Phylogeny, divergence times and species delimitation of Tonicia (Polyplacophora: Chitonidae) from the eastern Pacific Ocean

Abstract

Abstract The chiton genus Tonicia is composed of 12 species, 11 of which are found in the eastern Pacific, but the taxonomic status and geographical distribution of these species have long been controversial. In this study, we compare eastern Pacific Tonicia species using molecular systematics. The molecular markers cytochrome oxidase c subunit I (COI), 16S and 18S ribosomal RNA were used to estimate the phylogenetic relationships and divergence times among species. Species delimitations were inferred using a Bayesian generalized mixed Yule coalescent (bGMYC) and a Bayesian Poisson tree processes (bPTP) approach. We assigned southeastern Pacific specimens to nine recognized species based on their morphology, but these were resolved into only six species in our molecular phylogeny, suggesting that three of the recognized morpho species need to be synonymized. The bGMYC and bPTP analyses indicate the existence of six total coalescent groups interpreted as seven species in our gene tree results. Likewise, pairwise genetic distances, phylogenetic estimates and coalescent approaches support only seven lineages when considering specimens that match the nine currently recognized southeastern Pacific Tonicia species. We suggest that the colouration and morphological characters used to identify Tonicia species must be re-examined in depth.