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Exotic Aquatic Invertebrates in Illinois

Photos © photographer named. No photographs included within this information may be used on the internet, publications, or any other form of media without the photographer's express permission. All rights reserved.


Asian clam (Corbicula fluminea) photo © Kevin Cummings, Illinois Natural History Survey
The Asian clam has a small, yellow-brown to black shell with concentric, regularly spaced ridges. Its shell is rounded to triangular. The inside of the shell is white or purple. This organism may grow to one and one-half inches in length.

The Asian clam lives in the silt, mud, sand or gravel bottom of lakes and streams. This clam has a free-swimming larva. It filters small organisms from the water for its food. The Asian clam was introduced into western North America in the 1920s and 1930s and has spread throughout much of the United States.
rusty crayfish (Orconectes rusticus) photo © Chris Taylor, Illinois Natural History Survey
The rusty crayfish is green-brown to brown-red on the upper side of the body. Single, brown spots are located on each side near the middle of the animal. The claws are fairly smooth and gray-green to red-brown in color.
The rusty crayfish lives in creeks, rivers and lakes with rock or gravel bottoms. This crustacean reaches maturity at about 15 months of age. Mating occurs in the fall with egg-laying in late spring. The number of eggs produced is dependent on the body size of the female, ranging from about 50 to 350. Eggs hatch in May after being carried under the female's tail for about four to six weeks. The species overwinters in burrows it constructs in streambanks or in other places in the ground. The rusty crayfish hunts aggressively for food, feeding mainly on plants and dead organisms. It is native to the southern United States. This species was first collected from Illinois in 1973. It is believed to have been introduced into Illinois by fishermen who used it as fishing bait, releasing the remaining crayfish when they were done fishing. Current laws ban the sale and possession of this species in Illinois. Strictly enforcing these laws is the only way to stop its spread. The rusty crayfish is an aggressive, large species that takes over prime habitat from native crayfishes, forcing them into areas where they are preyed upon more easily. In other places it has caused the elimination of many native crayfish species from their original habitat.

spiny water flea (Bythotrephes longimanus)
The spiny water flea is less than one-half inch long. This tiny crustacean has a long, sharp, barbed tail spine that often makes up over 70 percent of the animal's length. Its head consists mainly of a large, single, black eye and a pair of jaws, or mandibles. Four pairs of legs are present, the first pair longer than the others. A pair of swimming antennae are located behind the head.
The spiny water flea lives in Lake Michigan and possibly some inland lakes. It uses swimming antennae to move in the water. As it grows, it sheds the outer shell over its body, but not over its spine. The barbed tail spine makes it difficult for predators to swallow this organism, and, after attempting to eat it, they often spit it back out. The life span of the spiny water flea lasts several days to about two weeks. Adults are present in water bodies from late spring until fall. This crustacean may produce up to 10 offspring every two weeks. Most of the spring, summer and fall populations are comprised of females. These adult females produce unfertilized eggs that develop into female offspring that are identical to the mother. This cycle continues as long as the water temperature is not too hot or too cold and food is plentiful. When the water temperature begins to cool in the fall, both males and females are produced, allowing sexual reproduction to occur. Eggs are then deposited over the bottom, where they will remain until conditions become favorable for them to hatch. Adults die after reproducing. The spiny water flea eats zooplankton. It may eat up to 20 prey items each day, using its jaws to pierce and shred the prey. The spiny water flea is native to northern Europe. It first appeared in the United States (Lake Huron) in 1984 and spread to all of the Great Lakes by 1987. It is believed to have been brought from Europe to the United States in the ballast water of ships. It may be competing with small fishes for food. It can be transported in bilge and bait bucket water or by being attached to fishing equipment or other items.

zebra mussel (Dreissena polymorpha) photo © Illinois Department of Natural Resources
The zebra mussel has a small, triangular shell with alternating light and dark bands. It grows to one and one-half inches in length. The inside of the shell is white.

The zebra mussel may be found in Lake Michigan and large rivers and lakes in Illinois. It attaches to nearly any hard, underwater surface by gluelike fibers called byssal threads. Areas with large numbers of these animals may have 30,000 to 70,000 zebra mussels per square meter. Zebra mussels attach to water intake pipes of power plants and water treatment plants. Millions of dollars are spent each year for cleanup and repair to these structures. Zebra mussels also attach to other mollusks, which may stop the native species from feeding or reproducing. The zebra mussel reproduces at an age of about one year. Each female may produce 30,000 to 1 million eggs per year. Breeding occurs from May through October. Males and females release eggs and sperm into the water. Fertilized eggs develop quickly into free-swimming larvae called veligers. Veligers form shells and, after about 10-15 days, settle on the bottom and attach to anything hard. Zebra mussels filter water to remove plankton for food. They are often found attached on native mussels near the incurrent siphon, the place where food is taken into the shell. Each zebra mussel may filter up to one liter of water per day. Filtering improves water clarity but reduces the amount of plankton available for native species. The zebra mussel is native to the Caspian Sea region of Asia. Zebra mussels were brought to the Great Lakes from Europe in the ballast water of ships in 1986. They may be spread by livewells, bilge water, boats and boating equipment.


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