The return of the clown: pseudocryptic speciation in the North Pacific clown nudibranch, Triopha catalinae (Cooper, 1863) sensu lato identified by integrative taxonomic approaches

The North Pacific nudibranch species Triopha catalinae (Cooper, 1863 ), also known as the clown nudibranch, includes two distinct morphotypes: the trans-Pacific morphotype, known from South Korea to Southern California, and the eastern Pacific-only morphotype from Southeast Alaska to Baja California. We tested the hypothesis that Triopha catalinae is a species complex by applying an integrative taxonomic approach that included (1) phylogenetic analyses of the mitochondrial 16S and COI, and the nuclear Histone H3 genes; (2) haplotype network analysis based on COI sequences; (3) Automatic Barcode Gap Discovery (ABGD) species delimitation analysis based on 16S and COI sequences; and (4) comparative internal and external morphological studies. Specimens of two morphotypes were found to be genetically distinct; they clustered into two well-supported clades in the phylogenetic analyses and two groups in the TCS haplotype network. Moreover, these two groups displayed morphological differences in the dorsal tubercles and radula structure: the trans-Pacific morphotype specimens possess relatively small and dendritic dorsal tubercles, two rows of arborescent tubercles on the dorso-lateral appendages of larger individuals, and radular formula 19–24 × (9–12.5–10.1.5–10.9–12). Specimens of the eastern Pacific-only morphotype have relatively large, conical or rounded dorsal tubercles, and radular formula 48–79 × (16–18.17–33.1.17–33.16–18). Finally, the ABGD analysis confirmed that these two morphotypes constitute different species. A review of the literature (including original descriptions) and available type material revealed that Triopha modesta Bergh, 1880 and T. catalinae are the valid names for the trans-Pacific and eastern Pacific-only morphotype, respectively.