Dead mussels as food-stepping stone habitats for deep-sea hydrothermal fauna.

Abstract

Hydrothermal vents are patchy ephemeral habitats which are hotspots of productivity in the deep sea. Vent endemic fauna may require intermediate stepping stone habitats to travel from vent to vent with these stepping stones typically comprising of decaying organic matter. Dead Bathymodiolus and Mytilus mussels were placed in ~2200 meters water depth at approximately 4 km distance from the Rainbow Vent Field for one year in order to measure the propensity for vent fauna to use dead mussels as stepping stone habitats. The vent endemic Dirivultidae copepods, Bathymodiolus mussels and generalist Hesionidae and Capillidae polychaetes settled among the mussels. Species richness and evenness was very low among the meiofauna with generalist Tisbe copepods accounting for almost all of the copepods. Macrofaunal samples were much richer and more even in comparison. The meat from the vent endemic Bathymodiolus mussels was consumed after one year while there was still some meat and a sulphurous smell present in the shallow water Mytilus mussels, indicating that decomposition was still taking place. This study indicates that dead mussels could act as a stepping stone habitat for some symbiotic and non-symbiotic vent fauna as such animals were found in the samples. Juvenile Bathymodiolus mussels settled among the dead mussels indicating that conditions were suitable for settlement for symbiotic fauna. It is possible that the dead mussels were an effective stepping stone habitat as Bathymodiolus mussels can reach sexual maturity in a matter of weeks and months. However, the habitat did not last long enough, nor was it probably productive enough to serve as an effective long-term stepping-stone habitat.