ORIGINAL PAPER The return of the clown: pseudocryptic speciation in the North Pacific clown nudibranch, Triopha catalinae (Cooper, 1863) sensu lato identified by integrative taxonomic approaches Dae-Wui Jung 1,2,3 & Terrence M. Gosliner 4 & Tae-Jun Choi 3 & Hyun-Jong Kil 5 & Anton Chichvarkhin 6,7 & Jeffrey H. R. Goddard 8 & Ángel Valdés 1 Received: 14 May 2020 /Revised: 30 July 2020 /Accepted: 18 August 2020 # Senckenberg Gesellschaft für Naturforschung 2020 Abstract 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. Keywords Systematics . Molecular taxonomy . COI . 16S . Histone H3 . Anatomy Communicated by V. Urgorri Electronic supplementary material The online version of this article (https://doi.org/10.1007/s12526-020-01107-2) contains supplementary material, which is available to authorized users. * Ángel Valdés aavaldes@cpp.edu 1 Department of Biological Sciences, California State Polytechnic University Pomona, 3801 West Temple Avenue, Pomona, CA 91768, USA 2 Korea Marine-Bio Lab, 308 Gajeong-Ro, Yuseong-Gu, Daejeon 34130, Republic of Korea 3 Department of Biotechnology, Sangmyung University, 20 Hongjimun 2-gil, Jongno-gu, Seoul 03016, Republic of Korea 4 Department of Invertebrate Zoology, California Academy of Sciences, 55 Music Concourse Drive, San Francisco, CA 94118, USA 5 National Institute of Biological Resources, 42 Hwangyeong-ro, Seo-gu, Incheon 22689, Republic of Korea 6 Far Eastern Federal University, 8 Ulitsa Sukhanova, Vladivostok 690091, Russia 7 A.V. Zhirmunsky Institute of Marine Biology, Russian Academy of Sciences, 17 Palchevskogo, Vladivostok 690041, Russia 8 Marine Science Institute, University of California, Santa Barbara, CA 93106, USA https://doi.org/10.1007/s12526-020-01107-2 /Published online: 18 September 2020 Marine Biodiversity (2020) 50: 84