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Species at a Glance
The fishhook waterflea is a tiny freshwater crustacean that threatens aquatic ecosystems and fishing. It is not actually a flea, but rather a predatory cladoceran that consumes native plankton and collects in cotton-like masses on fishing lines and downrigger cables.
Clumps of waterfleas look and feel like gelatin dotted with tiny black spots. Magnification is needed to see the transparent body, which is about 10 mm (0.4 in) in length, with the tail making up 80 percent of the total length. The tail spine is strongly angled at 90° away from the body, with 1-3 widely spaced pairs of barbs and a unique loop or “hook” at the tip. The head is composed primarily of a single large compound eye. The dorsal egg pouch is elongated and pointed.
Note: This species does have another morph, which usually occurs earlier in the spring season, but has a much shorter tail that lacks the loop at the end and has up to four paired barbs.
The spiny waterflea (Bythotrephes cederstroemi) is larger, with a similar delicate spine; however, it lacks the distinctive looped hook at the end of the tail. In addition, it has a bulbous “balloon-shaped” brood pouch, which is more elongated and pointed in the fishhook waterflea. The two species are difficult to identify without magnification.
While it can tolerate a wide range of temperatures, from 8-30°C (46-86°F), it prefers open, deep waters and is typically found in the upper, warmer water layer.
Fishing, boating, and other recreational equipment can transport fishhook water fleas and their eggs to new waters. In the absence of males, the females reproduce by a process called parthenogenesis, which requires no fertilization, and the offspring are clones of the mother.
Originally native to the Ponto-Caspian region of Europe and Asia, the fishhook waterflea arrived in the ballast water of ships. It has been found in lakes Ontario, Erie, and Michigan, and the Finger Lakes Region of New York. In Pennsylvania, the fishhook waterflea is present in Lake Erie.
Note: Distribution data for this species may have changed since the publication of Pennsylvania's Field Guide to Aquatic Invasive Species (Second Edition 2015), the source of information for this description.
Introduction of the fishhook waterflea into the Great Lakes continues to decrease populations of native zooplankton species. It competes directly with juvenile and small fish for food and is considered a nuisance to fishermen because the barbed tail can hook onto and clog fishing lines, nets, trawls, and other equipment used for recreational and commercial fishing.
Information for this species profile comes from Pennsylvania's Field Guide to Aquatic Invasive Species (Second Edition 2015).