Structure and Trophic Ecology of a Low Latitude Midwater Decapod and Mysid Assemblage

The micronektonic crustacean assemblage in the eastern Gulf of Mexico is an extension of the low latitude Atlantic and Caribbean faunas. Species showed highly varying diel distribution patterns ranging from a strong vertical migration to the epipelagic zone to absence of any migration resulting in a permanent residence deep in the mesopelagic zone. As in other low latitude areas, decapod species with variegated pigment patterns centered above 650 m during the day, whereas \u27all-red\u27 species centered below this depth. Standing stocks were estimated at 0.18 g dry wt m-2 and 3 ind. m-2 in the upper 1000 m. Diet analysis revealed that crustaceans dominate as the main food biomass of sergestids (e.g. copepods, ostracods, euphausiids) while aristeids (Gennadas spp.) and carideans feed heavily on both fish and crustaceans. Among mysids, Gnathophausia ingens ingests mostly fish while eucopiids are primarily copepod feeders. Other common diet items of the micronektonic crustacean assemblage are chaetognaths, molluscs (pteropods, heteropods) and large phaeodarian radiolarians. Olive-green debris containing phytoplankton and protists was encountered in the diets of all but 2 caridean species and the mysids. It was most prevalent in the diets of the Sergia species and the aristeids. A likely source of this material is \u27marine snow\u27. The predation impact of the decapod and mysid population on zooplankton is estimated at 1% of standing stocks and 18% of biomass production d-1. Combined results from cluster analyses of vertical distribution (space and time niche dimensions) and diet (food niche dimension) characteristics indicate that resource partitioning among the shrimp assemblage in the eastern Gulf of Mexico occurs at the between-species level