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

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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.