Molecular phylogeny of eastern Pacific sea cucumbers (Echinodermata: Holothuroidea) based on mitochondrial DNA sequence.

A molecular phylogenetic analysis of some Holothuroidea was undertaken in order to clarify the systematics and taxonomy of this class in the northeast Pacific. DNA sequence for portions of two mitochondrial genes, the large ribosomal subunit and cytochrome oxidase 1 was obtained from 16 species of sea cucumbers, including nine members of the family Cucumariidae. As reported for many mitochondrial genomes, a strong bias against G was noted in the coding strand at synonymous sites. In order to verify trees recovered in the presence of this bias, parsimony, maximum likelihood, distance, and log determinant methods were all employed. The resulting molecular phylogeny of the Cucumariidae, with a few exceptions, supported existing taxonomy which is based largely on the morphology of calcareous parts. In particular, evolutionary relationships among the brooding species were clarified. Our results confirm that C. pseudocurata Deichmann is distinct from C. curata Cowles and that these two species do not represent geographic variants of a single species. In fact C. pseudocurata appears to be conspecific with the more northerly distributed C. vegae Theel. We also clarify the identity of C. lubrica Clark and further suggest that C. fisheri astigmata (recently revised to Pseudocnus) should be considered a junior synonym of C. lubrica. The recent description of C. pallida Kirkendale and Lambert as a species, distinct from C. miniata (Brandt), is also supported. The resultant phylogenetic trees are consistent with multiple origins of direct development within the Cucumariidae. Finally, we suggest that an in-depth phylogenetic analysis of sea cucumber families will require examination of additional, more slowly evolving, regions of the genome(s).