ALBERT

Description: Hydrothermal vent sites along the Juan de Fuca Ridge in the north-east Pacific host dense populations of Ridgeia piscesae tubeworms that promote habitat heterogeneity and local diversity. A detailed description of the biodiversity and community structure is needed to help understand the ecological processes that underlie the distribution and dynamics of deep-sea vent communities. Here, we assessed the composition, abundance, diversity and trophic structure of six tubeworm assemblages, corresponding to different successional stages, collected on the Grotto hydrothermal edifice (Main Endeavour, Juan de Fuca Ridge) at 2196 m depth. Including R. piscesae, a total of 36 macrofaunal taxa were identified to the species level. Although polychaetes made up the most diverse taxon, faunal densities were dominated by gastropods. Most tubeworm aggregations were numerically dominated by the polychaete Amphisamytha carldarei and gastropods Lepetodrilus fucensis and Depressigyra globulus. The highest diversities were found in mature tubeworm aggregations, characterized by fairly long tubes. The high biomass of grazers and the high resource partitioning at small scale illustrates the importance of the diversity of free-living microbial communities in the maintenance of the food web. Although symbiont-bearing invertebrates R. piscesae represented a large part of the total biomass, the absence of specialized predators on this potential food source suggests that its primary role lies in community structuring. Vent food webs did not appear to be organized through predator-prey relationships. For example, although trophic structure complexity increased with ecological successional stages, showing a higher number of predators in the last stages, the food web structure itself did not change across assemblages. We suggest that environmental gradients provided by the biogenic structure of tubeworm bushes generate a multitude of ecological niches and contribute to the partitioning of nutritional resources, releasing communities from competition pressure for resources, thus allowing species co-existence.