Bythotrephes longimanus or Bythotrephes cederstroemi, also called a spiny water flea, is a tiny freshwater crustacean native to Asia and northern Europe. It arrived in the Great Lakes region of the United States through discharged ballast water during the early 1980s and spread rapidly throughout the area. This invasive species competes with local species for food and has no natural predators.
The tiny crustaceans arrived in the U.S. from Europe through water that ships carried in their tanks to help stabilize and balance their hulls during long ocean voyages. When ships arrive in ports, they discharge the water and refill the tanks, often releasing organisms that they collected at the last port. Many species that are introduced in this manner do not survive, but the spiny water flea thrived in its new environment.
It was first discovered in Lake Huron and Lake Ontario between 1982 and 1984. It quickly spread throughout all the major and minor lakes. The species was fully established in much of the region by 2011.
The spiny water flea resembles a tiny shrimp in appearance, with the addition of a single long, barbed tail. This spine prevents the spiny water flea from being eaten by any species except the largest fish, so they have no natural predators to limit their populations. They range in size from 0.25 to 0.625 inches (6.35 mm to 15.87 mm). They collect in bristly, jelly-like blobs on fishing nets and lines.
The Bythotrephes species reproduces either sexually or asexually, depending on the environment. Females can produce as many as 20 viable offspring per month. Spiny water flea eggs are capable of remaining dormant throughout the winter and hatch when the temperatures rise.
Eggs that are eaten by birds or fish can pass through the animal's body unharmed. The eggs resist both freezing and drying. Even if the mother dies out of water and her eggs dry out, the eggs can still hatch if they are returned to the water.
Spiny water fleas compete with other fish for food, particularly a type of zooplankton called daphnia. They have completely eliminated certain native zooplankton species in some regions, limiting the available food supply for juvenile fish that rely on zooplankton to survive. In addition, they clog up fishing rod eyelets and prevent fish from being caught.
This species is difficult to control after it is established. Spiny water fleas travel easily from one spot to another on bait buckets or in bilge water and quickly colonize new lakes after being introduced. They were listed as a regulated invasive species in Minnesota as of 2011.