Separating the effects of a habitat modifier, Modiolus modiolus and substrate properties on the associated megafauna

The effects of the habitat modification by the horse mussel Modiolus modiolus on the abundance and diversity of benthic fauna are poorly known. In this study we examined the effects of the abundance of M. modiolus, substrate properties, stone cover and spatial location on the abundance and species richness of the associated megafauna in Faxaflَi Bay, Iceland. Quantitative data on the megafaunal abundances and species richness was obtained by analysis of photographs (n = 855) collected with an underwater camera system in an area characterised by dense beds of M. modiolus. The abundance of M. modiolus and substrate coarseness acted both synergistically towards increasing the abundance of the dominant epifaunal taxa (Henricia sp., Actiniaria and Alcyonium digitatum), although the effect of M. modiolus was in all cases greater compared to that of substrate type. The distribution of Arctica islandica was predominately influenced by the substrate type. Species richness was strongly positively correlated with abundance of M. modiolus and stone coverage, whereas substrate type was of less importance. M. modiolus effects on the abundance of the dominant epifaunal taxa and the species richness were largely non-linear and became rapidly asymptotic with their increasing abundance. Habitat modification by M. modiolus had profound effects on the megafaunal epibenthic community structure, even when found at relatively low abundance. We conclude that M. modiolus can have a major influence on ecosystem functioning in coastal waters.