Plate Nº 12 · recorded October 10, 2026
Biology & EvolutionReported finding
Spiny water flea may have lived in North America for centuries
Spiny water flea DNA recovered from Ontario lake sediment dates to 1867, a PNAS study reports. The finding pushes the species' known North American presence back by more than a century.
By Elena Vasquez4 min read721 words
In brief
- Spiny water flea DNA recovered from Ontario lake sediment dates to 1867
- Researchers identified the species in 300+ lakes and rivers across North America
- Fossils recovered in 2019 from Three Mile Lake sat in sediment older than 350 years
- St. Lawrence Seaway, completed in 1959, ruled out as a delivery route to inland lakes
- Previous theory blamed cargo ships for introducing the species to Lake Ontario in the early 1980s
Spiny water flea DNA recovered from Ontario lake sediment dates to 1867, a study in the Proceedings of the National Academy of Sciences reports. The finding pushes the pest's known presence on the continent back by more than a century.
The spiny water flea, scientifically named Bythotrephes longimanus or Bythotrephes cederströmii, is a tiny crustacean roughly the size of a rice grain, with a long barbed tail spine. It has spread to more than 300 lakes and rivers across Canada and the United States over the past four decades. Scientists previously blamed cargo ships for carrying it from Europe to Lake Ontario in the early 1980s.
What did the new study find?
In 2019, co-authors sifting through lake-bottom mud from Three Mile Lake in Ontario's Muskoka region recovered jaws of the spiny water flea next to sediment dated to more than 350 years old, around the time Europeans founded Sainte-Marie, a settlement in what is now Ontario.
Lead author Danai Kontou and colleagues at McMaster University then used radiocarbon dating to confirm the age of the fossils directly. The chemical makeup of these tiny crustaceans had made such dating impossible until recently. New laboratory approaches, the authors write, solved the problem.
The team also measured environmental DNA, genetic material that organisms shed into water and that accumulates in lake sediment. Spiny water flea DNA appeared in mud layers as old as 1867, the year Canada was founded. The two methods correlated nearly perfectly, strengthening the case that the species was present long before its supposed introduction.
How might the flea have arrived?
The spiny water flea cannot survive long out of water. Its eggs die within four hours without moisture, and the animals themselves die when handled. Before the St. Lawrence Seaway opened in 1959, trans-Atlantic ships could not pass beyond Montréal, ruling out ballast water as a delivery route to inland lakes.
The researchers propose that a single flea hitchhiked to North America on a migrating bird. Millions of waterfowl travel between Eurasia and North America each year. One viable individual, repeated over thousands of years, could establish a population that scientists would not detect for centuries.
The study does not name a specific bird or migration route. It simply notes that rare natural dispersal events, while statistically unlikely, cannot be excluded over 10,000-year timescales.
What triggered the population boom?
If the spiny water flea lived in the Great Lakes region for decades at low density before its recent explosion, climate warming may be a more important driver than the moment of introduction, the authors argue. Fieldwork in Europe, where the species is native, links variable summer temperatures and longer growing seasons to population growth.
The study authors write that determining when a species arrived always requires proving a negative, and the "absence of evidence is not evidence of absence." The team concludes that scientists sometimes "need to dig a little deeper" in invasion biology.
What does this mean for management?
Governments and conservation groups spend millions of dollars each year to control invasive species. The new findings suggest that some invaders are not recent arrivals at all, and that environmental change, not just the species itself, can drive population booms.
The researchers do not argue against managing the spiny water flea. They write that the species still damages lakes where it becomes overabundant. Their point is that once an invader establishes itself, treating the underlying ecosystem disturbance, such as climate shifts, may work better than targeting the animal alone.
Detection is also difficult. Catching a spiny water flea requires pulling it from the water in a net, and several million individuals may need to be present before a single one turns up. Most lakes go unmonitored, so the 300-lake tally is likely a massive underestimate, according to the researchers.
What remains uncertain?
The study cannot prove that the species persisted continuously in North America for 350 years. The fossils and DNA only show it was present at certain times. The researchers call for sediment sampling in more lakes to fill in the gaps and to test whether similar patterns appear elsewhere on the continent.
The paper, "Rewriting the history of the invasive spiny water flea in North America," appears in the 2026 volume of PNAS (DOI: 10.1073/pnas.2622864123).
via Phys.org Biology (Source)
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