Asian clam (Corbicula fluminea)

Common Names: golden clam, freshwater golden clam, prosperity clam, good luck clam, basket clam

Description:

The Asian clam (Corbicula fluminea) is a small freshwater clam native to eastern Asia that has spread to rivers, lakes, and ponds across most of the United States since the 1930s [1]. It breeds fast, can carpet the bottom of a waterway in huge numbers, and clogs water-intake pipes ? problems that have cost industry about a billion dollars a year to manage [7]. MISIN tracks it because it competes with native clams and mussels and can reshape the bottom of a lake or stream [4].

Identification:

The Asian clam is a small bivalve with two matching, mirror-image shells that are rounded to triangular in shape. Most are under 1.5 inches across ? roughly between a dime and a half-dollar ? though some reach about 2 inches [3]. The outer shell is yellow-green to brown and covered in coarse, evenly spaced raised ridges; run a fingernail across it and the ridges feel and sound like a tiny washboard [3][5]. Unlike many native bivalves, this clam lives burrowed loose in sand or gravel on the bottom ? it does not attach to rocks, docks, or boats [3]. Because the shells are thick and long-lasting, they are easiest to find in late summer and early fall, when populations peak and empty shells wash up along shorelines [4].

At-a-glance cues:

1. Small, thick shell, usually under 1.5 inches across ? about the size of a coin [3]
2. Yellow-green to brown surface with coarse, evenly spaced raised ridges that feel like a washboard [3][5]
3. Rounded to triangular outline, with two matching mirror-image shells [3]
4. Found burrowed loose in sand or gravel on the bottom, never attached to a hard surface [3]

Habitat:

The Asian clam is native to eastern Asia, including China, Korea, and eastern Russia [1][5]. It now lives in at least 43 U.S. states, where it has been found in rivers, streams, lakes, ponds, canals, and reservoirs [1]. It burrows into sand, fine gravel, or mud, usually in shallow water less than 10 feet deep, but sometimes down to about 30 feet [4].

This clam needs clean, well-oxygenated water and does poorly where water is heavily polluted or low in oxygen [5]. It handles fresh and slightly brackish water but not salty seawater [4][5]. It is sensitive to temperature extremes: large numbers die during sustained heat above about 93 ?F (34 ?C) and during cold spells below about 41 ?F (5 ?C), which limits how far north it can survive harsh winters [4].

Reproduction:

Most Asian clams move between waters by hitchhiking with people. Tiny young clams ride along in bait buckets, live wells, and bilge water, cling to boats, trailers, and waders, or arrive when someone empties an aquarium or water garden near a waterway [4]. Once in a stream, the nearly microscopic juveniles also drift downstream on the current and start new colonies on their own [4].

This clam is built to spread quickly. Each one is a hermaphrodite ? it carries both male and female parts and can fertilize itself ? so a single clam can start a whole new population [4]. It broods its young inside its gills and releases live juveniles only about 1 millimeter long. A single adult can release hundreds of young in a day and up to roughly 100,000 in a year [3][5]. The young grow fast, reaching breeding age in about 3 to 6 months, and adults live around 6 years [3][4].

Because one clam can reproduce alone and the young are almost too small to see, prevention matters more than cleanup. The most effective step is to stop moving water, mud, and gear between lakes and rivers before any clams can be spotted [4].

Impact and Damage:

For people, the Asian clam is mostly a plumbing and property problem. Dense beds clog water-intake pipes, irrigation lines, pumps, and dock or pond water systems, and dead clams leave sharp shells and a foul smell along shorelines [4][7]. The clams are not considered toxic to people or pets, but their sheer numbers make them hard to keep out of any system that draws water [7].

In natural areas, the impacts run deeper. Asian clams are powerful filter feeders that strip tiny plants and animals from the water. In the Potomac River estuary, they cut the amount of floating algae by 40 to 60 percent along a 7-mile stretch as their numbers climbed from about 1 clam to nearly 1,500 clams per square meter [6]. That filtering removes food that young fish and native shellfish depend on. The clams also crowd out native fingernail clams and already-imperiled native mussels, and their constant burrowing stirs up the bottom and releases nutrients that can shift the balance of a waterway [4][1].

The economic toll is large. Clogged water systems at power plants and factories cost an estimated $1 billion a year in the United States, and clam build-ups have forced temporary shutdowns at nuclear plants and even weakened sand and gravel used to make concrete [7][4].

Similar species:

Native fingernail and pea clams are easily confused with young Asian clams because they are small, rounded freshwater clams that live in the same bottom sediments. You can tell them apart by their shells: fingernail and pea clams are smaller, thinner, and smoother or shinier, with fine bands rather than coarse raised ridges, and they never grow as large as an adult Asian clam [3].

Young native freshwater mussels can also be mistaken for Asian clams when they are small. Native mussels grow much larger as adults, have a pearly inner shell, and need a fish host to complete their life cycle. They lack the heavy washboard ridges of the Asian clam, whose shell stays small and ridged [3].

Zebra mussels are another invasive bivalve found in the same waters, but they are easy to separate. Zebra mussels are thin and D-shaped, often striped, and they attach firmly to rocks, docks, and boats with sticky threads. Asian clams stay loose in the sediment and never attach to hard surfaces [3].

Quagga mussels are close relatives of zebra mussels and cause the same kind of confusion. Like zebra mussels, they attach to hard surfaces and have a rounder, paler shell, while the burrowing, ridged Asian clam is never anchored in place [3].

Monitoring and rapid response:

Signs to watch for:

1. Patches of small, ridged clams burrowed in sand or gravel in shallow water
2. Empty shells washing up along the shoreline, especially in late summer and fall
3. Small clams turning up in bait buckets, live wells, or on waders and gear
4. Clogged or slow water-intake screens on a pond, dock, or irrigation line

What you can do:

1. Clean, Drain, and Dry every boat, trailer, wader, and bucket before moving between waters ? the young clams are tiny and easy to carry by accident. Let gear dry completely for at least five days, or rinse it with hot water above 104 ?F (40 ?C).
2. Never empty bait buckets, aquariums, or water-garden containers into or near a lake, pond, or stream.
3. In a small, contained spot such as a private pond intake, you can hand-collect clams and bag them for the trash. In open, connected waters, hand removal rarely clears them out, so focus on stopping the spread.
4. Natural-area crews can survey and map new patches early, target small or upstream populations before they seed waters downstream, and use bottom barriers or manual removal in small, isolated waterbodies. Coordinating monitoring across a whole watershed catches outliers while they are still small.

When to call a professional:

1. Large or well-established beds, or any clam fouling of an industrial or municipal water intake
2. Whole-pond or whole-waterbody situations, or anything needing in-water treatment ? contact a lake manager, land manager, or aquatic invasive species program
3. For chemical control options, contact a licensed applicator or your local cooperative extension office.

Report sightings to MISIN. Help track this species' spread by submitting observations through the MISIN web site or the MISIN mobile app (Android, iOS, Windows, and macOS). Photos at multiple angles paired with a precise GPS location are the most valuable data for confirming new occurrences and triggering rapid-response review.

Credits:

1. U.S. Geological Survey, Nonindigenous Aquatic Species Database. Corbicula fluminea (freshwater golden clam) Species Profile. nas.er.usgs.gov
2. U.S. Fish & Wildlife Service. Asian Clam (Corbicula fluminea). fws.gov
3. Missouri Department of Conservation. Asian Clam field guide. mdc.mo.gov
4. U.S. Geological Survey, Great Lakes Aquatic Nonindigenous Species. Corbicula fluminea Fact Sheet. nas.er.usgs.gov
5. IUCN/SSC Global Invasive Species Database. Corbicula fluminea. iucngisd.org
6. Phelps HL (1994). "The Asiatic clam (Corbicula fluminea) invasion and system-level ecological change in the Potomac River estuary near Washington, D.C." Estuaries 17(3):614?621. doi.org/10.2307/1352409
7. Pimentel D, Zuniga R, Morrison D (2005). "Update on the environmental and economic costs associated with alien-invasive species in the United States." Ecological Economics 52(3):273?288. doi.org/10.1016/j.ecolecon.2004.10.002

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