Spiny Water Flea

by Howard Webb (St. Louis, MO, USA)

Spiny Water Flea
Bythotrephes cederstroemi
click for full size image: 393K


The spiny water flea (Bythotrephes cederstroemi) is a non-native species of cladocera that has become quite common in the Great Lakes. These were first discovered in Lake Huron in 1984, and have since spread to the other lakes. It is assumed that the spiny water flea was introduced via ballast water from a trans-Atlantic freighter. The Great Lakes are considered an international waterway, as large ships can easily come up the St. Laurence Sea Way, and reach ports in Ohio, Michigan, Indiana, Illinois, Wisconsin and Minnesota. The spiny water flea is native to Great Britain and northern Europe, east to the Caspian Sea. It is possible that a ship in Europe pumped water into its hold for stability, then dumped the water in Lake Huron before loading cargo for the return voyage, releasing the Bythotrephes in the process. DNA analysis shows that the Great Lakes population is most closely related to those around Lake Ladoga in Russia. During the spring floods, the port of St. Petersburg changes from salt water to freshwater, which might have provided the opportunity for these freshwater cladocera to board an empty grain ship.

The impact of the spiny water flea on the Great Lakes fisheries is still debated. The long tail spine makes them hard for small fish to eat, and they are predators of the same zooplankton (daphnia) that small fish consume. Some studies show them hurting the food chain, though the sum total of all the research is inconclusive at this time.

Unlike most cladocera, which are filter feeders, this is a predator with a large eye, and legs designed for capturing and holding prey. The barbed tail spine, which makes them difficult for small fish to eat, is also what causes them to tangle in fishing lines.


While I have been interested in collecting the spiny water flea, I have considered it unlikely I would ever find any as they inhabit deep, open water. My collecting techniques are good for shorelines and waters safe for canoeing, but venturing onto the Great Lakes requires a bigger boat, or good conditions and kayaking skills (both of which I lack).

While on vacation this past September, my family and I were in Petosky, Michigan. By coincidence, I was down at the docks and happened to get into a conversation with Barry Aspenleiter, who operates the Ruddy Duck as a charter fishing boat. When I asked him about the spiny water flea, he knew immediately what I was talking about, and even started picking dried parts of them from his boat to show me samples. The barbs on the long tail spine tangle in fishing lines and down-rigger cables, sometimes being so thick that they jam in the rod tips, and the line cannot be reeled in. The population (and problem) varies from year to year, by location and over the season. This year has been relatively light compared to a year or two ago, and the population is larger in the spring than late summer. Barry was getting ready for an evening charter, and I asked him if he would be willing to save me a sample if he encountered any that evening. I met Barry later that evening when the Ruddy Duck returned to port, he had a 35mm film canister over half-filled with them, which he had picked off of his lines.


While the main body of Bythotrephes is no bigger than other large daphnia, the tail spine is relatively immense. With each molt, another set of barbs is added. This is the first cladocera I have encountered where I could not view the whole animal under 40x magnification. This picture is a composite of seven separate pictures, combined with Photoshop Elements.

Technical Details

Environmental Conditions:
Water temperature: 67F at the surface, 55F at 60ft.
Depth: 40-60 feet
Visibility: 20ft plus (incredibly clear!)
Location: lat: 45.3890, long: -84.9622

Microscope: Bauch & Lomb monocular, 10x ocular, 4x, 10x and 40x objectives.

Camera: Canon A70

Software: Photoshop Elements


Great Lakes Information Network: Invasive Species

Illinois-Indiana Sea Grant: This has a good explanation of a possible invasion route.

Minnesota Sea Grant: Life history and possible effects on the Great Lakes.

I owe Barry a real debt of gratitude for his kind assistance. If you are ever interested in chartering a boat for salmon fishing (or daphnia collecting) near Little Traverse Bay of Lake Michigan, give him a call:

Barry Aspenleiter
3175 Country Club Rd
Petosky, MI 49770



Comments to the author Howard Webb are welcomed.

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