The Asian clam is a nonnative, invasive species introduced to our continent from Asia. Like most other clams, it has two mirror-image shells that are hinged together and are generally rounded to triangular. The Asian clam’s shells are relatively strong and moderately inflated. The beak (or umbo, the rounded part near the hinge) is moderately inflated and slightly elevated, with a sculpture of concentric lines that continue onto the rest of the shell as well-defined, raised rings. The beak’s cavity is deep. The posterior ridge (a ridge or slope extending from the beak to the outer margin on the front part of the shell) is indistinct. The periostracum (the thin, usually brownish outer covering of the shell) is yellowish or brown, often green, becoming darker with age. It tends to flake off, revealing the whitish layer beneath. The nacre (the inner, “pearl” layer of the shell) is purple, white, bluish white, or purple changing to white.
Similar species: Our many native fingernail, pill, or pea clams (family Sphaeriidae) might be confused with small Asian clams, but fingernail clams tend to have comparatively thinner shells, with smoother and shinier outer layers, and they never reach the size of Asian clams.
Shell length: to about 1½ to 2 inches.
Potentially statewide, but mostly in the southwestern two-thirds of the state, particularly in the Ozarks.
Habitat and Conservation
Occurs in small to large rivers and streams, lakes, and ponds, in a variety of substrate types. Like other clams and mussels, they live buried or partially buried in the bottom substrates, using their muscular “foot” to position the rear (posterior) end of their bodies in the substrate so the front (anterior) end protrudes upward. That way, the clam’s tubelike siphons, which protrude from the anterior end of its body, have access to water, oxygen, and food.
Like most other clams and mussels, the Asian clam filters food particles — suspended algae, tiny animals, or organic materials — from the water or sediments. The clam’s incurrent siphon (tube) draws water and suspended food particles into the cavity within the clam’s two shells. A pair of wide, fleshy gills extract oxygen and food particles from the water. The excurrent siphon expels water, waste materials, and carbon dioxide from the clam.
Nonnative invasive species.
Asian clams are hermaphrodites, with each individual capable of producing both sperm and eggs. They are capable of fertilizing their own as well as each other’s eggs. The fertilized eggs are retained within the clam’s body, where they develop for 4 or 5 days before being released as live juveniles. The tiny juveniles can travel in water currents, which disperses them to new places away from the parents. The juveniles, in their next stage, settle to the bottom, seek a solid surface, and attach to it. They reach sexual maturity in about 3 to 6 months, before they reach ½ inch in shell length. Depending on temperatures and availability of food, they can be about an inch long at the end of their first year.
A single adult clam can produce up to 100,000 young each year. Because Asian clams reach maturity rapidly and can self-fertilize, it takes only a single clam one year to create a large population where none had existed before.
Similar to the invasive zebra mussel, Asian clam larvae don’t require a fish host to reach a juvenile stage, so they are able to reproduce at a much faster rate than our native freshwater mussels.
Asian clams are appreciated as food in their native regions of the world. Since it is an invasive species in Missouri, you may harvest as many Asian clams as you like. Note: you still need a license. Always keep up to date with the annual edition of the Wildlife Code of Missouri, and know how to tell the difference between Asian clams and our many types of native freshwater mussels, many of which are threatened or endangered.
It’s not entirely clear how Asian clams first arrived in North America. They might have been introduced purposefully as a food source, they might have arrived accidentally when other aquatic organisms were being imported for food, or they might have arrived — as microscopic larvae — suspended in the ballast water of large international container ships. They may have come via more than one route. The first U.S. collections were in the Columbia River, Washington state, in 1938. They have spread across the continent since then. It is estimated they will continue to spread until they occupy all suitable habitats that are not too cold for them to survive.
Invasive zebra mussels attach their shells to the solid surfaces of pipes, screens, and other parts of water treatment systems and power stations, blocking flow, but Asian clams do not affix their shells that way. However, Asian clams still cause problems. The tiny, drifting juveniles easily pass through intake screens and then settle to the bottoms of larger pipes with slower flow. There, with sand and silt, they grow to their adult size, begin to reproduce and accumulate in these systems. Their shells impede flow, cause more sediment to accumulate, and create blockages.
A population of Asian clams can consume a lot of food and channel it into explosive growth within a short period of time, rapidly colonizing a body of water. Asian clams can reach densities of up to 10,000 individuals per square meter. However, a period of unusually cold temperatures, low oxygen, or other adverse conditions can cause massive die-offs, which can further destabilize the water chemistry. In summer, especially, the decaying clams can foul the water for other species, including our native mussels, which cannot survive the high ammonia levels. Only a few surviving Asian clams, however, can quickly expand the colony back to a large size.
Asian clams are believed to compete with our native mussels for food and space. With their less complex life cycle, fast growth to maturity, and large numbers of offspring, Asian clams can reproduce at a faster rate.
When Asian clams occur in dense populations, they can excrete nitrogenous waste products into the water in such quantities that algae and other plants grow extra abundantly, altering the balance of the ecosystem.