Northern Distribution of the Seaweed Limpet Lottia insessa (Mollusca: Gastropoda) along the Pacific Coast

The marine seaweed limpet Lottia insessa (Hinds) is a habitat specialist on the kelp Egregia menziesii along the northeastern Pacific coast. The geographic range of L. insessa is commonly reported to extend from Alaska to Baja California, Mexico, but the northern distribution of this limpet has been poorly documented. We examined L. insessa specimens from museum collections and surveyed distribution of this species in the field. Results indicate that this species is common in California but is rarely found farther north. Some museum specimens collected from northern localities were misidentified individuals of other limpet species. The northern limit to continuous distribution of persistent L. insessa populations ends at Cape Arago in southern Oregon, a known biogeographic boundary for a variety of marine species. We suggest that individuals occasionally found in the Pacific Northwest and Alaska may have been vagrants that were transported to those locations during anomalous oceanographic events such as El Niño–Southern Oscillations, a phenomenon that has also been observed in other marine organisms. This study clarifies location of the northern range boundary of L. insessa and provides baseline data on its current distributional patterns, which are important for assessing effects of climate change on potential range shifts in the future.