A new species, *Adamsia obvolva* (Cnidaria: Anthozoa: Actiniaria), from the Gulf of Mexico, and a discussion of the taxonomy of carcinoecium-forming sea anemones

Abstract

*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.