The Spread ofElminius ModestusDarwin In North-West Europe

Abstract

Material collected prior to 1940 indicates thatElminius modestuswas not present on British coasts at that time.Elminiusincreased in abundance in south-east England from 1946 to 1950 and extended its range as far as the Humber, where it halted.Its advance westwards along the south coast was similarly halted at Portland, but by 1948 independent colonies had been established in several of the river systems of Devon and Cornwall, in Milford Haven, and in the Bristol Channel.The first populations in the Irish Sea were in Morecambe Bay. From thereElminiusspread rapidly south and west along the north coast of Wales, and more slowly north and west towards Galloway, eventually bridging the sea to the Isle of Man.Detailed observations showed thatElminiusadvanced along the uniformly favourable north coast of Wales as a definite front moving at a rate of approximately 20–30 km per year. Around Anglesey where tidal currents were stronger it appeared simultaneously in many scattered centres.A distinction is drawn between marginal dispersal taking place under the influence of normal agencies at the boundary of an existing population, and remote dispersal due to an artificial or freak transport over a long distance. In the case ofElminiusthe maximum distance that is likely to be bridged by marginal dispersal in the absence of strong residual drifts is about 30 miles.Elminiusprobably first appeared near Southampton, and was introduced into the Thames estuary area probably by remote dispersal. Thence it spread along the east coast and was transported to Holland. Its extension into south Devon, the Bristol Channel, the Irish Sea, and to the French coast must also be attributed to remote dispersal.The main ecological effects ofElminiusresult from competition for space withBalanus balanoides. SinceElminiusbreeds in summer, its dominance has a profound effect on the composition of the summer plankton, greatly increasing the number of barnacle nauplii, presumably at the expense of other larvae.