Associations between amphipods (Crustacea: Amphipoda) and sea anemones (Anthozoa: Actiniaria)
Published and unpublished records of amphipod-sea anemone associations are reviewed. They involve at least 22 amphipod species in 7 families, and 8 families of sea anemones. The associations are of 4 main types: protection only, ectocommensals, endocommensals and micropredators. Morphological adaptations are not conspicuous, except for the specialised mouthparts of Acidostoma spp., but most obligate symbionts show inborn immunity against the toxic substances released by the host. Sex ratios are normal, sexual dimorphism small, and fecundity low compared to related free-living species. The obligate commensal associates are usually host-specific, although able to survive in alternative hosts in the laboratory, while the micropredators and the facultative associates show low host specificity. The amphipod symbionts usually do not occupy the entire geographical and ecological range of their hosts' distribution. Amphipod-sea anemone associations have evolved independently many times and do not seem to be of great evolutionary antiquity.