Assemblage structure and trophic ecology of the tubeshoulders (Family: Platytroctidae), a biomass-dominant bathypelagic fish

Guy Harvey Oceanographic Center Facility

Members of the fish family Platytroctidae are circumglobal and primarily inhabit bathypelagic depths (i.e. water columns depths below 1000 m). Due to the lack of specimens this taxon has received little focused attention, despite recent evidence suggesting its predominance in the bathypelagic fish community. Within the Gulf of Mexico (GoM), a highly diverse deep-pelagic system, only four species had been reported prior to this study. An extensive bathypelagic trawl series in the northern GoM allowed a detailed examination of this family, including species composition, abundance, distribution, and trophic ecology. A total of 16 species were collected, 12 of which are new records for the GoM. The five most-abundant species collected were Mentodus facilis, Platytroctes apus, Barbantus curvifrons, Mentodus mesalirus, and Maulisia microlepis. All platytroctids were collected from depths below 700 meters. Platytroctids appear to be capable of actively avoiding standard research-sized midwater trawls, evidenced by higher catch rates using a large, high-speed rope trawl. Gut content analysis revealed that the dominant platytroctids are generalist zooplanktivores, consuming a wide variety of taxa such as, copepods, euphausiids, ostracods, chaetognaths, gelatinous taxa, and the occasional consumption of cephalopods. The majority (~70%) of platytroctids examined had empty stomachs, suggesting infrequent feeding. This study represents the first investigation into the diet of this fish family, and adds to the sparse community data of the bathypelagic zone by identifying nutrient pathways that connect this deep-sea ecosystem to the upper ocean.