Molluscan invasions in marine and estuarine communities

The distributions of many species of marine and estuarine mollusks have bee n altered dramatically by human movements over the past 2,000 and more year s. Vectors have included vessels, mariculture, the aquarium trade, intentio nal or accidental releases into the wild, and canals. Most marine mollusk d istributions are held to be "natural" prior to the 19th century, whereas mo llusk distributions during or since the 19th century are held to be potenti ally subject to human modification. However, that pre-19th century invasion s occurred is clear, suggesting that the antiquity of human-mediated mollus k introductions has been extensively underestimated. The Asian oyster Crass ostrea gigas was introduced to Europe by the 1500s, the Northern Hemisphere mussel Mytilus may have arrived in the Southern Hemisphere by the early 15 00s, and shipworms have similarly been widespread by shipping. A subset of 38 Northern Hemisphere introduced mollusks reveals distinct geographic patt erns: 63% originate in the North Atlantic Ocean/Mediterranean area, while 3 7% originate in the North Pacific Ocean. Within the Atlantic Ocean, the wes tern Atlantic is a significantly stronger donor area, accounting for 75% of those North Atlantic taxa that have dispersed globally. Similarly, the wes tern Pacific Ocean is also a strong donor region, exporting 93% of all thos e originating in the Pacific. Ecologically, in San Francisco Bay, Californi a, the introduced infaunal or near-surface bivalves Mya, Gamma, Venerupis, Musculista and Potamocorbula may be sufficiently abundant as to control wat er column productivity. The European snail Littorina littorea (Linnaeus, 17 58) has had vast and complex impacts on intertidal hard and soft bottom com munities from Canada to the mid-Atlantic America. In general, far more atte ntion must be paid to experimentally demonstrating the impacts of invasive species.