Taxonomic status of the three color variants in sea cucumber (Apostichopus japonicus): evidence from mitochondrial phylogenomic analyses

Color variation in sea cucumber is one of the most crucial traits affecting price and taste in East Asian countries. However, the relationship and taxonomic status of the three color variants are still unclear. We used 14 samples that covered all three color variants and their geographic distributions, to construct the first phylogeny for the color variants based on the complete mitochondrial genome sequence and a number of tree-building methods (maximum parsimony (MP), maximum likelihood (ML), and Bayesian inference (BI)). The divergence times within color variants were estimated by the Bayesian molecular clock approach using the BEAST program. Our results showed that the color variants were not monophyletic in the well-resolved phylogenetic tree, which strongly refuted their separate species status. The molecular dating estimate revealed that the sea cucumber was a young group, which originated in the early Miocene period (22.03 mya) and rapidly diverged after the late Miocene period. It is interesting that individuals within each variant or geographic distribution were not always closely related and thus did not share a common origin. We propose that although they differ in body color, the three color morphs all belong to a single species of Apostichopus japonicus and the historical marine climate and the hydrographic complexity of the ocean currents could be responsible for their present distribution patterns.