Molecular identification of two species of the carnivorous sea slug Philine, invaders of the US west coast

Open-coast and deep-water ecosystems have been less disrupted by invasion than estuaries, but a notable exception is the establishment of multiple species of the sea slug genus Philine in the northeastern Pacific. These large slugs spread from San Francisco Bay to the whole of the US west coast in 5 years during the 1990s, and are abundant from intertidal mudflats to soft-sediment bottoms >300 m deep along the open coast. Voracious predators that secrete acid, Philine spp. have few natural enemies and substantial impacts on native bivalve communities. Despite their ecological significance, the identity and number of invasive Philine spp. in the US has remained controversial. Here, we adopt a molecular approach to identify the species commonly found along the US west coast. We compared mitochondrial 16S gene sequences of reference specimens from the native range of seven possible invaders against sequences of 66 specimens collected from southern California to Oregon from intertidal and subtidal habitats. All slugs from southern California and Oregon were confirmed as P. auriformis , and most shared haplotypes with samples from New Zealand. Larger slugs from San Francisco, Tomales Bay and Bodega Harbor, believed to represent 3–4 species, were all identified as P. orientalis . Molecular data support a recent morphological analysis that erected two proposed species, P. paucipapillata from Hong Kong and P. quadripartita from Europe, which are genetically distinct from anatomically similar species. For taxonomically challenging groups like sea slugs, genetic data can illuminate cryptic invasions and provide a backdrop for further studies of the ecological impacts of introduced marine species.