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Species Composition and Food Habits of the Micronektonic Cephalopod Assemblage in the Eastern Gulf of Mexico

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Species Composition and Food Habits of the Micronektonic Cephalopod Assemblage in the Eastern Gulf of Mexico BULLETIN OF MARINE SCIENCE, 49(1-2): 638-659, 1991 SPECIES COMPOSITION AND FOOD HABITS OF THE MICRONEKTONIC CEPHALOPOD ASSEMBLAGE IN THE EASTERN GULF OF MEXICO Kenneth C. Passarella and Thomas L. Hopkins ABSTRACT Cephalopods are a common but not abundant element of the micronekton of the eastern Gulf of Mexico, an area hydrographically and biologically similar to low latitude oligotrophic oceans throughout the world. Forty seven species were identified from Tucker trawl collections in the vicinity of 27°N, 86°W, with seven new records for the Gulf. All species have been recorded from the Atlantic and 69% are pan-oceanic at low latitudes. The Teuthoidea were the largest fraction of the catch, particularly species of the families Enoploteuthidae and Cranchiidae. All but three species occurred in the epipelagic zone at night and diel vertical migration is suggested for many of the population. Closing trawl data indicate that most of the cephalopod population occurs shallower than 200 m at night and centers at 100 to 400 m during the day. Populations of several of the abundant, smaller species were greatest in July but this could not be linked, on the basis of size measurements, to recruitment of juveniles to the population. Diet analysis indicates that micronektonic cephalopods are crustacean feeders as juveniles, but rely more on fish as they mature (>4 cm mantle length). Some cannibalism is apparent. Cranchiids contained relatively little food which might result from a relatively inactive life strategy. The latter is suggested by rather flaccid musculature in comparison to other teuthoids. The copepod genus Pleuromamma is highly selected for by a number of species, perhaps a function of the strong bioluminescent signal produced by members of this genus. Cluster analysis revealed several feeding guilds among the abundant species, though intracluster diets usually exhibited strong overlap. Given the relatively low abundance levels of cephalopods (50-70-lO^km"^; 0-1,000 m), trophic competition may stem primarily from more abundant ( > 1 0 x) micronektonic groups such as midwater fishes and shrimps than from other ceph­ alopods. Cephalopods are a conspicuous element of the oceanic fauna, ranging in size from planktonic to some of the largest of nekton (Roper et al., 1984). We report herein on the micronektonic component of the pelagic assemblage in the eastern Gulf of Mexico. Composition of the fauna in this region is fairly well known through the efforts of Voss and his colleagues (Voss, 1954, 1956a, 1956b, 1962, 1963a, 1964; Voss and Voss, 1962; Roper, 1964; Roper etal., 1969;Caims, 1976) and from Lipka’s (1975) dissertation research. The present work is based on two large collections made with opening-closing Tucker trawls in the vicinity of 27°N, 86®W in a water depth of 3,200 m. The first sample set is from 1975-1977 R/V Columbus Iselin cruises during which 20 depth horizons were sampled day and night. These collections provided diel vertical distribution information in the epi- mesopelagic zone (0-1,000 m). The second set comes from 1984-1987 R/V SuNCOASTER cruises which concentrated on the upper 200 m at night. Samples from these cruises were used to further examine vertical distribution patterns in the epipelagic zone and especially for cephalopod trophic studies. Since little has been reported on diets of micronektonic cephalopods, the present results have special value in elucidating their role in oligotrophic tropical-subtropical ecosys­ tems. M e t h o d s Sampling took place within a 20 km circle centered at 27°N, 86°W. Collecting gear on RA'C o l u m b u s Ise lin cruises was 3.2 and 6.5 m^ nets (Tucker trawl) o f 4 mm mesh, with a 1 mm mesh codend. 638 DWH-AR0127433 PASSARELLA AND HOPKINS: GULF CEPHALOPOD ASSEMBLAGE 639 Trawls were opened and closed with messenger-operated, double-trip mechanisms, and volume of water filtered was measured with dial-type meters which recorded only when the net was fishing (Hopkins et al., 1973). Depth was monitored through a deck readout using a depth transducer con- ducting-cable-system. A depth trace of each tow was obtained with a time-depth recorder, Trawling speed was 1.5 to 3.0 kn. A coarse mesh (4 mm) “fishcatcher” sleeve was inserted in the trawl body ahead of the codend net to retain micronekton while allowing zooplankton to pass through to the codend. This was done to minimize bias in diet analysis resulting from post-capture feeding in the trawl (Hopkins and Baird, 1975). Also, collapsible 44-44 cm or 66-66 cm, 163-^rm mesh plankton nets were suspended in the mouth of each trawl to capture zooplankton concvnrently with micronekton. Flowmeters used with the plankton nets recorded only when the nets were open. Similar Tucker trawl-plankton net com­ binations were used on R/V Su n c o a ste r cruises, thou^ with the addition of a 3.2 m^, 1.6-mm mesh trawl and the substitution of clock release mechanisms (Davies and Barham, 1969) for the messenger operated net releases. R/V Columbus Iselin tows were at discrete horizons and had a depth variance of ± 15 m at depths <300 m and ±25 m at greater depths. R/V Suncoaster tows were of two kinds, 0-200-0 m oblique sweeps and 25 to 50 m discrete zone hauls in the upper 200 m. A summary of the collection data is in Passarella (1990). Samples were initially preserved in 5-10% v/v formalin-sea water solution buffered with borax and subsequently transferred to 50% isopropanol. Cephalopods were sorted to species in the lab and mantle lengths (ML) were measured. Most (>95%) of the individuals in our collections were micronektonic in size, 0.5-20.0 cm ML, though the larger cephalopods were included in the study to provide additional information on species composition, vertical distribution and feeding habits of the assemblage. The protocol for biomass determination was to first weigh alcohol preserved specimens (=AW), soak them in water for 24 h, then reweigh (=WW). Regressions of WW on ML were established for five of the numerically dominant species (Pterygioteuthis gemmata, P. giardi, Pyroteuthis margaritifera, Heli- cocranchia pfefferi and Egea inermis), the P values ranging from 84 to 99%. Individuals of less abundant species were weighed separately, with their weights adjusted to WW using a WW = 1.4 AW correction factor. For diet analysis, the cephalopods examined were subdivided into four size classes based on mantle length, < 1 cm ML, 1-2 cm ML, 3-4 cm ML and >4 cm ML. Within each size class digestive tracts (esophagus, stomach, caecum and intestine) were removed and gut contents were distributed as evenly as possible in water on a microscope slide. Examination was at 100 to 400 k magnification. Food, mostly crustacean, was fragmented, therefore prey identification was usually based on diagnostic body parts, In most instances prey were identified to family; often it was possible to classify prey to species and to determine their numbers. In the case of the copepod genus Pleuromamma. the metasomal organ (spot) was measured and its shape recorded to estimate copepod size (using a spot dimension- copepod length regression) and to determine species. Statistical treatment of the data included using the Kolmogorov-Smimov test (Sokal and Rohlf, 1981) in comparing catches of cephalopods by trawls of different meshes (1.6 mm and 4.0 mm). ANOVA statistics were applied to seasonal data (R/V SLntcoASTER collections) to discern significant population size changes in the upper 200 m at n i^ t from cruise to cruise. This was done for three of the most common species,Pterygioteuthis gemmata. P. giardi and Pyroteuthis margaritifera. ANOVA was also used to test differences in mantle length of these three species from cruise to cruise to determine if there were seasonal recruitment patterns. Diets of seven of the abundant species were compared with the Bray-Curtis similarity index (Bray and Curtis, 1957). This index was applied to percent values for each of 21 diet categories where the number of items in each category was expressed as a percent of the total food items recorded for the sample. The similarity indices were then grouped by average distance linkage cluster analysis (Sarle, 1982; Field et al., 1982; Romesburg, 1990) to discern feeding guilds. The cutoff for diet dissimilarity was arbitrarily set at two thirds of the total average linkage cluster distance. Diet diversity was calculated with a variant of the information measure (Travers, 1971); D = 1/N [loga N - 2 n, log; n,] where N = total incidences (=100%) of all food categories (21), n, = incidence (%) of a single food category and D = 0-4.45. Physical data at 27°N, 86°W were obtained through numerous XBT casts on R/V C o lu m b u s Iselin cruises, and from XBT and CTD casts on R/V Su n c o a st e r cruises. R e s u l t s Hydrography. — The dominant but variable feature of the eastern Gulf circulation is the Loop Current. It is the anticyclonic extension into the eastern Gulf of Mexico DWH-AR0127434 640 BULLETIN OF MARINE SCIENCE, VOL. 49, NO. 1-2, 1991 by Caribbean water which forms the Florida Current. The core of the Loop which centers at -2 0 0 m, is characterized by high salinity, >36.5%o, and warm tem­ peratures, >22® (Nowlin, 1971). The surrounding residual water in the eastern Gulf is both cooler and less saline. Intrusion into the Gulf, which is intermittent and unpredictable, is on an axis oriented from the center of the Yucatan Straits to the mouth of the Mississippi (Maul, 1977; Molinari and Mayer, 1980; Sturges and Evans, 1983).
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