIENCE, VOL. 36, NO.2, 1985

Table 1. Hatchetfish collection data

No. tows with hatch- A. A. hem;· S. S. pseudob· Cruise Date Location etfishes aculealUS gymnus diaphana SCUTa D-I 7/1971 27"00'N, 86"00'W 1 M-II 3/1972 25°50'N,85°30'W 2 2 8 B-1 8/1972 27"00'N,86"00'W 9 2 2 26 M-III 8/1973 27"00'N, 86000'W 4 2 2 I 27°36'N, 88°40'W 5 7 12 20 28°28'N, 88°56'W 5 6 20 48 29°19'N, 87"01 'W 3 1 1 3 B-II 8/1973 24°30'N, 84°IO'W 2 I 2 24°38'N,85°IO'W 1 I B-III 8/1974 27"00'N, 86"00'W 10 23 31 28 5 C-I 6/1975 27"00'N, 86000'W 10 70 2 I C-II 6/1976 27"00'N,86"00'W 37 20 50 15 2 27006'N,85°16'W 2 4 B-IV 5/1977 27"09'N, 85002'W 1 I 27"09'N, 85006'W 6 2 2 1 5 27"06'N,85°16'W 2 4 27"04'N, 85°40'W 2 8 10 1 C-III 9/1977 27"00'N, 86000'W 31 7 8 21 93 B-V 6/1978 27"00'N, 86"00'W 1 1 B-VI 6/1980 27"00'N,86"00'W 10 7 13 68 8 B-VIII 9/1981 27000'N, 86"00'W 19 20 50 51 43 Totals 163 174 209 299 164

the esophageal opening to the origin of the thin-wall, slightly pigmented intestine. Separate records were kept on stomachs and intestines. Intestinal caecae were not analyzed as they contained little recognizable food. Prey items were identified to species when possible and measured to the nearest 0.1 mm. Copepod prey were measured from the anterior end of the metasome to the tip ofthe furcae exclusive of spines. Euphausiids, amphipods, decapods, mysids and stomatopods were measured from the anterior end of the eyes to either telson tip or the end of the uropods depending on which was longest. Valve length was recorded for ostracods. Total body length was measured for chaetognaths, polychaetes and tunicates, and standard length for fishes. Length including tentacles was recorded for cephalopods and maximum body or shell length for other molluscs. The maximum dimension was measured for individual siphonophore units found in stomachs. Prey biomass was obtained through substitution of the above measurements into plankton size vs. dry weight regressions (Hopkins and Baird, 1981). Average ash content of zooplankton, principally crustacean in composition, has been previously determined as 8% dry weight (DW) (Hopkins, 1982). Food items in stomachs were graded according to state of decomposition, the five categories being fresh, slightly digested, digested, well digested and very well digested (Hopkins and Baird, 1981). The material in intestines was not identified and only arbitrary estimates of intestine fullness were recorded (e.g., 1f4, 'h or 3f4 full). To reveal feeding chronology patterns, stomach contents of similar sized (5 mm size range for each species; see Table 4) fishes collected within the time periods 0500-0800, 0800-1300, 1300-1800, 1800-2100, 2100-0\00, and 0100-0500 were pooled and dried to constant weight at 60°C. Each pooled sample weight was divided by the number of individuals, and this value (average stomach content) was then compared with those for all other time periods. Length vs. weight regressions were established for each of the four hatchetfish species. Frozen individuals of a broad size range were measured to the nearest mm SL, dried to constant weight at 600c, and combusted in a muffle furnace at 4500C to determine ash content and ash free dry weight (AFDW). All weights were determined to the nearest 0.01 mg. The regression equations for the four species are:

Argyropelecus hemigymnus: y = 1.63 X 1O-5x2.ss (r2 = 0.98); Argyropelecus acu/eatus.· y = 8.94 X \O-6X2.88 (r2 = 0.99); Sternoptyx diaphana.· y = 3.78 X \O-6X3.19 (r2 = 0.97); Sternoptyx pseudobscura: y = 3.78 X \O-6X3.19 (r2 = 0.97) where x = mm SL and y = mg AFDW. The following morphological characteristics of the four hatch- HOPKINS AND BAIRD: HA TCHETFISH FEEDING ECOLOGY 263

4202468 6 4 2 2 4 4 2 0 2 4 8 6 2 2 A. ACULEATUS A. HEMIGYMNUS S.DIAPHANA S.PSEUDOBSCURA 100 D N D N D N //- 21.5 200

300 ,...... E 400 J: I- 500 a. W o 600

700 +

800

900

1000 Figure 1. Vertical distribution of hatchetfishes around 27°N, 86°W in the eastern Gulf of Mexico based on discrete depth trawls with opening-closing nets. + = <0.2/10' m'. D = day; N = night.

etfish species and three myctophids (Benthosema suborbitale, Ceratoscopelus warmingii and Lam- panyctus alatus) were measured for comparisons and for analysis of modes of feeding: jaw length (mandible); gill slit height (straight line distance from dorsal to ventral origin of gill slit); number of gill rakers on first arch; gill raker interspace; and standard length. Zooplankton in aliquots of the plankton net samples were identified, counted and measured using procedures outlined in Hopkins and Baird (1981) and Hopkins (1982). This provided prey availability data for each depth stratum at which hatchetfishes occurred. Prey species diversity in stomachs in terms of biomass was calculated using a modification of the information index, ])', as set forth in Travers (1971). The non-parametric Kolmogorov-Smirnov test (Sokal and Rohlf, 1969) was used to compare (at P < 0.05) taxonomic composition (biomass) of the diets of individual collections of hatchetfishes with that of net-caught zooplankton captured at the same time and depth. This test was used in comparing diets of various size classes within species and diets of different species of the same size class. Numbers and biomass of hatchetfish species per Jan2 in the upper 1,000 m were obtained by determining the number of each size class for each species at the depth horizons sampled, using the size vs. weight regressions above to calculate AFDW of the median sized fish in each size class, then multiplying by the number of individuals of each size class and summing. ANOV A statistics were used to test for significant (P < 0.0 I) trends in hatchetfish length vs. depth of occurrence both during the day and night.

RESULTS Population Structure. - The horizontal discrete depth tows indicated that Argy- ropelecus was a shallower dwelling genus than Sternoptyx. The former occurred primarily above 500 m and the latter below that depth (Fig. 1). Though there was considerable overlap in the vertical distributions of the two Argyropelecus species during daylight hours, at night A. aculeatus was most abundant between 100 and 200 m and A. hemigymnus between 300 and 400 m. During the day A. aculeatus 264 BULLETIN OF MARINE SCIENCE, VOL. 36, NO.2, 1985 and A. hemigymnus occurred at 150-500 m and 350-500 m, respectively. The diel ranges suggested a 200-300-m vertical migration of a considerable portion of the A. aculeatus population and that some individuals of A. hemigymnus migrated as well, though over a much smaller range. Our discrete depth hauls at deeper horizons were too few to clearly resolve diel migration patterns in either species of Sternoptyx. Most of the S. diaphana population was broadly distributed between 500 and 900 m. Sternoptyx pseudobscura occurred deeper, its population centered below 800 m. The most abundant sizes of the four species were 10-19 mm SL (Fig. 2). Argyropelecus hemigymnus was the species with the smallest maximum length, 35 mm. The largest individuals taken were A. aculeatus, with several specimens exceeding 60 mm SL. Because of the preponderance of immature hatchetfishes in the population, the mean size of all four species was small and fell within a narrow range, 17.3-21.3 mm SL. Larger individuals, while relatively scarce, contributed significantly to total biomass. This was especially apparent in A. aculeatus and S. pseudobscura. The ANOV A tests indicated that there were no depth related trends in size of A. hemigymnus and S. diaphana, whereas the larger S. pseudobscura were found with significantly greater frequency at the deep end of its vertical range. Size vs. depth patterns for A. aculeatus could not be tested statistically because of inadequate sample size at critical depths. Standing stock estimates of A. hemigymnus, S. diaphana and S. pseudobscura. based on the oblique O-l,OOO-m hauls, revealed close similarity in population sizes, 44,600-52,900 km-2 sea surface. Argyropelecus aculeatus was less than half as abundant (20,800 km-2). Argyropelecus hemigymnus contributed least in terms of biomass (1.9 kg AFDW km-2) and S. pseudobscura the most (5.3 kg AFDW km-2). Diet Composition. - The complexity of hatchetfish diets is apparent, with over 180 prey taxa identified (Table 2; Appendix Table 1). The most important prey taxa in terms of biomass were molluscs for A. aculeatus, ostracods for A. hemigymnus, amphipods for S. diaphana and alciopid polychaetes for S. pseudobscura. The greatest total variety occurred in S. diaphana stomachs; prey diversity indices calculated from both total prey species and major prey groups showed A. aculeatus and S. diaphana had the most varied diets in terms of biomass distribution among the prey taxa (see Appendix Table I and Table 2). Copepods were the most numerous prey in stomachs of all species but A. aculeatus, which most frequently ingested ostracods (Table 2). In all four species, copepods and ostracods combined were 53-95% of the total number offood items. Other numerically important groups were molluscs (pteropods) in the diet of A. aculeatus, amphipods in S. diaphana and alciopid polychaetes and euphausiids in S. pseudobscura. Differences in the food of the four hatchetfish species are readily discernible within the crustacean prey (Appendix Table 1). For example, among the copepods A. aculeatus ingested relatively large numbers of Pleuromamma spp. (P. abdomi- nalis in particular), Oncaea spp. and Undinula vulgaris, though the latter were all found in one stomach which contained over 700 individuals. A. hemigymnus fed heavily on Eucalanus spp., especially E. monachus, and on oncaeids. Both Ster- noptyx species took large numbers of candaciids with Candacia bipinnata being most abundant in S. diaphana stomachs and C. pachydactyla in S. pseudobscura stomachs. S. diaphana also ingested many sapphirinid cyclopoids and S. pseudobscura fed heavily on the pontellid Labidocera acutifrons, which was the principal copepod in its diet. HOPKINS AND BAIRD: HATCHETFISH FEEDING ECOLOGY 265

SIZE CLASS II III IV V VI VII VIII X XI XII

IZI= NO. .=WT. A. HEMIGYMNUS X=20.3mm 30

A. ACULEATUS

X=21.2mm 30

.-Z W 10 U a: w a.. S.DIAPHANA X=17.3mm 30

S.PSEUDOBSCURA X:: 21.3mm 30

60 70 80

SIZE (mmSL) Figure 2. Size distribution of hatchetfishes in the eastern Gulf in terms of numbers and biomass. Biomass calculated using size vs. weight regressions (see Methods). Biomass for each size class estimated by multiplying weight of median sized fish of each size class by number of individuals in that size class. Values were then converted to percents of total species biomass. 266 BULLETIN OF MARINE SCIENCE, VOL. 36, NO.2, 1985

Table 2. Summary of hatchet fish diet characteristics. * Value does not include data for one 50 mm A. aculeatus which had> 700 Undinula vulgaris in its stomach

% Stomach contents A. aculealUS A. hemigymnus S. diaphana S. pseudobscura (174 fish) (154 fish) (263 fish) (150 fish)

Prey group No. Wt No. Wt No. Wt No. Wt Copepods 25.9 8.2 59.9 30.9 38.0 13.8 51.2 22.8 Ostracods 36.9 15.6 35.2 46.9 22.4 13.8 1.5 0.3 Amphipods 7.9 7.4 2.3 <0.1 16.1 23.8 9.5 8.1 Euphausiids 2.5 14.5 0.5 <0.1 1.8 4.5 11.2 17.7 Decapods 2.6 13.8 0.1 <0.1 4.6 9.2 3.5 3.5 Other crustaceans 14.8 0.1 1.0 0.1 1.9 Polychaetes 1.7 0.8 0.1 -4.7 7.4 -18.7 23.6 Molluscs 14.4 20.3 0.1 1.4 4.2 0.4 1.0 Chaetognaths 2.5 5.0 0.1 4.7 4.8 1.4 0.7 Tunicates 1.5 1.7 0.7 0.1 <0.1 <0.1 Fishes 1.0 9.5 0.4 3.4 12.0 1.7 17.7 Other 3.1 3.2 1.3 7.4 2.1 5.4 0.8 2.7 Total No. prey items 1,440* 741 2,432 3,142 Total No. prey taxa 107 59 132 91 Prey species biomass diversity (D') 5.9 4.3 5.9 4.3 Prey major group biomass diversity (D') 2.5 1.3 2.5 2.1

Conchoecia secernenda was the most abundant ostracod in stomachs of both species of Argyropelecus while C. atlantica was the most frequently encountered ostracod in S. diaphana. S. pseudobscura ate relatively few ostracods. Phrosinids (Primno spp.) were the most prevalent hyperiid amphipods in the diets of the Argyropelecus species, though amphipods were relatively rare food items in A. hemigymnus. Members of the families Hyperiidae and Platyscelidae were the most abundant amphipods in the diets, respectively, of S. diaphana and S. pseudobscura. Euphausiids were relatively infrequent prey in stomachs of Argyrope- lecus species whereas the genus Stylocheiron was an abundant component of the food of S. pseudobscura. A study of diet composition by fish size class indicated that diets changed with hatchetfish growth (Table 3). Larger fish of all species ingested larger (> 5 mm) and heavier food items. The dominant groups of zooplankton encountered in the stomachs of both species of Argyropelecus, i.e., copepods and ostracods, were the same for the first two size classes, but the relative percentages changed from the first to the second size class and were markedly different for each species, this being mo~t apparent in the second size class. In the diets of the larger size classes of A. aculeatus, copepods and ostracods were replaced as dominant food items by euphausiids, molluscs and fishes. Amphipods were an important diet component in all three size classes of S. diaphana, with euphausiids replacing copepods and ostracods as dominants in the largest fish size class. In S. pseudobscura, polychaetes, copepods and euphausiids were the most prevalent taxa in stomachs of the first three size classes, while fishes became important in the diets of the two largest size classes. Kolmogorov-Smimov tests indicated that diet compositions were significantly different for all pairings of size classes in Table 3 within species and for all pairings of the same size class of different species. HOPKINS AND BAIRD: HA TCHETFISH FEEDING ECOLOGY 267

Table 3. Diet patterns related to hatchetfish ontogeny

Percent of biomass of prey in stomachs for Fish Fish prey sizes (mm) of: Mean weight sample length (mg OW) per Dominant prey groups in stomach Species size (mm SL) <5 >5 prey item (% biomass) A. hemigymnus 81 10-19 97.7 2.3 0.054 Ostracods (69.0) Copepods (27.4) 72 20-29 72.4 27.6 0.188 Ostracods (42.1) Copepods (31.4) A. aculeatus 102 10-19 90.2 9.8 0.103 Ostracods (61.3) Copepods (15.7) 52 20-29 38.8 61.2 0.443 Ostracods (17.4) Copepods (13.4) 12 30-39 19.5 80.5 0.484 Euphausiids (26.1) Molluscs (19.4) 14 >40 17.4 82.6 0.785 Fishes (33.5) Molluscs (19.5) S. diaphana 183 10-19 52.6 47.4 0.217 Copepods (21.6) Amphipods (17.3) 65 20-29 47.7 52.3 0.420 Amphipods (33.5) Ostracods (25.4) 8 30-39 29.3 70.7 0.581 Amphipods (38.5) Euphausiids (21.4) S. pseudobscura 69 10-19 79.5 20.5 0.356 Polychaetes (42.4) Copepods (40.1) 40 20-29 55.6 44.4 0.576 Copepods (42.3) Euphausiids (23.7) 22 30-39 12.1 87.9 1.646 Polychaetes (35.5) Fishes (15.3) 7 >40 1.9 98.1 4.699 Fishes (67.9) Amphipods (20.4)

Feeding Chronology.-Although data are not available for all time periods, A. aculeatus stomachs were fullest in the midnight 2100-01 OO-hperiod; they were least full and intestines were fullest in the subsequent night period of 0100-0500 h (Table 4). A. hemigymnus, on the other hand, had fullest stomachs in the afternoon period of 1300-1800 h, though fullness of intestines showed relatively little time-related variation. The two Sternoptyx species exhibited less diel variation, particularly concerning biomass of stomach contents. Additional sampling is necessary to adequately address feeding chronology in this genus and to defin- itively confirm cycles suggested for Argyropelecus. Selectivity.-Food size selection was apparent from comparisons of size distributions of stomach contents with that of plankton collected concurrently in plankton nets (Fig. 3). The data for all four species indicated strong selection for prey > 1 mm while plankton < I mm were predominant in the plankton catches. Taxonomic selection was apparent as well (Table 5). This particular data set revealed ostracods to be the most prevalent item in diets of A. hemigymnus and A. aculeatus, with copepods ranking second. In plankton net catches the reverse was true. Also, in the diet of S. diaphana both ostracods and hyperiid amphipods were found in much greater frequency than in the plankton. Comparisons (Kol- 268 BULLETIN OF MARINE SCIENCE, VOL. 36, NO.2, 1985

Table 4. Diel feeding patterns. Value in parentheses represents number of observations. ID = in- sufficient data

Time period 0500- 0800- 1300- 1800- 2100- 0100- Diet characteristic 0800 1300 1800 2100 0100 0500 A. acu/eatus [13-18 mm SL] it Stomach contents (mg DW) ID 0.30 (11) 0.04 (5) 0.43 (15) 0.13 (30) it Items/stomach ID 6.0 (13) ID 13.1 (13) 2.1 (30) % Very well digested prey in stomachs ID 41 (13) ID 40 (13) 62 (31) % Intestine fullness ID 38 (9) ID 38 (9) 62 (13) A. hemigymnus [18-23 mm SL] it Stomach contents (mgDW) ID 0.01 (13) 0.36 (21) 0.09 (3) 0.04 (10) it Items/stomach ID 3.9 (18) 5.7 (26) ID 5.0 (14) % Very well digested prey in stomachs ID 42 (18) 47 (26) ID 51 (14) % Intestine fullness ID 39 (16) 44 (21) ID 41 (13) S. diaphana [14-19 mm SL] it Stomach contents (mgDW) ID 2.21 (13) 1.24(41) ID 1.98 (5) 2.34 (6) it Items/stomach ID 11.2 (13) 8.6 (42) ID 29.6 (5) 11.0 (7) % Very well digested prey in stomachs ID 41 (13) 61 (42) ID 10 (5) 51 (7) % Intestine fullness ID 35 (10) 27 (30) ID ID 22 (5) S. pseudobscura [10-15 mm SL] it Stomach contents (mgDW) 3.05 (6) 2.84 (8) 2.00(12) ID ID 3.04 (10) it Items/stomach 23.3 (10) 17.2 (6) 15.1 (13) ID ID 19.3 (11) % Very well digested prey in stomachs 26 (10) 19 (6) 23 (9) ID ID 38 (11) % Intestine fullness 18 (6) ID 42 (9) ID ID 43 (9)

mogorov-Smirnov test) of the percentages of major crustacean groups in stomachs vs. plankton samples revealed the diet compositions were significantly different from that of the plankton in each of the three species. In comparing taxonomic composition of the diet of S. pseudobscura with that of the zooplankton, best coincidence was with zooplankton species characteristic of the 0-50-m zone which were relatively uncommon below 100 m (e.g., a1ciopid polychaetes, Stylocheiron carinaturn, Luclfer faxoni, Labidocera acutifrons; see Appendix Table 1). This striking lack of concurrence in vertical distributions between S. pseudobscura and its prey will be addressed in the Discussion.

DISCUSSION The standing stock of each of these four hatchetfish species in the eastern Gulf is in the tens of thousands per km2 and the total sternoptychid population ap- proximates two hundred thousand per km2• Maynard et al. (1975) found the same in waters adjacent to Hawaii. To put this population size in perspective, our data HOPKlNS AND BAIRD: HATCHETFISH FEEDING ECOLOGY 269

70 A. HEMIGYMNUS 20-27 mmSL 9 FISH; 56 FOOD 50 ITEMS • STOMACH CONTENTS; TOW 211 30 o PLANKTON; TOW 211 f.;:! PLANKTON; TOW 10 269

70 A. ACULEATUS 15-19mmSL en 6 FISH; 72 FOOD a: 50 ITEMS W al .STOMACH CONTENTS; TOW ~ 30 ::> 237 Z ~ PLANKTON; TOW 10 237 U. 0 S. DIAPHANA I- 11-15mmSL Z 50 5 FISH; 63 FOOD W ITEMS U • STOMACH a: CONTENTS; TOW W 30 370 a. ~ PLANKTON; TOW 370 10

70 S.- PSEUDOBSCURA 10-13 mmSL 5 FISH; 119 FOOD 50 ITEMS .STOMACH CONTENTS; TOW 30 388 ~ PLANKTON; TOW 388 10

< 10 >10

Figure 3. Comparison of size distribution of prey in hatchetfish stomachs and of plankton taken concurrently with nets at the same time and depth. Tow 269 was included in the A. hemigymnus data set because Tow 211 failed to close and fished on ascent. Tow 269 is from the same depth (400 m) and time of day (afternoon). 270 BULLETIN OF MARINE SCIENCE. VOL. 36. NO.2. 1985

Table 5. Comparison of major components of hatchetfish diets with their respective abundance in the plankton. Tow 211 was open on ascent hence Tow 269, a discrete depth tow from the same depth and time of day, was included in the comparison

% of numbers Collection Hyperiid Species Sources (tow No.) Copepods Ostracods amphipods A. hemigymnus Stomach contents 211 35.7 55.3 20-27 mm SL; 9 fish, Plankton 211 87.9 2.8 56 food items Plankton 269 86.6 4.6 A. aculeatus Stomach contents 237 20.8 48.6 15-19 mm SL; 6 fish, Plankton 237 80.4 9.6 72 food items S. diaphana Stomach contents 370 50.8 19.0 17.4 11-15 mm SL; 5 fish, Plankton 370 97.4 1.8 0.1 63 food items

(Hopkins and Lancraft, 1984) and Maynard et al.'s (1975) show myctophids to be 4-5 times and Cyclothone gonostomatids to be 10-15 times more abundant than hatchetfishes. These estimates, however, are subject to the usual problems associated with trawl samples, e.g., escapement of juvenile stages, avoidance, seasonal bias and sample size. Further, the entire vertical range of S. pseudobscura probably was not sampled since our discrete depth collections were limited to the upper 1,000 m. Baird (1971) notes that the vertical range of this species throughout its geographical occurrence is 500-1,500 m. S. pseudobscura, then, is likely the most abundant hatchetfish at this locale. Hatchetfishes clearly partition resources in the mesopelagic environment in the eastern Gulf. This is apparent in depth distributions, diet composition, to some degree in diel feeding patterns, and is reflected in functional morphology. Spatial separation is obvious, as has been noted in other oceanic regions (Baird, 1971; Badcock and Merrett, 1976), the species of Argyropelecus occurring shallower than Sternoptyx and with little overlap. Also, within Argyropelecus there is vertical separation during the period of foraging. While A. aculeatus and A. hemigymnus co-occur during the day, most of the A. aculeatus population migrates at night into the epipelagic zone «200 m) to feed. Although limited migration occurs in A. hemigymnus, as Badcock (1970) also found in the northeast Atlantic, this species remains below 300 m throughout the diel period and feeds primarily during the day (afternoon). Badcock and Merrett (1976) also noted a separation at night of population centers in the eastern North Atlantic (300N, 23°W), with the abundance peak of A. aculeatus occurring shallowest. Feeding studies at that location by Merrett and Roe (1974) indicate that A. hemigymnus forages late in the afternoon and at dark, and A. aculeatus during dusk and early evening, patterns similar to our results. The two Sternoptyx species are alike in that clear evidence is lacking for a distinct feeding cycle, both possibly foraging throughout the diel period. The two species do have different depth centers, S. diaphana occurring mostly above and S. pseudobscura below 800 m. The time-space separation of the four species of hatchetfishes during foraging is important in terms of potential interspecific competition since there is considerable overlap of population size distributions. Most hatchetfishes are 10-24 mm SL, and the mean size of individual within the four species varies little. As indicated in results (Tables 2, 3; Appendix Table 1) there are major differ- HOPKINS AND BAIRD: HATCHETFISH FEEDING ECOLOGY 271

ences in diets, especially when the food of the entire size range of each species is considered. Copepods are dominant food items in the immature stages of all four species and ostracods in the diets of three (S. pseudobscura excepted). More mature hatchetfishes, however, show greater interspecific variation, with relatively little close correspondence in percentages of major taxa in diets of fishes larger than 30 mm SL (Table 3). For example, molluscs and fishes are important components in the food of the larger A. aculeatus; amphipods and euphausiids in the larger S. diaphana; and polychaetes, fishes and amphipods in the more mature S. pseudobscura. A. hemigymnus rarely exceeds 30 mm SL and its food consists primarily of copepods and ostracods throughout its life. Each ofthe hatchetfishes considered here eats a wide range of prey species. This is apparent in the long lists of copepods in Appendix Table 1 found in stomachs of all four hatchetfishes, of ostracods eaten by the two Argyropelecus species and of amphipods in the diets of the two Sternoptyx species. No two arrays are the same, however, and while the number of prey types found in each of the hatchetfish species is high, our evidence (Table 5) indicates that food choice is not random. This is not unexpected since selective feeding has been reported for a number of myctophids (Hartmann and Weikert, 1969; Gorelova, 1974; Clarke, 1980) and gonostomatids (Merrett and Roe, 1974; Baird and Hopkins, 1981). It has been reported for hatchetfish as well, Merrett and Roe (1974) finding as we did that A. aculeatus positively selects ostracods as prey. These authors, however, suggested A. hemigymnus feeds randomly, whereas our data indicate that in the eastern Gulf this species is selective for ostracods. Other than the present study no published information is available on selective feeding in Sternoptyx. The plankton nets are a potential source of bias and possibly could mask true selection patterns. We have no data testing the fishing characteristics ofthese nets. However, mesh size was 162 J,Lm, sufficiently small to retain quantitatively the sizes of all prey found in these hatchetfish stomachs. Also, because the net was suspended in the mouth of the trawl (i.e., a "nested" Tucker trawl design), there was no bridle ahead of the net opening to disturb the water immediately in front of the net. Thus warning cues for avoidance behavior, at least from this cause, were minimal. Further, the data indicate selective feeding on the smaller prey, i.e., copepods and ostracods, which we presume would be well sampled by the nets used in this study. Regional variations in the diets of these four hatchetfish species have been documented (Hopkins and Baird, 1973, tables 5, 7; Hopkins and Baird, 1977, tables 1, 3), though there is some apparent consistency in diet pattern, at least at higher taxonomic levels. A. hemigymnus, for example, primarily ingests cope pods and ostracods in all regions for which data are available (6 oceanic areas). A. aculeatus, while generally feeding heavily on these groups as juveniles, takes proportionately large percentages of molluscs, euphausiids and fishes at sizes larger than 30 mm SL in three areas investigated. The larger S. diaphana in six regions include a high percentage of amphipods in the diet. While little comparative data are available for S. pseudobscura, polychaetes are important in the diet of this species both in the eastern Gulf and the Gulf of Guinea. It is significant from the present and earlier feeding studies that when these species are found together in a region, their diets are conspicuously different (Merrett and Roe, 1974; Hopkins and Baird, 1977). Depth of habitat is reflected in the functional morphology of the two genera. The Argyropelecus species, which are exposed to higher light intensities, have more irregular and broken pigment patterns and more (50 from side view) and 272 BULLETIN OF MARINE SCIENCE, VOL. 36, NO.2, 1985 larger photophores than does Sternoptyx. A. hemigymnus, in fact, is able to adapt the intensity of its pigment bars to the diel cycle of the photoenvironment (Bad- cock, 1969). The Argyropelecus species have large telescopic, dorsally oriented eyes and strongly angled mouths armed with the small teeth characteristic of most other mesopelagic fishes which feed primarily on small crustaceans. These two species are well camouflaged predators apparently well adapted for visually de- tecting and approaching prey from beneath. The Sternoptyx species, which are exposed to lower light intensities, have a more uniform pigmentation and the photophores are smaller and fewer (38 from side view; see Baird, 1971) in number. The eyes are less conspicuously telescopic and more laterally oriented. The jaw is strongly angled as in Argyropelecus, but is shorter. The teeth, while numerous, are also shorter. Also in contrast to Ar- gyropelecus, the body has a "stepped" configuration in that the centers of the mouth and tail are not in horizontal alignment, the tail centering well above the mouth. This morphology combined with the shorter post-abdominal trunk in Sternoptyx suggests a decreased locomotory ability with less effective thrust for quick pursuit or predator avoidance. These characteristics point to a more sedentary behavior than in Argyropelecus and perhaps a greater reliance on ambush capture of food. Also, in keeping with more laterally positioned eyes a greater proportion of prey may be taken in "head on" encounters, with less reliance on approaching prey from beneath. Certain characteristics of the diets of the Sternoptyx species are puzzling and questions arise as to foraging depth and feeding behavior. The diet of S. pseudobscura consisted largely of epipelagic and even neustonic (e.g., Labidocera acutifrons) prey. Sternoptyx pseudobscura, however, was never taken in our neuston hauls and, in fact, has not been collected here or reported elsewhere shallower than 700 m (Baird, 1971; Badcock and Merrett, 1976; and Badcock and Baird, 1980 regarding the vertical range of this species). Post-capture feeding in the trawl seemed an obvious initial explanation since net feeding (though to a very limited extent) has been recorded for S. diaphana (Lancraft and Robison, 1980). This seems unlikely in the present case for the following reasons: (1) this species was taken in discrete depth tows and neustonic plankton species were rarely found (as contaminants) in either plankton net or trawl catches, (2) S. pseudobscura was invariably moribund on arrival in surface waters where neuston would be encountered, and (3) when the two Sternoptyx species occurred in the same catch their stomach contents were markedly different and consistent with the general diet characteristics of each species. While presence as secondary food is a possi- bility, stomachs contained no remains of prey sufficiently large to have contained the quantities of neuston (e.g., L. acutifrons) encountered. The genus Sternoptyx typically has the large swim bladder and poorly developed gas gland (Baird, 1971) characteristic of many non-migrating species. Therefore, rapid migration cycles from below 700 m to the surface undetected by our trawling seem unlikely. Finally, XBT and CTD temperature traces gave no evidence of downwelling of near- surface water to the depths inhabited by Sternoptyx. We have no explanation, then, for the diet composition of S. pseudobscura in the eastern Gulf. The high percentage of amphipods and Sapphirina copepods in stomachs of S. diaphana merits comment. Harbison et al. (1977) suggest that virtually all hyperiids are associated with gelatinous zooplankton at some period in their life history. This is apparently the case as well for Sapphirina copepods (Heron, 1973). Their gelatinous hosts, however, were rarely encountered in stomachs of S. diaphana (gelatinous organisms are recognizable in digestive tracts of mesopelagic HOPKINS AND BAIRD: HATCHETFISH FEEDING ECOLOGY 273

Table 6. Selected morphometric measurements of hatchetfishes (all length measurements in mm; data on myctophids included for comparison)

No. gill Jaw length Gill slit rakers Gill raker Fish length Species (mandible) height (I st arch) interspace X(Rn) Hatchetfishes Argyropelecus aculeatus 8.2 9.9 16 0.56 28.0 (27-29) Argyropelecus hemigymnus 6.7 7.8 20 0.43 27.5 (27-28) Sternoptyx diaphana 5.0 12.7 7 0.65 28.3 (28-29) Sternoptyx pseudobscura 6.5 13.9 7 0.78 27.5 (27-28) Myctophids Benthosema suborbitale 5.4 5.6 13 0.28 27.3 (27-28) Ceratoscopelus warmingii 6.7 5.2 14 0.41 27.5 (27-28) Lampanyctus alatus 6.0 6.0 14 0.49 27.5 (27-28) fishes; Cailliet, 1972; Hopkins and Baird, 1981). This implies a free-swimming period for these crustaceans and/or browsing by the hatchetfishes while the prey is external on the host. Shulenberger (1978) presented evidence for diel migrations in hyperiids which suggests independent movements of at least a portion of the hyperiid population and therefore accessibility in the plankton to hatchetfishes. The diversity and abundance of amphipods in the S. diaphana diet is high but does not appear to be the result of net feeding since our percentages of hyperiid families in stomachs closely matched those found in S. diaphana taken from stomachs of line-caught Alepisaurus (Repelin, 1972). Of special importance in the diet of the Sternoptyx species in the eastern Gulf are alciopid polychaetes, with fragments in stomachs often being several times the length of the Sternoptyx individual swallowing them. An obvious question is how are such long food items captured? We suggest the mouth and gill construction may be adapted to ingesting such prey through suction. Compared to myctophids (Table 6) and even the two Argyropelecus species, mouth gape is small in relation to the gill opening, as judged from gill slit height, and Sternoptyx has fewer gill rakers with larger interspaces, which presumably would offer less impedence to water flow through the branchial chamber. Sudden flaring of the relatively large gill complex could generate strong currents through the mouth which would fa- cilitate indrafting of long flexible prey such as polychaetes and highly mobile organisms such as amphipods. This feeding mode would be consistent with the sedentary ambush strategy postulated for this genus. In summary, the principal hatchetfish species in the eastern Gulfare essentially crustacean feeders taking mostly small prey. The diets are diverse, species-specific and show selectivity in relation to the available array of sizes and taxa in the plankton. The four' species are vertically separated during periods of foraging. Depth, diet and morphological characteristics of the four species are consistent with the concept that evolution has occurred within the group to reduce competitive interactions.

ACKNOWLEDGMENTS

This research was funded by National Science Foundation contracts DES 75-03845 and OCE 75- 03845. Shiptime was made available through UNOLS, the Florida Institute of Oceanography and the Naval Research Laboratory. I wish to thank T. Lancraft for his help in the preparation of this manuscript. 274 BULLETIN OF MARINE SCIENCE. VOL. 36. NO.2, 1985

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DATEACCEPTED: January 4, 1984.

ADDRESS: Department of Marine Science, University of South Florida, St. Petersburg, Florida 33701. 276 BULLETIN OF MARINE SCIENCE, VOL. 36, NO.2, 1985

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