Southeastern Wisconsin Invasive Species Consortium, Inc.


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Meet a New Invader

A new invasive has recently arrived in Wisconsin. According to Lisie Kitchel, Wisconsin DNR Conservation Biologist, the Asian clam (Corbicula fluminea) has spread both intentionally and unintentionally through typical invasive aquatic pathways such as boating, bait bucket disposal and aquarium releases.

Asian clam Corbicula flumineaAsian clam (Corbicula fluminea)Corbicula was first reported in Wisconsin in 1977 from the St. Croix River area. Subsequent reports include the Mississippi River (1981), Lake Superior (1997), and Lake Michigan (2001). In 2012 it was reported in the Mukwonago River and associated lakes; the first time it was confirmed in a Wisconsin inland waterway.

The greatest threat Asian clams pose is from sheer numbers. They are hermaphroditic, can self-fertilize, and reproduce twice a year resulting in 70,000 young per year, according to Kitchel. Although not as prolific as zebra and quagga mussels (Dreissena polymorpha and D. bugensis), they still can overwhelm the bottom of a waterbody, becoming the primary component of the substrate and competing with native species for food and space.

Asian clams affect water quality, releasing high levels of phosphorus and nitrogen that promote algal blooms and affect the taste of the water. They are also known biofoulers in water treatment plants, power plants, pipes and irrigation systems.

Wisconsin is at the northern edge of the temperature range for Asian clams, and that limits how abundant Asian clams will become. However, they are known to overwinter at power plants in the warmer effluent, giving them a head start on next year's invasion, Kitchel said. They also appear to be surviving our winters better than expected.

Zebra left and Quagga right musselsZebra (left) and Quagga (right) musselsAsian clams can grow to the size of a quarter, but usually are closer to the size of a nickel or dime. Their yellow-to-brown colored shells are thick with distinct ridged concentric rings. Our native fingernail or pea clams (Sphaeriidae) are similarly shaped, but thinner-shelled and the lines are not raised, and only grow as big as a pea or dime.

The invasive zebra and quagga mussels are also similar to Asian clams in size but are triangular or D-shaped, often black and white striped, and when live are attached by black threads to surfaces. Asian and native clams are more round and symmetrical and never attach to surfaces.

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