Lake-bed DNA puts the spiny water flea in Ontario more than a century before its accepted arrival

The researchers set out their evidence in a first-person account on phys.org. They rank the flea among North America's most damaging invaders and say it may have been on the continent for centuries longer than believed ^[1]. The dates behind that claim do not carry equal weight. The oldest, more than 350 years, is the age of the mud lying beside the jaws found in Three Mile Lake ^[3]. The authors themselves note that waves and burrowing organisms mix fossils between layers, and that the best way to age a fossil is to date it directly ^[4]. Radiocarbon dates on the fossils themselves put them before the 1950s ^[5], a far more recent bound than the mud beside them.

Direct dating was hard for a practical reason. The animals are so small that hundreds of individuals go into one age estimate, and their chemistry interferes with the measurement; the authors say new approaches solved both problems ^[7]. The account does not describe those methods, give uncertainties on the dates, or say how many lakes were cored.

The DNA work is the most carefully built part of the study. Shed DNA settles into lake beds and can survive for centuries in cold, dark sediment, and its quantity can estimate the size of populations that are hard to count ^[8]. The team checked that molecular proxy against a physical one. DNA abundance tracked fossil abundance nearly perfectly ^[17]. Flea DNA appeared in layers dated as far back as 1867 ^[6]. Measured against an arrival in the early 1980s ^[2], that is a gap of at least 113 years ^[1].

Detection explains how the gap went unnoticed. Surveys find the flea in nets or fish stomachs, and a lake may need several million individuals before one is caught ^[9]. The reports from more than 300 lakes and rivers over four decades ^[10] are a record of dense populations. The authors conclude the flea lived at low densities in the Great Lakes region for decades before anyone detected it ^[12].

The cargo-ship account also has a geography problem for these lakes. Until the St. Lawrence Seaway was completed in 1959, trans-Atlantic ships could not travel past Montreal ^[11]. There were no canals or roads to carry the flea inland ^[13]. It dies when handled, and its eggs last no more than four hours out of water ^[14]. The authors suggest a migrating bird as the carrier. Millions of aquatic birds move between North America and Eurasia each year, and a single individual can found a population ^[15]. Over 10,000 years, they wrote, such an event cannot be excluded ^[15].

The management case asks more of the evidence. The authors wrote that millions of dollars go to controlling the spread of invasive species, and that for native species that benefit from environmental change, "this strategy may misplace limited funds" ^[16]. The sediment record places the flea in Muskoka more than a century before the ships ^[1]; whether it is native is a separate question the cores cannot answer. The authors' own text has the flea arriving in central Ontario's small lakes "in the early 20th century" ^[13], and a founder carried by a bird from Eurasia would still be a newcomer. In my view the defensible conclusion is narrower. A flea that sat at low density for decades before becoming a pest ^[12] points managers toward what changed in the lakes, and money spent slowing its spread between lakes does not answer that question.