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The Asian clam Corbicula fluminea is a freshwater bivalve that is native to south-east Asia, but is now well-established in Europe, Australia, Africa, and North and South America. It was probably introduced to South America in the late 1960s or early 1970s in the ballast water of ocean-going ships that visited estuarine ports on the Atlantic coast. It then used boats plying the region’s rivers to spread inland, and now occurs from Argentina’s Patagonia region in the south to Venezuela in the north, including the lower reaches of Brazil’s Amazon Basin.
In its native range, the Asian clam is collected and cultivated for human consumption and for feeding to domestic fowl. Indeed, its introduction into North America in the latter half of the 1800s is attributed to Chinese immigrants, reluctant to forsake a favourite food. Away from its natural enemies the population was able to grow and spread rapidly, largely due to its exceptional reproductive capacity. The species is hermaphrodite – having both male and female sex organs – and is capable of selffertilisation, which means that a single juvenile can initiate the formation of a new colony. Colonies made up of 10 000 to 20 000 clams per m2 are not uncommon, and densities as high as 131 000 per m2 have been recorded in California. Clearly, there is a high potential for the Asian clam to compete with indigenous bivalves for space and food.
Of more concern from an economic viewpoint is the biofouling problem posed by this invader. In many areas where it has become established, the Asian clam clogs water intake pipes and obstructs irrigation and drainage canals, sometimes with disastrous consequences. Mitigation measures are expensive, time-consuming and often environmentally undesirable. For example, thermal regulation – using heated water to kill the clams – is often not practical, while chemical control with chlorine or bromine treatment may kill many other organisms in the vicinity. Screens and traps can be used in water systems to inhibit the entry of mature clams and the accumulation of shells, but the most effective way of avoiding biofouling problems is to prevent the introduction and spread of the species into new areas. Preventative actions include implementing ballast and bilge water control measures and limiting the clam’s transport as a food source or as live bait.
In 1980, Asian clams clogged the water system of a nuclear power plant in Arkansas in the USA, jeopardizing its fire protection plans. The United States Nuclear Regulatory Commission shut down the plant and ordered an inspection of every nuclear plant in the country, to determine the threat to safety that fouling by this invasive alien species posed. Clams were found at 19 of 32 plants, with another 11 at risk because of the proximity of clams to the plants. Compliance with this directive cost an estimated $4.5 million. During the l980s, losses caused by utility and industry down-time, corrective actions and maintenance costs were estimated to be $1 billion annually.
Reference: Matthews S. & Brandt K. South America Invaded: the growing danger of invasive alien species. Global Invasive Species Programme 2006