Fate and legacy of an invasion: extinct and extant populations of the soft-shell clam (Mya arenaria) in Grays Harbor (Washington)

1. The soft-shell clam, Mya arenaria, was intentionally introduced to Washi ngton State during the mid-1870s. Presently, extensive shell deposits cover parts of the intertidal flats of Grays Harbor, an estuarine ecosystem that has received much attention because of its role as nursery ground for Dung eness crab (Cancer magister), a commercially significant species. 2. Preliminary observations revealed that shells in the deposits belonged t o clams that were much larger than extant conspecifics. Shell deposits and extant populations have spatially disjoint distributions; few individuals a re found living within the deposits. The purpose of our study was to determ ine the cause of the different size characteristics between extinct and ext ant clams. when and how the deposits originated, and why the extant populat ion does not prosper in what appears to be prime habitat. 3. We (i) surveyed extant populations and shell deposits, (ii) followed the extant population through periodic sampling over a 3-year period, (iii) in vestigated growth and survival in extant and extinct populations, (iv) expe rimentally manipulated growth conditions, and (v) searched archival sources for information about the origin of the deposits. 4. We conclude that extinct clams grew faster and lived longer, apparently because they occupied the best habitats available. Archival records suggest that the population expanded rapidly during the period around 20 years aft er its introduction, but was decimated between 1895 and 1897 by a mass-mort ality episode that resulted in the formation of the deposits. The populatio n since then has never rebounded in its prime habitat, ill spite of potenti al settlers being regularly available. 5. Dungeness crab larvae settle preferentially in the shell deposits, where the abundance of 0 + age juveniles is orders of magnitude higher than in t he adjacent flats. We hypothesize that predation by juvenile crabs limits c lam recruitment. 6. The introduction of Mya to Grays Harbor during the 1870s has had long-la sting effects, which persist a century after their once large populations w ere decimated by a mass-mortality event. Copyright (C) 2000 John Wiley & So ns, Ltd.