Reversing a reported case of transoceanic dispersal: Nudibranch identifications among tsunami debris

In the aftermath of the 2011 east Japanese earthquake and tsunami, anthropogenic debris from the east coast of Japan floated across the Pacific Ocean to the west coast of North America. One such vessel from Iwate Prefecture arrived on the coast of Oregon, and the fouling community included specimens identified as the nudibranch Hermissenda crassicornis, which was previously thought to range from Japan to Baja California but has since been split into three species: H. crassicornis (northeastern Pacific), H. opalescens (southeastern Pacific), and H. emurai (western Pacific). Also aboard were nudibranchs of the genera Dendronotus and Eubranchus. Previous work suggested that all of the motile invertebrates found in the tsunami debris fouling community were either pelagic or Japanese in origin. Our study sought to determine whether the Hermissenda nudibranch specimens collected from the Iwate vessel were, according to the most updated classification system, only H. emurai as would be the case if the nudibranchs were Japanese in origin. In addition, we also sought to identify the Dendronotus and Eubranchus aboard. Results from DNA sequencing and limited morphological analysis indicate that specimens of H. crassicornis, as it is currently recognized, and H. opalescens were found on the vessel. Morphological or genetic data resolved the other nudibranchs as the Eastern Pacific Dendronotus venustus and Eubranchus rustyus. These findings indicate that these species settled after arrival to the west coast of North America. Data shared on GBIF and the iNaturalist platform were also used to map where eastern Pacific Hermissenda are currently understood to occur.