Introduction, dispersal and naturalisation of the Manila clam Ruditapes philippinarum in British estuaries, 1980-2010

The introduction of the Manila clam into British coastal waters in the 1980s was contested by conservation agencies. While recognising the value of the clam for aquaculture, the government decided that it posed no invasive risk, as British sea temperatures would prevent naturalisation. This proved incorrect. Here we establish the pattern of introduction and spread of the species over the first thirty years of its presence in Britain. We report archival research on the sequence of licensed introductions and examine their relationship in time and space to the appearance of wild populations as revealed in the literature and by field surveys. By 2010 the species had naturalised in at least eleven estuaries in southern England. These included estuaries with no history of licensed introduction. In these cases activities such as storage of catch before market or deliberate unlicensed introduction represent the probable mechanisms of dispersal. In any event naturalisation is not an inevitable consequence of introduction and the chances of establishment over the period in question were finely balanced. Consequently in Britain the species is not currently aggressively invasive and appears not to present significant risk to indigenous diversity or ecosystem function. However it is likely to gradually continue its spread should sea surface temperatures rise as predicted.