Bythotrephes cederstroemi (Spiny waterflea)

Animal Crustaceans
Family: Cercopagidae
Classification: Prohibited

Spiny Water Flea

Like the fish hook flea, Bythotrephes cederstroemi was introduced into the Great Lakes in the 1980's, and rapidly spread throughout the lakes. Most likely transported from Europe in ballast water or mud. The distinguishing feature that separates it from other swimming invertebrates is its tail spine, which comprises about 70 percent of its total length. When the animal molts (sheds its exoskeleton in order to grow) it does not shed the tail spine. The spine contains from one to four pairs of thorn-like barbs. Juveniles have just one pair, so these barbs can be used to determine the age of the animal. The animals propel themselves through the water with antenna located just behind the head. Bythotrephes have a very rapid reproduction rate. Most of the time they reproduce asexually, producing one to ten eggs that develop into new females without mating or fertilization. In warm weather, these clones may be produced in less than two weeks. When food becomes scarce, or the weather turns cold, the females produce male offspring. These males mate with the surviving females, producing resting eggs. The resting eggs are first carried as orange-brown spheres in the brood pouch on the females back, and are later released to fall to the lake bottom. These resting eggs can remain dormant form long periods of time.

Bythotrephes compete directly with young fish for food, monopolizing the food supply to the determent of the fish. Although larger fish may eat them, small fish have difficulty swallowing them.