Placobdellids

Placobdella group

Cluster of Placobdella parasitica on a snapping turtle

Mark E. Siddall *



___.___.___ Desserobdella phalera
   |   |___.__ Desserobdella phalera
   |       |___ Placobdella transluscens
   |___.___.___ Placobdella parasitica
       |   |___.___ Placobdella papillifera
       |       |___ Placobdella ornata
       |___.___ Placobdella pediculata
           |___.___ Placobdella montifera
               |___.___ Desserobdella picta
                   |___ Oligobdella biannulata

Based on: CO-I, ND-1, and morphology in Light and Siddall, 1998

Discussion of Relationships

The placobdellid group consists of the genera Placobdella, Desserobdella, and Oligobdella species. All members of this clade, except Oligobdella biannulata , have two pairs of coalesced eyes wherein the posterior pair is larger than the anterior pair . The genus Desserobdella is problematic within the placobdellid group. The Desserobdella congener, D. phalera , failed to group with the type species, D. picta, rendering the genus Desserobdella polyphyletic. The genus Desserobdella was established by Barta and Sawyer (1990) with their redescription of Clepsine picta as "an attempt to more accurately reflect relationships of species which have, until now, been placed in the paraphyletic genus Placobdella ." The defining characteristics of the genus Desserobdella differ only in the presence of diffuse salivary glands instead of the compact tissue of placobdellid species (Barta and Sawyer, 1990). Jones and Woo (1990) placed Desserobdella phalera in the newly established Desserobdella genus in light of its also having diffuse salivary tissue. Insofar as this trait proves to be plesiomorphic for all glossiphoniids, there are no other morphological synapomorphies that would support a recent common ancestry for members of the genus Desserobdella. Grouping the two Desserobdella species together relies only on symplesiomorphies. Furthermore, not all Placobdella species have compact salivary glands. Placobdella parasitica, P. papillifera, and P. ornata are the only Placobdella species included here with compact salivary glands. They do form a monophyletic group. Interestingly, each of these species feeds on non-avian reptiles.

Klemm's 1982 description of Desserobdella ( Batracobdella) phalera was non-descript (Appendix I). The two representatives of D. phalera used in this study do not wholly resemble each other . The representative from Tennessee possessed the characteristic small proportions, translucent body, and unpigmented neck ring typical of D. phalera. In contrast, the representative from Virginia, although not readily attributable to a species other than D. phalera, was more robust, opaque, and lacked the characteristic neck ring. Furthermore, according to Klemm's (1982) character izations, P. translucens is indistinguishable from D. phalera. Placobdella translucens and D. phalera vary only in degree of pigmentation and papillation. Phylogenetic data further supports a close relationship between the typical Tennessee D. phalera and P. translucens .

Barta and Sawyer (1990) stated that: "Although the definition of the genus Placobdella could have been broadened to encompass the characteristics of Desserobdella (Clepsine) picta, redefinition of the genus would have continued an unfortunate tradition of using the genus Placobdella as a repository for poorly known species (Soós, 1969)." Our data suggest that in order for the genus Placobdella to be monophyletic, one of two options can be pursued. The first option in reaching a monophyletic status would be to expand the genus Placobdella to include all taxa with the characteristic placobdellid eye morphology. The second option would be to narrow the scope of the genus Placobdella considerably, as previously suggested by Soós (1969) and Barta and Sawyer (1990) but with the attendant establishment of several monotypic genera. Given these two options, we believe it would be best to expand the genus Placobdella to include all taxa with the characteristic eye morphology which is corroborated here as uniquely derived for the clade.

Placobdella group

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