Scientists have long believed that the spiny water flea arrived in the Great Lakes in the early 1980s with trans-Atlantic ships before spreading through inland lakes across Ontario and beyond. Now, new research involving Dr. Andrew Tanentzap, Canada Research Chair of Climate Change and Northern Ecosystems at Trent University, is offering a new timeframe for the crustacean's origins in North America. 

Published in the Proceedings of the National Academy of Sciences, the study found evidence that the invasive species (Bythotrephes longimanus) was already established in Ontario lakes at least a century before scientists first detected it in North America, challenging a widely accepted invasion history in freshwater ecology. 

"For decades, scientists believed the spiny water flea arrived through ballast water released by trans-Atlantic ships entering the Great Lakes," said Professor Tanentzap. "That timeline shaped how government and conservation organizations have managed the species' spread across North America, which has come at considerable cost and caused significant economic and environmental damage to Ontario’s freshwaters." 

The team used a new method to estimate the age of living remains from insect fossils preserved in mud at the bottom of lakes. Prof. Tanentzap’s group also searched for molecules of DNA that the spiny water flea left behind in the lake bottom. Using these two approaches, researchers found evidence that the spiny water flea was present in Ontario lakes decades, and in some cases more than a century, before its widely accepted discovery in Lake Ontario in 1982.  

“Lake sediment profiles can act like ‘time machines’, slowly accumulating libraries of information that we can then use to undertake retrospective environmental assessments,” added Dr. John P. Smol, a biology professor at Queen’s University and co-author on the study. “Such historical perspectives can often provide key information for effective lake management.” 

The findings suggest the species may have existed as a so-called "sleeper population," remaining rare and largely undetected until environmental conditions allowed numbers to surge.  

The Invasion Nobody Could See Coming

Traditional monitoring of aquatic species relies on physically capturing organisms in net samples or observing them in the water column. The spiny water flea is roughly the length of a fingernail and nearly transparent in the water, easily missed by anyone peering into a lake. Past research has shown that millions of individuals may be required in a lake before even one water flea could be captured in a net sample of a tiny fraction of a lake. 

The crustacean's most distinctive feature is the long, needle-like tail spine that can cover most of its body length and is lined with sharp barbs. This feature makes it difficult for many small fish to consume the flea safely, helping protect it from predation. 

A Tiny Predator with Outsized Impacts

The spiny water flea, however, is a formidable predator. It feeds on native zooplankton, microscopic animals that form the foundation of freshwater food webs. These tiny organisms are critical to lake health, grazing on algae and providing food for young fish. Once established, the spiny water flea can dramatically reduce both the abundance and diversity of native zooplankton populations, disrupting food webs from the bottom up.  

Studies cited in the paper by the researchers—including colleagues from Queen's University, Laurentian University, the University of Leeds in the UK and Scottish Universities Environmental Research Centre—have documented declines of up to 34 percent in native zooplankton biomass and 54 percent in species diversity following invasion.  

"The ecological consequences can ripple throughout an entire lake," said Prof. Tanentzap, who received Canada's Arthur B. McDonald Fellowship in 2025 for his pioneering work on ecosystem ecology and evolutionary biology. "As native grazers disappear, algae can become more abundant, affecting water quality. The loss of these ecosystem services can cost tens or even hundreds of millions of dollars per lake." 

Professor Tanentzap has also shown that the invader is driving evolutionary change in native species, forcing some organisms to adapt to a predator they had never encountered before. 

How to Uncover the Invisible

The study's findings raise a pressing question about ecological and species management: how can we detect invasive species before they become an environmental problem if they are essentially invisible? 

"We need to be looking for threats before we even know they are there," said Prof. Tanentzap. "At Trent, we're helping to develop and apply new ways to detect environmental change before it becomes visible through traditional monitoring." 

Prof. Tanentzap and collaborators combined cutting-edge environmental DNA techniques with paleolimnology, the study of ecological history preserved in lake sediments. The approach allows researchers to reconstruct decades and even centuries of environmental change from biological traces left behind in the mud at the bottom of lakes. 

The result is a much longer ecological memory than conventional monitoring programs can provide. 

The study found that sediment DNA could detect the spiny water flea long before it appeared in net surveys. This highlights the potential for this type of sampling, combined with sediment analysis and modern environmental DNA monitoring, to be a critical early-warning system for future invasions. Ultimately, these findings show the tools are available to help scientists uncover the hidden histories of ecosystems and gain a better chance of protecting them. 

Learn more about the School of the Environment, Biology and Chemistry at Trent University.