SPECIES COMPOSITION, VERTICAL-DISTRIBUTION, AND FOOD-HABITS OF THE SERGESTID SHRIMP ASSEMBLAGE IN THE EASTERN GULF-OF-MEXICO

Eighteen species of sergestid shrimps are found in the upper 1,000 m of the eastern Gulf of Mexico (27-degrees-N, 86-degrees-W). Species composition of the assemblage is most closely allied with that of subtropical western North Atlantic. Most of the group migrate into the epipelagic zone at night. During the day the mature stages of all species center below 200 m. Seven species have bimodal distributions at night with early juvenile stages occurring in the upper 50 m, and the older stages between 100-200 m. Two species retain bimodal distributions during the day. The genus Sergia is centered deeper in the water column than Sergestes. Standing stocks in the upper 1,000 m are 1.14 x 10(6) individuals km-2 and 41 kg dry wt. km-2. The three most abundant species are Sergia splendens, Sergestes pectinatus, and Sergestes atlanticus; the principal biomass species are Sergia splendens, Sergestes henseni, and Sergia robustus. Sergestids are zooplanktivores, with crustaceans as the predominant food. Important individual diet categories are copepods, ostracods, euphausiids, coelenterates, chaetognaths, and olive-colored debris. The latter, possibly marine snow, contains a spectrum of phytoplankton and protozoans. Comparisons of vertical distributions and diets suggest resource partitioning among sergestids. One species pair, Sergestes henseni and Sergestes paraseminudus, members of the "corniculum" group, had closely overlapping diets and vertical distributions. In terms of the two major niche parameters, food and space, they appear to be ecologically the same, and given their minute morphological differences, may represent different morphs of the same species.