Sea anemones in genera Adamsia, Calliactis and Paracalliactis (family Hormathiidae) engage in a mutualistic symbiosis with hermit crabs. The anemone gains substrate and food in exchange for defending the crab. Some of the sea anemones also expand the living space of the crab by producing a carcinoecium, a chitinous structure that overlies the initial gastropod shell in which the hermit crab lives. The symbiosis is initiated either by the crab or by the anemone. Although behavioral and physiological aspects of this symbiosis have been studied, interpretations cannot be generalized because no previous taxonomic or molecular-based studies have focused on this group of sea anemone or implemented an evolutionary framework.
To explore evolutionary hypotheses for the group, I reconstructed relationships among members of Hormathiidae using DNA sequences (12S, 16S, 18S, 28S and COIII). I found that the association between sea anemones and hermit crabs has evolved at least twice: Adamsia nests within Calliactis in a single clade, and Paracalliactis belongs to a different clade within the family. The carcinoecium and complex behavioral and anatomical features associated with the symbiosis are interpreted as having evolved at least twice within Hormathiidae and seem to be phylogenetically labile. Both parsimony and model-based analyses of these DNA sequences recover similar relationships between members of Hormathiidae and closely related families. None of the markers separately or in combination support a monophyletic origin for Hormathiidae due to the inclusion of Actinoscyphia plebeia (Actinoscyphiidae), Bathyphellia australis (Bathyphellidae), and Nemanthus nitidus (Nemanthidae) within Hormathiidae. Most of the characters used in the circumscription of genera within the family, such as fertility of perfect mesenteries, number of fertile and sterile mesenteries and number of mesenteries distally and proximally, are inferred to be convergent in the hormathiids included in the analysis.
To evaluate genetic diversity and phylogenetic utility of the internal transcribed spacer (ITS) region in sea anemones, five populations of the most widespread species of sea anemones symbiotic with hermit crab, C. polypus, and two closely related species of Calliactis were analyzed. I found extensive intragenomic variation, but this does not overlap with intraspecific or inter-specific variation. Thus, variation does not obscure phylogenetic signal, making ITS a potentially useful marker for analysis of closely related species of sea anemones. In addition, no population structure was found in a parsimony analysis of sequences from 36 individuals of C. polypus from five populations. Thus, ITS sequences were not variable enough for studies below the species level in C. polypus. Finally, to understand the diversity of sea anemones symbiotic with hermit crabs in family Hormathiidae, I undertook a monograph of Adamsia, Calliactis, and Paracalliactis, The monograph provides a comprehensive perspective on diversity and variation within these genera and includes important taxonomic tasks such as the validation of names and circumscription of genera. Eighteen species of sea anemones symbiotic with hermit crabs are described in detail, including two new species and one new combination.