Shelled opisthobranchs

Abstract

In his contributions to the monographic series "Manual of Conchology", Henry Pilsbry reviewed the subgroup Tectibranchiata, comprising those opisthobranch snails that (at least primitively) still possess a shell (Pilsbry, 1894-1896). Exemplified by the Cephalaspidea (bubble shells), others included in this group at Pilsbry's time and since were Anaspidea (sea hares) and the shelled members of Notaspidea (side-gilled slugs) and Sacoglossa (leaf slugs). Pilsbry (and others since his time) considered tectibranchs to be the "root stock" from which more advanced gastropods such as Nudibranchia and Pulmonata were derived. Tectibranch systematics is firmly based on conchology and most species were originally described from empty shells. However, soft-anatomical characters were acknowledged quite early on as equally important in tectibranchs, due to the reduction of their shells and their evolutionary proximity to unshelled gastropods. Today, Tectibranchiata is not recognized as a natural taxon although the word "tectibranch" (like "prosobranch" and "mesogastropod") continues in vernacular use. Shelled opisthobranchs have been redistributed among various taxa, including several new ones--the unresolved basal opisthobranchs (Architectibranchia) and the "lower Heterobranchia", an enigmatic and currently much-studied group of families considered basal to all of Euthyneura (Opisthobranchia and landsnails (Pulmonata)). Despite their polyphyletic status, shelled opisthobranchs remain important subjects in evolutionary studies of gastropods--as the most basal members of nearly every opisthobranch clade and as organisms with mosaic combinations of primitive and derived features within evolutionary "trends" (e.g., loss of the shell, detorsion, concentration of the nervous system, ecological specialization, etc.). Although they play a pivotal role, the shelled opisthobranchs have received minimal attention in more comprehensive gastropod studies, often relegated to token representatives at the derived end of prosobranchs or at the basal end of nudibranchs. The choice of this representative in a larger study is critical if its morphology and/or molecules are to adequately exemplify a larger group. This review explores the shelled opisthobranchs, including their history, current status and presumed synapomorphies, and emphasizes the importance of anatomical data to our current understanding of these "transitional" forms. A synthetic phylogenetic analysis, based on a combination of characters used in four published phylogenies involving tectibranchs, shows the current state of our knowledge and emphasizes areas for future study. The results indicate that Opisthobranchia, Cephalaspidea and Sacoglossa are monophyletic taxa, and that Acteon, the traditional basal opisthobranch, is convincingly a lower heterobranch. In most of the resulting cladograms, Anaspidea formed a monophyletic group with Cephalaspidea, as did pleurobranchoidean Notaspidea with Nudibranchia (the latter recently named as Nudipleura Wägele and Willan, 2000).