Chemosymbiotic species from the Gulf of Cadiz (NE Atlantic): distribution, life styles and nutritional patterns

Abstract

Abstract. Previous work in the mud volcanoes from the Gulf of Cadiz revealed a high number of chemosymbiotic species, namely bivalves and siboglinid polychaetes. In this study we give an overview of the distribution and life styles of those species in the Gulf of Cadiz, determine the role of autotrophic symbionts in the nutrition of selected species using stable isotope analyses (δ13C, δ15N and δ34S) and investigate the intra-specific variation of isotope signatures within and between study sites. Twenty siboglinid and nine bivalve chemosymbiotic species have been identified and were found living in fifteen mud volcanoes during our studies. Solemyids bivalves and tubeworms of the genus Siboglinum are the most widespread, whereas other species were found in a single mud volcano (e.g. "Bathymodiolus" mauritanicus) or restricted to deeper mud volcanoes (e.g. Polybrachia sp., Lamelisabella denticulata). Species distribution suggests that different species may adjust their position within the sediment according to their particular needs and intensity and variability of the chemical substrata supply. Isotopic values found for selected species are in accordance with values found in other studies, with thiotrophy as the dominant nutritional pathway, and with methanotrophy and mixotrophy emerging as secondary strategies. The heterogeneity in terms of nutrient sources (expressed in the high variance of nitrogen and sulphur values) and the ability to exploit different resources by the different species may explain the high diversity of chemosymbiotic species found in the Gulf of Cadiz. This study increases the knowledge of the chemosymbiotic species in the Gulf of Cadiz, highlight the relevance of seep chemoautolithotrophic production in this area and provide a starting point for future trophic ecology studies.