Bythotrephes longimanus (also Bythotrephes cederstroemi), or the spiny water flea, is a planktonic crustacean less than 15 millimetres (0.6 in) long. It is native to fresh waters of Northern Europe and Asia, but has been accidentally introduced and widely distributed in the Great Lakes area of North America since the 1980s. Bythotrephes is typified by a long abdominal spine with several barbs which protect it from predators.
Bythotrephes longimanus, the spiny water flea, is a predatory cladoceran native to northern Europe and Asia. It was introduced to the North American Great Lakes through ballast water and has since spread to a number of inland lakes. B. longimanus competes directly for prey with juvenile and small fish along with predatory zooplankton. It can foul fishing lines and downrigger cables, and can have substantial impacts on zooplankton community structure.