Pending possible establishment of Littorina littorea in the Pacific: to know the remedy, know thine enemy

Abstract

The number of human-mediated marine introductions is constantly growing globally, and whenever a non-indigenous species survives the most critical filters it encounters, such as tolerance to conditions during transport and survival in the new habitat with novel species interactions (Elton 1958; Mack et al. 2000; Lenz et al. 2011; Sareyka et al. 2011), it may become permanently established and in the worst case proceed to impose clearly negative impacts on its new surroundings. One such introduction may be exemplified by the common periwinkle, Littorina littorea, which supposedly was introduced to the NW Atlantic from Europe in the 1840s as a food source or in rock ballast (Chapman et al. 2007). Now, L. littorea constitutes one of the most abundant mollusks in its invaded NE American range with extensive ecological effects such as reduced recruitment of algae and invertebrates as well as generally altered rocky shore community structure and diversity (see Harley et al. 2013 for references, and Diaz et al. 2012 for more on its effects in its native European range). During the past few decades, L. littorea has also appeared in several locations on the North American Pacific coast (see Harley et al. 2013 for references), although it does not yet seem to be self-recruiting (Chang et al. 2011). This time, the main vector is probably the export of live seafood followed by people’s diverse reasons (ethical and religious considerations or a personal desire to establish a harvestable resource) for releasing living snails (Harley et al. 2013). Why are then some introductions immediately successful, while others are not? Understanding the factors restricting the establishment of a species is arguable one of the holy grails of invasion biology, and the unique opportunity to focus on a species that seems likely to establish, but has not yet done so, is very exciting from an strictly ecological point of view (e.g. Stachowicz et al. 2002), despite all the possible downsides associated with a permanent establishment. In their original article, Harley et al. (2013) provides a comprehensive case study of the current status for L. littorea in Vancouver, where L. littorea has been found only at a couple of sites, without detectable community impact, this far. Whenever encountered, the species has been eradicated by the authors and co-workers, and to date, L. littorea has not been found again during repeated revisits over the past 1.5 years. Regarding the collected individuals, their morphology and the presence of possible hitchhikers were examined and compared to market snails. The laboratory examination indicated that the field specimens likely originated from the live seafood trade and that cooccurring organisms, living on the gastropod shells and as gut parasites, could still be introduced even if a L. littorea introduction would fail. A set of experiments, focusing on potential future success and impact of L. littorea, like comparing its grazing effects with those of a native gastropod, determining its survival under low salinity and its susceptibility to native predators, rendered significant and interesting results. L. littorea had a similar dietary preference as the native gastropod Chlorostoma funebralis suggesting possible competition. L. littorea was also capable of surviving at seasonal low salinity in the area. Most Communicated by U. Sommer.