The Model Organism Hermissenda crassicornis (Gastropoda: Heterobranchia) Is a Species Complex

Abstract

Hermissenda crassicornis is a model organism used in various fields of research including neurology, ecology, pharmacology, and toxicology. In order to investigate the systematics of this species and the presence of cryptic species in H. crassicornis, we conducted a comprehensive molecular and morphological analysis of this species covering its entire range across the North Pacific Ocean. We determined that H. crassicornis constitutes a species complex of three distinct species. The name Hermissensa crassicornis is retained for the northeast Pacific species, occurring from Alaska to Northern California. The name H. opalescens is reinstated for a species occurring from the Sea of Cortez to Northern California. Finally, the name H. emurai is maintained for the northwestern species, found in Japan and in the Russian Far East. These three species have consistent morphological and color pattern differences that can be used for identification in the field.

Range extension for the region of sympatry between the nudibranchsHermissenda opalescensandHermissenda crassicornisin the northeastern Pacific

“…However, unverified photographic records from the world wide web suggest H. opalescens may be present farther north (inaturalist.org/taxa/494603-Hermissenda-opalescens). The photos show specimens lacking white stripes on their cerata, matching the only morphological character distinguishing H. opalescens (stripe absent) from H. crassicornis (stripe present), as described by Lindsay and Valdés (2016). The northwestern Pacific species, H. emurai, also lacks the white stripe, but has a different body shape, colouration, and different spacing between groups of cerata, eliminating it from consideration.…”

“…The geographic distribution and structure of the haplotypes was visualized by producing a haplotype network using the program PopArt v. 1.7 (Leigh et al 2015) using the TCS option. Sequences were compared with data from both species reported by Lindsay and Valdés (2016) and obtained from GenBank (Table S2).…”

“…The evidence presented by Lindsay and Valdés (2016) indicated that both H. crassicornis and H. opalescens are present along the Pacific coast of North America. H. opalescens is a more southern species, found between Baja, California, Mexico, and Bodega Bay, California, USA, whereas H. crassicornis ranges from Alaska to central California.…”

“…Cooper, 1863;andH. emurai Baba, 1937 (Lindsay andValdés 2016). The three species were distinguished by genetic analyses, morphological characters, and geographic ranges in the North Pacific (Table 1).…”

“…Furthermore, our initial observations in the summer of 2016 suggested that H. crassicornis (with a stripe) and H. opalescens (without a stripe) were both common on the west coast of Vancouver Island, British Columbia, Canada. Thus, our goals for this study were (i) to survey these locations on Vancouver Island to determine which species were present, and (ii) to confirm whether any morphological differences between species were consistent with those noted by Lindsay and Valdés (2016).…”