CYLINDROBULLA AND ASCOBULLA IN THE WESTERN ATLANTIC (GASTROPODA, OPISTHOBRANCHIA, SACOGLOSSA) - SYSTEMATIC REVIEW, DESCRIPTION OF A NEW SPECIES, AND PHYLOGENETIC REANALYSIS

Two often-confused western Atlantic shelled opisthobranchs - Cylindrob ulla beauii P. Fischer, 1857, and Ascobulla ulla (Marcus & Marcus, 197 0) - are redescribed and compared, via a full literature review; scann ing 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 pro vides the first larval development data for the genus. The overlapping classificatory histories of the genera Cylindrobulla and Ascobulla ar e summarized and discussed with reference to the Cephalaspidea, Sacogl ossa (= Ascoglossa), Anaspidea, and Diaphanoidea. Two of the most-rece nt studies (Jensen 1996a, Jensen 1996b; Mikkelsen 1996) have resulted in conflicting conclusions about the relationships and classification of Cylindrobulla. In the author's earlier phylogenetic analysis (Mikke lsen 1996), Cylindrobulla formed a monophyletic group with the shelled Sacoglossa with synapomorphies in digestive, reproductive, pallial, a nd nervous system characters. Jensen's two-part study (Jensen 1996a, J ensen 1996b) resulted in removal of Cylindrobulla to a new opisthobran ch order, Cylindrobullacea. These two divergent cladistically produced classifications were tested by critically re-evaluating Jensen's data set, especially with regard to Cylindrobulla, and re-running the analy sis. Thirteen of Jensen's 52 characters were recoded for Cylindrobulla according to new morphological data and the new species described her e. Other coding changes were required to update anatomical data for ot her taxa, add supplementary characters and taxa, more clearly define t he outgroup, and to critically re-evaluate the cladistic criteria behi nd several characters. The resultant reanalysis produced a shorter tre e using Fewer a priori assumptions, but nevertheless preserved the bas ic tree topology, including three monophyletic sacoglossan clades, pre sented by Jensen (1996b). Cylindrobulla, however, rejoined the shelled sacoglossans (Oxynoacea), supported by two synapomorphies. This analy sis therefore reconfirmed Cylindrobulla's proper placement in the Saco glossa, rather than in the traditional Cephalaspidea or segregated in its own monogeneric opisthobranch order. (C) 1998 The Norwegian Academ y of Science and Letters.