CylindrobullaandAscobullain the western Atlantic (Gastropoda, Opisthobranchia, Sacoglossa): Systematic review, description of a new species, and phylogenetic reanalysis

Abstract

Two often‐confused western Atlantic shelled opisthobranchs ‐Cylindrobulla beauiiP. Fischer, 1857, andAscobulla ulla(Marcus & Marcus, 1970) ‐ are redescribed and compared, via a full literature review, scanning electron microscopy of shells and radulae, and serial histological sectioning of soft anatomy. A third sympatric species, C.gigas, from Florida, the Caribbean and Bermuda, is described as new, and also provides the first larval development data for the genus. The overlapping classificatory histories of the generaCylindrobullaandAscobullaare summarized and discussed with reference to the Cephalaspidea, Sacoglossa (= Ascoglossa), Anaspidea, and Diaphanoidea. Two of the most‐recent studies (Jensen 1996u, Jensen 1996h; Mikkelsen 1996) have resulted in conflicting conclusions about the relationships and classification ofCylindrobulla.In the author's earlier phylogenetic analysis (Mikkelsen 1996),Cylindrobullaformed a monophyletic group with the shelled Sacoglossa with synapomorphies in digestive, reproductive, pallial, and nervous system characters. Jensen's two‐part study (Jensen 1996a, Jensen 1996b) resulted in removal ofCylindrobullato a new opisthobranch order, Cylindrobullacea. These two divergent cladistically produced classifications were tested by critically re‐evaluating Jensen's dataset, especially with regard toCylindrobulla, and re‐running the analysis. Thirteen of Jensen's 52 characters were recoded forCylindrobullaaccording to new morphological data and the new species described here. Other coding changes were required to update anatomical data for other taxa, add supplementary characters and taxa, more clearly define the outgroup, and to critically re‐evaluate the cladistic criteria behind several characters. The resultant reanalysis produced a shorter tree using fewer a priori assumptions, but nevertheless preserved the basic tree topology, including three monophyletic sacoglossan clades, presented by Jensen (19966).Cylindrobulla, however, rejoined the shelled sacoglossans (Oxynoacea), supported by two synapomorphies. This analysis therefore reconfirmedCylindrobulla's proper placement in the Sacoglossa, rather than in the traditional Cephalaspidea or segregated in its own monogeneric opisthobranch order. © 1998 The Norwegian Academy of Science and Letters