Asian clams: the most successful aquatic invaders

Some scientist considers Asian clams (Corbicula fluminea) to be the most successful aquatic invasive species, as evidenced by their being found on every continent except for Antarctica (1, 2, 3, 4, 5, 6). In addition, the species results in the loss of a billion dollars a year in the U.S. from clogging intact pipes (7). The clams are small, typically ranging from the size of a penny to a quarter, brownish yellowish, and triangular oval. They have distinctly raised growth groves. The species may be confused with native finger clams, but native species are more oval, found buried entirely, and have very fine growth ridges (8). While multiple species of Asian clams are invading aquatic systems, the most common species is C. fluminea (9, 10).

Biology

Asian clams are filter feeders but can also feed on the substrate called pedal feeding (11). The clam has one of the highest rates of filter-feeding of any bivalve (8). Unfortunately, the high filtration rate also means that they excrete many nutrients that can cause algal blooms (12,13).

The clams are found in the bottoms of water bodies, often buried in the substrate. Consequently, they are adapted to a wide range of substrates. Clams can also survive up to 36 days without being submerged in water (11).

The clams are prone to die-offs due to silt load from spring runoff, temperatures extremes, and low dissolved oxygen, which is usually associated with low water flows (12, 14). In these conditions, the clams will die and start to decompose, releasing compounds that may be harmful to other aquatic life, like native freshwater mussels (15).

Life Cycle

Asian Clam. Photo credit Lauren Schramm

Asian clams can reproduce in large numbers as they can start reproducing at around 3 to 6 months (8). The adult lives 3 to 4 years and can produce 100,000 juveniles per year (8). Adults can self fertilize, and the fertilized eggs develop into juveniles in the gills in 4 to 5 days. (8)

Range

In the United States, Asian clams can be found throughout 44 states but have the highest numbers in the southeast, Texas, and southwest. Significant populations globally can be found in South America, Europe, and Asia. Native populations also exist in Africa and Australia.

Researchers have found that various climate change models will increase the suitable habitat for Asain clams from 6.6 percent of aquatic habitats to 12.7 percent by 2050 in the U.S. (16).

Map of significant populations of Asain Clams (solid circles), sourced from Mackie & Brinsmead, 2017.

Introduction history

It is widely theorized that they were introduced as a food source on the west coast of the United States and were first discovered in the country in 1938 in California (17, 18). Another theory is that they were hitchhikers with imported Giant Pacific oysters (Crassostrea gigas) (8). Several factors have led to successful invasions of Asian clams, including its ability to produce a high number of offspring, self-fertilization, rapid sexual maturity, lack of a parasitic life stage (most other freshwater clams have this), filter-feeding, and the ability to disperse across long distances (11). In addition, juvenile clams can travel long distances using mucus “parachutes” (8).

Management options

Unfortunately, there are limited management options as once an invader enters an aquatic system, there are several different pathways it can travel. For example, Asian clams can be disturbed via fish eating them and then being excreted in waste (19). Other dispersal methods are not fully understood but likely include; bait buckets, bilge water, live wells, water currents, and transportation by wildlife (8).

Due to the rapid spread, there have been few peer-reviewed management techniques for the clam (8). However, a physical barrier can be installed to stop the spread, the clams can be dredged for and removed, and molluscicides (a chemical that specifically kills mussels) can be used (7). The latter two options should only be limited in use as they can potentially harm native mussels.

Interactions with native mussels

Regarding interactions with other species, an emphasis should be placed on Unionidae, or the family of freshwater pearly mussels. Freshwater mussels are considered the most imperiled group of organisms in North America, with only a quarter of species having stable populations (20, 21, 22, 23, 24, 25, 26, 27).

Their declines are due to previous commercial harvesting, invasive species, sedimentation, declines in fish host species, urban development, disease, predation, poor water quality/eutrophication, dams resulting in habitat alterations, habitat destruction, and channelization (21, 28, 29, 30, 31).

Historically, native mussels dominated the streams and rivers of eastern North America (32). In addition, mussels are long-lived, with some species living up to 100 years (33). On average, many do not reproduce until age 7, which increases their susceptibility to threats contributing to their decline (34). Conversely, Asian clams typically live 3 to 4 years and reproduce early (35, 36).

Asian clams and mussels occupy the same physical space in rivers, streams, and lakes. They also are both filter feeders. It was initially thought that Asian clams did not impact native mussels (37). Although some early researchers (38) did state there were declines in native mussels due to new populations of Asian clams; these researchers were in the minority. In addition to differing habitat preferences, their ranges did not appear to overlap (39, 37, 40, 41).

Current research points to several ways Asian clams can negatively impact native mussels, including direct competition for food, displacing of juvenile mussels, and ingestion of mussel sperm. Additionally, Asian clams are prone to die-offs that produce excess ammonia, which can be fatal to native mussels (42, 43).

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Sources:

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Map source: Mackie, G. L., & Brinsmead, J. K. (2017). A risk assessment of the golden mussel, Limnoperna fortunei (Dunker, 1857) for Ontario, Canada. Management of Biological Invasions8(3), 383.

Asian clam ID chart: https://adkinvasives.com/Invasive-Species/Detail/31

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