Asian Clam (Corbicula fluminea)

Description

Asian clams are small, light-colored bivalves with distinct elevated conentric growth rings. The exterior shells of asian clam are typically light tan or brown, but color can vary significantly between waterbodies. The interior of the shell is generally of white to light blue coloration. Adult asian clams are typically 20mm or less, but size can also vary significantly for this species. Asian clams can be differentiated from native fingernail clams by the number of cardinal teeth present. Invasive Asian Clams will have 3 cardinal teeth, while native fingernail clams only have 1 or 2. Asian clams' life cycles and physiological adaptations make them very effective invaders. These bivalve mollusks are hermaphroditic which can self-fertilize. An average individual can produce around 35,000 larvae per breeding season. However, Asian clams have a low juvenile survival and a high mortality rate throughout its lifespan which lead to populations dominated by high proportions of juveniles. Asian clam's life history adaptions and reproductive traits allow massive population densities to form when invading a new habitat or after subjection to environmental stress.

Native Range and Distribution

This bivalve mollusk is indigenous to Asia, Australia, and Africa, but now currently inhabits freshwater habitats in the Americas and Europe. Asian clam has been found outside of its original distribution  since the 1920s on the Pacific Coast in the United States. Initial Asian clam establishment in North America is thought to be due to transoceanic ballast water exchange and Chinese immigration for a food resources. With rapid, long distance colonization through ballast/bilge/engine water transport; food resource trade; bait release; aquarium industry; and anthropogenic mediated hitchhiking, Asian clam has quickly spread throughout the United States. Currently it inhabits water bodies in nearly all 50 States and throughout New York . The Adirondack Park is surrounded by successful Asian clam populations in the Erie Canal at Utica, Champlain Canal at Fort Edward, the Finger Lakes, and St. Lawrence River in Quebec. As of 2010 Lake George, on the exterior of the Adirondack Park, has supported Asian clams with growing distributions throughout the lake .

Threats and Impacts

Asian clams' economic and ecological impacts are large. They cost industry around a billion dollars a year from clogging intake/water filtration pipes, damaging electric generating plants through biofouling, in management, and tourism industry losses. For example the Lake George Asian Clam Rapid Response Task Force (LGACRRTF) has spent over a million dollars on management since 2010 with over $600,000 in 2011. While the infrastructures lost from Asian clam's destructive qualities are expensive but recoverable, the damage to the environment caused by this invader and its management may be irreversible.

Asian clam's negative effects are most noted on the native mollusk populations. Firstly Asian clam's burrowing and bioturbation activity in to the sediments may displace or reduce habitats of the native bivalves. They also directly compete and limit planktonic food availability to native mussels. Furthermore, when Asian clams form dense populations, they may ingest large amounts of unioid sperm, glochidia, and newly metamorphosed bivalve juveniles.This invasive not only directly impacts native species but also can indirectly affect flora and fauna by altering the water chemistry. Due to its high filtration rates, Asian clams excrete elevated levels of nitrogen and phosphorus in the lake-sediment interface, leading to green algae blooms and dissolved oxygen depletion. Similarly, algae blooms and dissolved oxygen exhaustion can transpire from an Asian clam mass die-off. Water quality is further impacted by the increased levels and bioconcentration of calcium due to intense shell deposition (up to 4.7cm deep has been observed). This calcium accretion may lead to the invasion of other nonindigenous species with higher calcium requirements like zebra or quagga mussel. Additionally, changes in water chemistry by Asian clams have been associated with the decrease in aquatic submerged macrophyte populations. This can be followed by modifications throughout the entire aquatic ecosystem, from zooplankton to migratory birds.

Management

Since Asian clam colonization has negative ecological effects and cost industry huge amounts of money, it is important to develop a comprehensive management plan. The Asian Clam Work Group in Lake Tahoe, NV, CA has developed a four phase management system for dealing with established Asian clam populations which follows:

1. The initial management response, urgent actions, and immediate science need for surveying and understanding of the species' biology to fully evaluate the invasion so attainable management goals can be set/established and resources can be allocated properly

2. Selection of preferred management practices based on the above results to achieve management goals at the lowest environmental and economic cost

3. The implementation of selected control actions

4. Long-term monitoring to evaluate success and adapt future management

The three main methods used for controlling or eradicating (depending of invasion and reactiveness) established Asian clams include:

• Benthic Barriers

• Suction Dredging

• Chemical Molluscicides in the form of potassium

It is suggested that the integration of these methods could have the most effective results. However all management practices for controlling established Asian clam populations are fiscally expensive, require tons of man hours, and come with environmental degradation.