The Asiatic clam
The Asiatic clam (Corbicula fluminea) arrived in North America from Asia in the 1930s. Due to a lack of natural predators and a suitable environment, it has since spread to at least 35 of the contiguous United States and into Mexico. South America and Europe also suffer from the Asiatic clam, while a similar species is endemic to Africa, so the problem is nearly worldwide.
These organisms are bivalve mollusks of the phylum Mollusca, class Pelecepoda. As is the case with the most troublesome marine macrofoulers, mobile larvae (called "veligers") are released into the water as temperatures rise in the spring and again as water temperatures drop in autumn. Veligers are tiny (approx. 0.01 inches in length) and can readily pass through most screens or racks.
Having arrived at a favorable habitat, the larvae settle and begin to mature. Most control methods are better when used against the larval or juvenile stages. Once the clams settle, they can grow at rates of up to 2.1 mm per month in their first year. Asiatic clams become sexually mature at 6-13 mm (in 6-9 months) and live in dense populations of up to 12,000 organisms per square foot. During peak spawning periods, each hermaphroditic clam can release 250-380 veligers per adult per day in the spring, and 200-320 per adult per day in autumn.
Corbicula prefers a well-oxygenated water and requires water temperatures between 2°C and 35°C (35°F to 100°F). High organic content water, or water with salinity greater than 5000 mg/1 inhibit clam survival. Short-term control strategies for Corbicula are difficult because the organisms can close and seal their shells for several weeks, preventing access of biocide to the organism.
Control technologies generally involve the use of oxidizing biocides. Chlorine applied continuously at free available chlorine levels of 0.2 to 0.5 mg/1 will eventually kill exposed Corbicula. However, concerns over accelerated corrosion damage to the system and potential environmental damage, including the formation of trihalomethanes (THMs), often render this method unacceptable. Alternative oxidizers, including activated bromide and chlorine dioxide appear to be effective with reduced environmental impact. Other control technologies include heat (above 35°C) and some newly developed nonoxidizing products.