Site-specific and ontogenetic variations in nutrition of mussels (Bathymodiolus sp.) from the Lucky Strike hydrothermal vent field, Mid-Atlantic Ridge

Lucky Strike mussels (Bathymodiolus sp.) support two metabolically distinct (methanotrophic and thiotrophic) prokaryotic endosymbionts in their gills. Differences in source inorganic carbon isotope ratios and in carbon fixation pathways between these two symbionts typically result in organic carbon with distinctive delta(13)C values. Site-specific differences in isotopic compositions of host mussels may therefore reflect differences in sulfide and methane availability. Large differences in mean carbon and nitrogen isotopic compositions were observed in adult mussels collected from two chemically distinct vents at Lucky Strike (Sintra: delta(13)C = -21.3 parts per thousand, delta(15)N = -4.5 parts per thousand; Eiffel Tower: delta(13)C = -30.7 parts per thousand, delta(15)N = -10.5 parts per thousand). These values are consistent with the hypothesis that Sintra mussels are more dependent on methanotrophy than are Eiffel Tower mussels. Relative abundances of the two types of endosymbionts in mussel gill tissues (Sintra: mean = 15% methanotrophs; Eiffel Tower: mean = 6% methanotrophs) provide further support of this hypothesis, suggesting that Lucky Strike mussels express a nutritional response to environmental variations. Within sites, there were small but significant correlations between isotopic composition and mussel size over the shell lengths of 15-80 mm. but these shifts are so small that they are attributable to factors other than ontogenetic shifts in nutritional strategy. In contrast, large nitrogen isotope differences were observed between larval and adult stages. Based on delta(15)N values, mussel larvae appear to rely very little on photosynthetically derived organic material. Observations of the demersal nature of mussel larvae and isotopic similarities between Sintra and Eiffel Tower larvae and Sintra adults suggest that the potential of stable isotopes as useful tracers of larval sources within the Lucky Strike vent field should be considered.