Use of chemosynthetic biomass by large, mobile, benthic predators in the Gulf of Mexico

Abstract

The Gulf of Mexico supports communities of chemoautotrophic and heterotrophic fauna associated with hydrocarbon seeps. The chemoautotrophic invertebrates, mostly dense epifaunal assemblages of mussels and tubeworms, derive their nutrition from symbiotic relationships with sulfide- or methane-oxidizing bacteria. The extent to which benthic predatory fauna feed on the chemoautotrophic assemblages has been an open question. Owing to differences in stable isotope values between chemosynthetic- and photosynthetic-derived biomass, isotopic characterization of predatory fauna would be useful in determining their dependence upon chemoautotrophs for food. Carbon and sulfur stable isotope analyses reveal that fishes with similar feeding strategies (rat tail [Nezumia sp.] and eels [Synaphobranchus sp., Ophicthus cruentifer and Dysomma rugosa]) have similar δ 13 C values (-32.7 and -42.5%o, respectively), reflecting chemoautotrophic carbon. Large vagrant predators/scavengers such as isopods Bathynomus giganteus, hagfish Eptatretus sp. and spider crabs Rochina crassa, have isotope values closer to oceanic ranges (δ 13 C: -20 to -18‰; δ 34 S: 18 to 20%o), although some individual Eptatretus sp. and R. crassa show a chemosynthetic component. Colonist invertebrates, such as the sea star (Sclerasterias cf. tanneri) and a predatory snail Buccinum canetae, have greatly depleted δ 13 C and δ 34 S values, indicating an almost 100% reliance on seep production. Nitrogen isotope ratios are depleted (-19%o) in some seep areas, and B. canetae and Synaphobranchus sp. reflect the 15 N depletion (-10 and -3%o respectively). On a species-specific basis, some mobile benthic predators from the background fauna obtain close to 100% of their nutrition from seep production, indicating that they are residents of the seeps. There is a high degree of movement in and out of the seep habitat by vagrant benthic predators, and although the majority derive most of their nutritional needs from photosynthetic production, the incorporation of chemosynthetic production is substantial.