Too cold to prosper—winter mortality prevents population increase of the introduced American slipper limpet Crepidula fornicata in northern Europe

Abstract

The distribution of the introduced American slipper limpet Crepidula fornicata (L.) on the Atlantic coast of Europe shows a clear latitudinal gradient. In the south, C. fornicata may reach abundances of several thousand individuals per meter squared accompanied by diverse ecological and economic effects. In contrast, abundances at northern waters as in Germany, Denmark and Norway barely reach 100 ind. m −2, indicating one or several limiting factors for population increase. By studying a population in the northern Wadden Sea of Germany, we examined four factors that we regarded as likely to potentially limit population increase of C. fornicata: (1) high predation by the main benthic mollusc predators, (2) high infestation by parasites, (3) low reproductive output and growth in cold waters and (4) high winter mortality during freezing winters. Our results do not indicate biotic restrictions: The main benthic mollusc predators, shore crabs ( Carcinus maenas) and sea stars ( Asterias rubens) strongly preferred the dominant blue mussel ( Mytilus edulis) in choice experiments and no infestation with parasitic trematodes were observed. Further, there was no indication of limitation in reproduction and growth since abundant larvae were observed in the water column and the period of reproduction (April to September) as well as growth rates (recruits: 9–14 mm first summer) match with data from southern areas. In contrast, population dynamics of C. fornicata was strongly affected by cold winters: During the two winters investigated, mortality amounted to 56–64% with up to 97% on single mussel beds contrasting to 11–14% yearly mortality in areas without frost in southern Europe. Negative winter effects are also suggested by remarkably low larval abundances after an exceptionally severe winter. Our results suggest that winter mortality is the main limiting factor for population increase of C. fornicata in the study area. We propose that milder winters as a corollary of global warming may allow for an increase in the abundance of northern populations combined with a northward shift of the concomitant negative ecological and economic effects.