Establishment of an extensive breeding population of a marine pulmonate snail far poleward of its previously documented range

The California salt marsh snail Melampus olivaceus , a species from southern California, has established a thriving population in the Elkhorn Slough estuary (Monterey Bay), hundreds of kilometers north of where it is long-established and regularly found. Since the late 1800s this high littoral zone salt marsh snail has occasionally been reported in central California, but only as isolated individuals or shells. We first observed a few individuals of M. olivaceus in Elkhorn Slough in 2013, and the species is now abundant and reproducing throughout the estuary in Salicornia pacifica (pickleweed) marshes. We also document for the first time that this species has a planktonic, feeding larva. Salt marsh snails may have been transported northward as planktonic larvae during periodic El Niño warm water events. Such transport has been reported for other species, but what makes this case unusual is the persistence, reproduction and abundance of the snail following transport.