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

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 (50–70·10³km⁻²; 0–1,000 m), trophic competition may stem primarily from more abundant (>10×) micronektonic groups such as midwater fishes and shrimps than from other cephalopods.