Spiny waterflea (Bythotrephes longimanus)

Spiny water flea on fishing line.
On a fishing line, spiny water fleas look like bristly gobs of jelly with black spots.

Photo of spiny waterflea magnified, actual size is 3/8 inch.
Adult size of the spiny water flea is 3/8 inch.

Species and Origin: Spiny waterfleas are zooplankton (microscopic animals). Native to Europe and Asia, they were introduced into the Great Lakes by ballast water discharged from ocean-going ships. They were first discovered in Lake Ontario in 1982 and spread to Lake Superior in 1987. Adults range from 1/4 to 5/8 inch long. Spiny waterflea have a single long tail with small spines along its length.

Impacts: Spiny waterfleas eat small animals (zooplankton), including Daphnia, which are an important food for native fishes. In some lakes, they caused the decline or elimination of some species of native zooplankton. They can clog eyelets of fishing rods and prevent fish from being landed.

Status: They have spread throughout the Great Lakes, and are established in some inland lakes and rivers in Minnesota. See US map  .

Means of spread: They can spread by attaching to fishing lines, downriggers, anchor ropes, and fishing nets. While female waterfleas die out of water, under certain conditions they produce eggs that resist drying and freezing, and can establish a new infestation. They also can be unintentionally transported in bilge water, bait buckets, or livewells.

Where to look: They collect in gelatinous globs on fishing lines and downrigger cables. They prefer deep lakes, but can be found in shallow lakes and rivers.

Regulatory Classification: Spiny waterfleas are a regulated invasive species in Minnesota (DNR), which means introduction into another waterbody is prohibited.


How can you help?

  • Learn to recognize them
  • Inspect and remove gelatinous material from fishing lines
  • Drain water before transporting boats, personal watercraft, and bait containers
  • Report new infestations to the DNR Invasive Species Program

Further information: