ST. CHARLES, MO. ? Thanks to a vigilant fisherman, the Missouri Department of Conservation received a heads-up on the unwanted rusty crayfish.

The dreaded critter was spotted at a bait company in Monroe County.

“The person who called us knew what a rusty crayfish looks like and he knew it was potential trouble for Missouri,” state official Bob DiStefano said. “We really appreciated his quick action.”

Rusty crayfish are bad news because they are what scientists call invasive. Removed from their original areas, without normal population controls such as diseases, parasites and specialized predators, invasive exotics can run rampant. That has been true of the rusty crayfish in other areas.

The medium-sized crayfish originally was found only in the Ohio River Basin. In recent years, however, the species has turned up in lakes or streams in 18 other states and Canada.

Outside its original range, it has caused a variety of ecological problems.

One problem is the rusty’s appetite for fresh-water snails and mussels. In some areas, it has decimated native frogs, plants and crustaceans, undermining the natural food chain. In extreme cases, it has caused numbers of popular sport fish species to crash.

Wisconsin has experienced such serious damage from exotic crayfish that it has banned the use of live crayfish as bait.

Arizona won’t even allow crayfish to be transported through the state. Some fisheries professionals are calling for a national ban on the use of crayfish as bait.