Diversity and abundance of invertebrate epifaunal assemblages associated with gorgonians are driven by colony attributes

The present study aimed to explicitly quantify the link between the attributes of shallow-water gorgonian colonies (Octocorallia: Alcyonacea) and the ecological patterns of associated non-colonial epifaunal invertebrates. Based on multiple regression analysis, we tested the contribution of several attributes (colony height, width, and area, fractal dimension as a measure of colony complexity, lacunarity as a measure of the heterogeneity, and colonial epibiont cover) to abundance and taxonomic richness of associated assemblages. The results highlight the variation in the response of epifaunal assemblages to the gorgonian colony characteristics. The nature and intensity of the relationships were gorgonian species-dependent and varied from one taxonomic group to another. For both gorgonian species analyzed, the strongest predictor of species richness and abundance of the epifaunal assemblages was colonial epibiont cover, possibly due to a trophic effect (direct or indirect enhancement of food availability) combined with the surface available for colonization (species-area effect). Although structural complexity is usually indicated as the main driver for rich and abundant coral-associated assemblages, no significant relationship was observed between fractal dimension and the community descriptors; lacunarity, which reflects the sizes of the inter-branch spaces, was only linked to taxonomic richness in the assemblages associated with Leptogorgia lusitanica. The validity of the paradigm that structural complexity enhances biodiversity may be scale-dependent. In the case of gorgonians, the effect of complexity at the garden level may be more relevant than at the individual colony level. This reinforces the need for the conservation of gorgonian aggregation areas as a whole in order to preserve host diversity and size structure.