Balcom, N. C.,  1994.  "Aquatic Immigrants of the Northeast, No. 4: Asian Clam, Corbicula fluminea", Connecticut Sea Grant College Program

"Aquatic Immigrants of the Northeast, No. 4: Asian Clam, Corbicula fluminea"

The Asian clam (Corbicula fluminea) is a small freshwater bivalve mollusk. It has two thick, hinged shells, characterized by a series of distinctive concentric ridges. Adults rarely grow larger than 40 mm (1.5"), and are commonly about the size of a nickel. Shell coloration can be yellowish-gold, or light brown. One incurrent, tubelike siphon draws water into the body, bringing food and dissolved oxygen, and a second excurrent siphon expels the water along with wastes. It moves by means of a strong, muscular foot.

The Asian clam originated from Southeast Asia. First introduced on the west coast of North America around 1924, it was discovered in Washington state in 1937. By the 1970s, the clam occupied most of the Mississippi Basin, the Gulf Coast and eastern United States. It was discovered in Delaware in 1972, in New Jersey ten years later, and on Long Island in New York in 1984. Although New England was considered environmentally inhospitable, Asian clams were discovered in the lower Connecticut River, near Haddam Neck, Connecticut in 1990.

This mollusk prefers running water, and sand or gravel substrates. It feeds on plankton, requires high levels of dissolved oxygen, and is intolerant of pollution. Water temperatures greater than 30 C (86 F) impair the organism’s metabolic and reproductive functions, while water temperatures less than 2� F) can kill the clam. Although it is considered a freshwater mollusk, the Asian clam’s physiology enables it to withstand slightly brackish water (5-8‰).

The Asian clam is hermaphroditic – meaning both sexes are found in the same animal – and is capable of self-fertilization. Larvae brooded in the parent’s gills, are released through the excurrent siphon into the water column as active post-larval juveniles, with the ability to resist downstream transport by currents. A single, prolific clam can release hundreds or even thousands of juveniles per day, up to 70,000 per year. Spawning can occur almost continuously at water temperatures exceeding l6�� F). Asian clams can reach densities of 10,000 to 20,000 per square meter, potentially releasing several million juveniles daily into the same area of the water column.

In warmer waters, spawning can continue year round, however in the Connecticut River, spawning appears to occur from July through September. Although spawning activity ceases when the water temperature drops too low, eggs and sperm remain in the reproductive tissues, signaling that the mollusks are ready to spawn as soon as the environment becomes favorable.

Once released, the juveniles are weak-swimming usually found near the bottom of the water column. This is one of the reasons why they readily cause the problems they do, because intake pipes are generally placed near the bottom of the water column also.

Humans are responsible for the dispersal of this clam. There have been inadvertent releases by researchers. Anglers use the clams as live bait, and whole clams sometimes survive. Aquaria releases into lakes or rivers also contribute to the spread of these clams. In the aquarium trade, Asian clams are known as "pygmy" or "gold" clams.

Although at least a decade elapsed between the establishment of the Asian clam in the United States and the appearance of its harmful effects, today, this clam is a serious biofouler of raw water intake pipes, affecting power and water suppliers and other industries. Asian clams are drawn into intake pipes, and the live animals, empty shells and body tissues obstruct water flow through condenser tubes, valves and service water systems. Buoyant dead clams can clog intake screens.

Nuclear service water (fire protection) systems are most vulnerable. In 1980, Asian clams clogged the service water system of a nuclear power plant in Arkansas, jeopardizing all 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 to what extent fouling by this mollusk threatened their safety. 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.

In 1990, Asian clams were discovered fouling the Connecticut Yankee Atomic Power Company facility in Haddam Neck, Connecticut. They are currently being controlled with continuous low-level chlorination, particularly for the service water system. If the clams continue to spread northward in the Connecticut River, they will create costly fouling problems for other utilities and industries.

There are several ways the Asian clam can be controlled to minimize its impact on a power plant, water supplier or industry. Thermal regulation, heating the water in the pipes to temperatures exceeding 37� F), is one method; however, this is not possible in most existing water systems.

Mechanical regulation, using screens and traps to prevent mature clams from entering the water system and to remove clam bodies and shells from the system, is effective for dealing with older clams. Chemical regulation, using small concentrations of chlorine or bromine to kill the juveniles, and in some cases, adults, is also very effective. However, state and federal environmental regulatory agencies are becoming increasingly restrictive as to the amount of chemicals such as chlorine that can be discharged from a facility. The current trend is to search for more environmentally-sound, yet cost-effective and efficient treatment methods, and thus move away from the use of chemicals altogether.

Some states have taken steps to try to prevent the introduction of the Asian clam to their waters through legislation. In Massachusetts, for example, a 1987 regulation prohibits the importation, transportation or possession of the Asian clam anywhere in the Commonwealth.

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Key Words: Bivalve, Outreach, Basic_biology
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