Amphipods and sea anemones, an update

We present an updated survey of the Amphipoda that live in association with sea anemones. These amphipods can be divided into four groups: 1) symbiotic amphipods using sea anemones mainly for protection, but feeding largely independently; 2) amphipods feeding on sea anemones, but not permanently associated; 3) symbiotic amphipods living permanently among the tentacles of the sea anemones; and 4) symbiotic amphipods living permanently in the gastrovascular cavity of the sea anemones. Contrary to previous speculations, it appears that the amphipods in groups 3 and 4 mainly feed on host tissue, and the anemone-eating amphipods can therefore generally be classified as micropredators (group 2), ectoparasites (group 3), and almost endoparasites (especially those species in group 4 that spend their entire life cycle inside their hosts). Although the associates in the latter two groups show various minor morphological, reproductive, and physiological adaptations to the symbiosis, these associations evolved many times independently. We provide new information on feeding ecology and a discussion of the evolution of these associations.