The spiny water flea (Bythotrephes cederstroemi) is not an insect but a small crustacean, about half an inch long with a long, sharp, barbed tail spine. It is native to Great Britian, and its home range extended through northern Europe east to the Caspian Sea. It was first found in Lake Huron in 1984, and was most likely introduced from ballast water discharge from a trans-oceanic freighter. Since then, populations of the crustacean have exploded and it can now be found throughout the Great Lakes and in many inland lakes and waterways.

The impact of the spiny water flea on the Great Lakes ecosystem is still largely unknown. Resource managers have expressed concern that the animal may compete directly with young perch and other small fish for food, such as the Daphnia zooplankton.

Spiny water fleas also reproduce rapidly. During warm summer conditions each female can produce up to 10 offspring every two weeks. As temperatures drop in the fall, so do their reproductive rates. However, the eggs that are produced can lie dormant all winter, and grow into adults as soon as springtime brings warmer weather.

High numbers of the animal would not pose a problem if they were heavily consumed by predators. But the sharp spine makes it extremely difficult for smaller fish to eat, leaving only a few larger species of fish to feed on them. As a result, their populations remain high, while the populations of native plankton, which they feed on, have declined. To the left a spiny water flea attached to a fishing line