New scud invading Illinois River

New scud invading Illinois River

By Gary Beaumont, University of Illinois College of ACES

Urbana—Another exotic species, a scud or sideswimmer, has been detected in the Illinois River. It’s a crustacean called an amphipod in the family Corophiidae, normally a saltwater, coastal inhabitant.

“Many species of corophiids cause tremendous ecological damage when introduced out of their natural range,” said Edward DeWalt of the Illinois Natural History Survey. “Its cousin, Corophium cuvispinum, introduced from the Caspian Sea area to the Rhine River in Western Europe, has drastically reduced the population of zebra mussels and native mussels.”

DeWalt says the scud smothers mussels with tubes and competes with mussels for fine particulate food to the point of clearing the water of much of its suspended particles.

“While the demise of zebra mussels might be viewed as a wonderful development in the U.S., the potential competitive interactions with native mussels, other invertebrates, and fish might have severe repercussions for native species in the Illinois River,” he said.

The current known locations in the Illinois River are at Pekin in Peoria County, Goofy Ridge in Fulton County, Hardin in Calhoun County, Havana in Mason County, and Valley City in Pike County.

Specimens of this unknown crustacean were collected by Jim Hefley, Illinois EPA, and Matt O’Hara, Illinois Natural History Survey. Dr. Edward A. Hendrycks, Canadian Museum of Nature, finally identified the exact species, Apocorophium lacustre (Vanhoffen 1911), which is native to the Atlantic Coast of North America from the Bay of Funday to Florida and is a dominant species in estuaries such as the Chesapeake Bay.

DeWalt says it is not known how it got into the Illinois River, but he speculates it was likely in a ship’s ballast water and entered through the St. Lawrence Seaway to the Port of Chicago. From there, ballast water was released into the harbor with larval and subsequently flowed into the Illinois River through the Sanitary and Ship Canal.

“This species is unlike any other native scud,” DeWalt said. “It is flattened from top-to-bottom (natives are flattened side-to-side), the second antennae (especially in males) are enlarged and modified (natives have antennae narrow throughout their length), and they are darkly pigment on top (natives are often white-opaque, or slightly orange colored). This species is a comparatively small scud, only 3-4 mm length; however, it is possible these might be juveniles.”

Editor’s Note: A JPG image of the new species is available at

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