Tuesday, December 17, 2013

A New Classification of the Chromodorid Nudibranchs

With over 300 described species, the chromodorid nudibranchs are one of the most species rich families of gastropods.  Their bright colors and interesting morphology appeal to underwater photographers and scientists alike.  Their aposematic coloration has also drawn the attention of scientists interested in natural products chemistry.  In spite of their conspicuousness and appeal, there has been no comprehensive, well-supported phylogeny of the chromodorid nudibranchs.  This hinders progress being made in other biological disciplines where one may assume that species from the same genus represent a monophyletic group.  In a recent issue of PlosOne, Rebecca Johnson and Terrence Gosliner set out to remedy this situation and generate a phylogeny of the chromodorid nudibranchs and present a classification that accurately reflects the evolutionary history of the group.  

In the paper entitled “Traditional Taxonomic Groupings Mask Evolutionary History: A Molecular Phylogeny and New Classification of the Chromodorid Nudibranchs,” Johnson and Gosliner assembled the most comprehensive dataset to date including 244 specimens (142 new), representing 157 species (106 new) chromodorid species and several other taxa.  They used two mitochondrial genes (16S rRNA and cytochrome oxidase I) to reconstruct a phylogeny of the group.  The results revealed that currently recognized genera were either polyphyletic or nested within another genus rendering the other genus paraphyletic.  Extensive homoplasy of morphological attributes that have been thought to be synapomorphies for a particular genus seems to be the cause of the unnatural groupings that have been recognized in the past.  This pattern of polyphyly of recognized genera has been observed for unionid bivalves as well (Campbell et al., 2004).  

One appealing outcome of the study by Johnson and Gosliner (2012) is that they took the time to propose a new classification of the group based on their findings.  As Johnson and Gosliner (2012) indicate, “the translation of phylogenetic hypotheses into classifications is the best way to communicate results to a larger community” and “communicating these new hypotheses is one of the main contributions systematics can make to the scientific community.”  Their study now provides a sound phylogenetic framework from which morphological, chemical and behavioral attributes can be examined.       

Literature Cited
Cambpell, D. C., J. M. Serb, J. E. Buhay, K J. Roe, R. L. Minton, and C. Lydeard.  2005.  Phylogeny of North American amblemines (Bivalvia, Unonoida): prodigious polyphyly proves pervasive across genera.  Invertebrate Biology 124:131-164.
Johnson, R. F., and T. M. Gosliner.  2012.  Traditional taxonomic groupings mask evolutionary history: a molecular phylogeny and new classification of the chromodorid nudibranchs.  PlosOne 7(4):e33479.