A New Species, Adamsia Obvolva (Cnidaria: Anthozoa: Actiniaria), from the Gulf of Mexico, and a Discussion of the Taxonomy of Carcinoecium-forming Sea Anemones

Adamsia obvolva is a new species of sea anemone (order Actiniaria, family Hormathiidae) from the Gulf of Mexico, symbiotic with the hermit crab Parapagurus pictus (Smith, 1883). The pedal disc of the anemone enwraps the gastropod shell in which the hermit crab lives, and secretes a thin, chitinous carcinoecium that lies between the pedal disc ectoderm of the anemone and the gastropod shell. The description of A. obvolva n. sp. highlights many of the problems confounding the systematics of actiniarians symbiotic with hermit crabs. Adamsia obvolva, which has been misidentified as Paracalliactis involvens (McMurrich, 1893), presents a suite of attributes that blur the distinction between the genera Adamsia Forbes, 1840, and Calliactis Verrill, 1869. We review definitions of these genera and emend Adamsia to include only those species that form a carcinoecium and have a lobed pedal disc. To better differentiate between A. obvolva and P. involvens, we investigate the syntypes of Adamsia involvens McMurrich, 1893, which had been transferred to Paracalliactis, and of its putative synonym Urticina consors Verrill, 1882; we redescribe the species as Paracalliactis consors (Verrill, 1882). Adamsia obvolva and P. consors can be distinguished by tentacle number, the cinclides on the lower column of specimens of A. obvolva, and cnidae distribution and size. The taxonomic changes we propose are summarized in a diagnostic key to actiniarian families and genera that live attached to gastropod shells.