Shelled Opisthobranchs

Opisthobranch gastropods comprise a large and diverse group of marine snails and slugs, including some of the most beautiful and most specialized forms. The most recognizeable to non-specialists are perhaps the sea slugs, or nudibranchs, and the pelagic sea butterflies, or pteropods. Morphologically intermediate between the more traditional snails and the shell-less nudibranchs are the "bubble-snails," belonging to the Order Cephalaspidea. Challenges to studying the relationships among these snails are: (1) species descriptions based almost exclusively on shells, which are themselves reduced (i.e., relatively featureless, paper-thin) compared to those of other gastropods; (2) high phylogenetic importance of characters from soft-body morphology, necessitating live observation, gross dissection, and histology; (3) reduction and loss of many morphological features, such as the shell, operculum, radular teeth, jaws, and stomach chamber; and (4) high incidence of parallelism and/or convergence in characters, especially those related to the burrowing habit. Phylogenetic analyses from these studies have confirmed the "rampant parallelism" suspected in cephalaspid morphology, but have nevertheless identified useful characters to suggest new classifications and re-defined clades.