Paradoris, BERGH, 1884

Dayrat, Benoît, 2006, A taxonomic revision of Paradoris sea slugs (Mollusca, Gastropoda, Nudibranchia, Doridina), Zoological Journal of the Linnean Society 147 (2), pp. 125-238 : 226-228

PHYLOGENETICS OF PARADORIS

A list of Paradoris species names could not be established for a long time because authors would use arbitrary, nonphylogenetic diagnoses to decide whether a species should be classified within Paradoris . For example, Valdés (2001) originally allocated araneosa to Paradoris for wrong reasons: the ‘combination of characters’ that he provided to classify araneosa in Paradoris did not include any of the synapomorphies of Paradoris . For the same wrong reasons, this author claimed that: ‘Due to the lack of these structures [accessory glands and stylet sacs], it is likely that P. leuca does not belong to the genus Paradoris ’. However, the present cladistic analysis demonstrates that the presence/absence of those accessory structures is not a criterion that should be used to decide whether a species should be classified within Paradoris .

Miller (1995) classified five valid species within Paradoris : granulata , indecora , leuca , mulciber , and tsurugensis . He also argued that Platydoris galbanus Burn, 1958 and Peltodoris fellowsi Kay & Young, 1969 ‘might be better placed within the genus Paradoris ’. However, contrary to what Valdés (2001) asserted, Miller did not formally include these two species in Paradoris . Dayrat & Gosliner (2005) demonstrated that fellowsi is not part of Paradoris . Also, the radular formula of galbanus −18 × (30-0-30) – indicates that this species does not belong to Paradoris .

Unresolved relationships within Paradoris are the result of the small number of characters that could be used at this phylogenetic level, and the fact that the character-state distributions contradict each other. For example, the presence of indecora -like tubercles on the dorsal notum (character 3) provides a phylogenetic signal that is not congruent with the presence of a dorsal spur on the hook of the outermost teeth (character 13) or the presence of stylet sacs in the distal part of the reproductive system (character 16). The fact that some characters had to be coded as polymorphic in several cases also introduced some noise in the phylogenetic analysis. However, I have performed several additional analyses without polymorphism (by choosing the general character state or removing the characters that were polymorphic in some taxa), but these analyses gave the same unresolved relationships within Paradoris .

Generally speaking, such an absence of resolution does not surprise me: the number of morphological characters available to resolve interspecific relationships is limited in discodorids ( Dayrat & Gosliner, 2005). Many characters traditionally used to resolve interspecific relationships, such as the relative size of the spermatic pouches and the length of the deferent duct, could not be used because they present a great deal of infra-specific variation. Adding more characters will be the only solution to resolving relationships within Paradoris . New morphological features may be discovered in the future, but I really doubt that there will be enough of them to give a complete answer, even if new techniques are explored, such as histology. Obviously, DNA sequencing data will have to be explored at some point.

Relationships of Paradoris with other discodorids, which I did not mean to resolve here, are also uncertain. The sister-group relationship of Geitodoris and Paradoris is exclusively supported by the presence of wide holes in the dorsal notum of the species of both clades. The exact function of these holes is largely unknown, although they seem to be openings of secretory mantle glands ( Dayrat & Gosliner, 2005). Gilianne Brodie and I intend to compare the microanatomy of those wide holes in Geitodoris and Paradoris , using microhistological sections, in order to see whether the hypothesis of primary homology, which is only based on a general, morphological similarity, is supported at a microstructural level. However, it is obvious that DNA sequencing studies will also be necessary to address the position of Paradoris within the phylogenetic hierarchy of dorids.