Spiny waterflea (Bythotrephes longimanus)

Synonyms: Bythotrephes cederstroemii, Bythotrephes brigantinus

Common Names: Eurasian spiny water flea

Description:

Spiny waterflea (Bythotrephes longimanus Leydig, 1860), also called Eurasian spiny water flea, is a tiny predatory crustacean native to lakes in northern Europe and Asia [1]. It was introduced to the Great Lakes in ships' ballast water, first found in Lake Huron in December 1984 [2]. MISIN tracks it because it preys heavily on native zooplankton, disrupting the base of the aquatic food web, and because its barbed tail spine fouls fishing gear [3][4].

Identification:

Spiny waterflea is easy to overlook on its own ? the body is only about 1/16 to 1/4 inch (1.5?5 mm) long ? but its tail gives it away: a long, straight spine, barbed with several pairs of hooks, that can be longer than the rest of the animal combined [4]. A single large, dark eyespot is usually visible near the head [3]. Individually the animals are hard to spot in open water, but they show up on fishing gear: when many get tangled together on a line, downrigger cable, or net, they clump into a gelatinous mass that looks and feels like wet cotton or jelly, often peppered with tiny black dots ? their eyespots [3]. Populations build through the open-water season, since a single female can produce roughly ten clones in about two weeks under warm conditions, so gear picked up later in summer and fall is more likely to carry a noticeable clump [2]. Spiny waterflea lives in the open water of a lake rather than close to shore, so check any line, net, or cable that's been out over deep water [4].

At-a-glance cues:

1. Long, straight, barbed tail spine, often longer than the rest of the body [4]
2. One large, dark eyespot near the head [3]
3. In large numbers, clumps into a gelatinous mass on fishing line, nets, and downrigger cables that looks and feels like wet cotton [3]
4. Found in open water, not close to shore [4]

Habitat:

Spiny waterflea is native to lakes across northern Europe and Asia, in a range centered on the Ponto-Caspian region of Eastern Europe and Western Asia [5][1]. It was introduced to Lake Huron in December 1984, most likely in ballast water discharged by an oceangoing ship, and spread to every one of the Great Lakes within about three years [5][2]. It has since moved into a growing number of inland lakes, mostly by hitching a ride on boats, trailers, and fishing gear moved between waterbodies [2][3]. It prefers large, deep, clear lakes with cool bottom water, where it can retreat into deep water during the day to avoid being eaten by fish, and it's generally absent from shallow, murky, nutrient-rich lakes [4].

Reproduction:

Spiny waterflea most often reaches a new lake attached to fishing and boating gear ? a gelatinous clump on a line, net, anchor rope, or downrigger cable can carry live animals or resting eggs to the next lake a boat visits [3]. Bait and bait water can also carry it, which is why it should never be moved from one waterbody to another [3]. For most of the growing season, females reproduce by cloning themselves, and a single female can produce roughly ten offspring in about two weeks under warm conditions ? a pace that lets a population explode over a single summer [2]. Late in the season, some individuals switch to sexual reproduction and produce tough, resting eggs that can survive a lake freezing over or drying out, keeping the population going into the next year [3].

Impact and Damage:

Spiny waterflea isn't a health hazard to people or pets, but it's a real nuisance for anyone who fishes: it clogs the eyelets of fishing rods and can prevent a hooked fish from being landed, and heavy clumps on a line or downrigger cable can also damage a reel's drag [3].

The bigger, harder-to-see impact is on the lake itself. Spiny waterflea is a predator, not a competitor ? it eats the same native zooplankton, especially Daphnia, that young fish depend on for food, and it eats a lot of them [4][2]. A peer-reviewed study of two northern Minnesota lakes found that zooplankton biomass dropped 40 to 60% in the years after spiny waterflea became established, and that young-of-the-year yellow perch ? which rely on that zooplankton for food in their first summer ? came out about 10% shorter by the end of the season than perch from a nearby lake spiny waterflea hadn't reached yet [6].

Because it competes directly with young sport fish for the same food supply, its effects ripple through a lake's fishery even though the waterflea itself is barely visible [2][6].

Similar species:

Fishhook waterflea (Cercopagis pengoi) is spiny waterflea's closest look-alike and fellow Great Lakes invader, but the tail gives them away under magnification: spiny waterflea's tail is straight and spine-studded, making up a little over half its total length, while fishhook waterflea's tail bends at a sharp angle partway along its length and ends in a distinctive hook-shaped loop, making up around 80% of its length. Spiny waterflea is also the larger of the two, and the egg-carrying pouch on a female's back is rounded and balloon-shaped in spiny waterflea versus pointed in fishhook waterflea [7].

Monitoring and rapid response:

Signs to watch for:

1. Gelatinous, cotton-like clumps ? sometimes speckled with tiny black dots ? on fishing line, downrigger cables, nets, or anchor ropes after time on open water [3]
2. A rod that keeps snagging at the first eyelet, or a line that feels unusually heavy or slimy when reeled in [3]
3. Most likely to show up from mid-to-late summer through fall, once populations have had a season to build [2]

What you can do:

1. After every outing, clean any clumps of gelatinous material off your line, net, downrigger cable, and anchor rope before leaving the water access [3]
2. Drain all water from your boat, livewell, and bait bucket before leaving ? resting eggs can survive in just a few drops [3]
3. Never move bait or bait water from one lake to another [3]
4. Rinse gear with hot water when possible, and let boats and gear dry completely before using them on a different lake ? thorough drying is the surest way to kill any resting eggs left behind [3]
5. Natural area and lake-association volunteers can help most by promoting the same clean-drain-dry habit at boat launches and by reporting new detections quickly, since there's currently no effective way to remove spiny waterflea from a lake once it's established [3]

When to call a professional:

1. A lake association, natural resources agency, or state aquatic invasive species program is the right first call for a new detection in a lake that hasn't reported spiny waterflea before
2. Large-scale monitoring or lake-wide assessment is a job for a professional or agency biologist, not a DIY project
3. For chemical control options, contact a licensed applicator or your local cooperative extension office.

Report sightings to MISIN. Help track this species' spread by submitting observations through the MISIN web site or the MISIN mobile app (Android, iOS, Windows, and macOS). Photos at multiple angles paired with a precise GPS location are the most valuable data for confirming new occurrences and triggering rapid-response review.

Credits:

1. GBIF Secretariat. Bythotrephes longimanus Leydig, 1860. gbif.org
2. Minnesota Aquatic Invasive Species Research Center (MAISRC), University of Minnesota. "Invasive Spiny Water Flea Disrupting Food Chain In the Great Lakes." maisrc.umn.edu
3. Minnesota Department of Natural Resources. Spiny waterflea (Bythotrephes longimanus). dnr.state.mn.us
4. IUCN Invasive Species Specialist Group, Global Invasive Species Database. Bythotrephes longimanus. iucngisd.org
5. New York Invasive Species Information (NYIS.info). Spiny Waterflea. nyis.info
6. Staples DF, Maki RP, Hirsch JK, Kerfoot WC, LeDuc JF, Burri TM, Lafrancois BM, Glase JD (2017). "Decrease in young-of-the-year yellow perch growth rates following Bythotrephes longimanus invasion." Biological Invasions 19:2197?2205. doi.org
7. Fisheries and Oceans Canada. Spiny Water Flea and Fishhook Water Flea (identification comparison). dfo-mpo.gc.ca

---