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Spiny waterflea (Bythotrephes longimanus)
Spiny waterflea (Bythotrephes longimanus)
Photo credit: Emily DeBolt, Lake George Association
Photo credit: Emily DeBolt, Lake George Association

Spiny Waterflea

(Bythotrephes longimanus)

Report this Species!

If you believe you have found this species anywhere in Pennsylvania, please report your findings to iMapInvasives by submitting an observation record.

Species at a Glance

The spiny waterflea is a tiny freshwater crustacean that consumes native plankton and collects in cotton-like masses on fishing lines and cables. It has the potential to outcompete native fish and can threaten a lake’s ecosystem.


This species is not actually a flea, but rather a transparent crustacean or cladoceran that sticks on fishing lines and forms clumps that look and feel like gelatin with tiny black spots (eyes). Magnification is needed to see the transparent body, which ranges from 6-16 mm (0.2-0.6 in) long. The single, long, straight tail, which makes up to 70 percent of its length, has several spikes or barbs. The ball-shaped head with large eyes is clearly separated from the body.

Similar Species

The fishhook waterflea (Cercopagis pengoi), which is smaller, also has a similar delicate spine; however, its tail has a distinctive looped hook at the end. It also has a more elongated and pointed brood pouch, in contrast with the spiny waterflea’s bulbous, “balloon-shaped” brood pouch. The two may be difficult to identify without magnification.


Prefers large, deep, clear lakes with summer bottom temperatures of 10-24°C (50-75°F). It can also be found in wetlands, estuaries, rivers, and congregating in marinas. The spiny waterflea does not tolerate warm lake temperatures.


Fishing, boating, and other recreational equipment can transport the spiny waterflea and its eggs to new waters. Enacting stricter ballast water regulations, avoiding the release of bait, rinsing boats and equipment with hot water sprayed at high pressure, and drying boats and equipment for five days before reentering the water could help prevent further invasion of this species.


While native to Europe and Asia, the spiny waterflea was most likely brought to the Great Lakes in the ballast water of ships traveling internationally. It was first found in 1984 in Lake Huron but is now found in all of the Great Lakes, including the Pennsylvania and New York portions of Lake Erie, and several inland lakes in New York.


Note: Distribution data for this species may have changed since the publication of the Mid-Atlantic Field Guide to Aquatic Invasive Species (2016), the source of information for this description.

Environmental Impacts

The spiny waterflea decreases populations of native zooplankton and competes directly with juvenile and small fish for food, although it can be a food source for several freshwater fish. Anglers consider it a nuisance because the tail spines hook onto and clog fishing lines and downrigger cables.


Information for this species profile comes from the Mid-Atlantic Field Guide to Aquatic Invasive Species (2016).

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