Invasive 'bloody red shrimp' that form dense swarms spread across Ontario waters

A new study confirms that an invasive crustacean species has now established a self-sustaining population in yet another one of the Great Lakes.

Hemimysis anomala (Crustacea, Malacostraca), more commonly known as "bloody red shrimp," is a macrozooplankton that's native to the Black Sea and Caspian Sea. The species was first detected in North America in 2006 in Lake Ontario and Lake Michigan, and was most likely introduced to the Great Lakes by ballast water from transoceanic cargo ships.

Just two years later, the shrimp had established populations in all of the Great Lakes, except for Lake Superior.

Now, a self-sustaining population of the species has been detected in Lake Superior in the major commercial and recreational port of Duluth-Superior Harbor, according to a new study published in the Journal of Great Lakes Research.

While individual cases of the shrimp's presence were recorded in the Duluth-Superior Harbor in 2017 and 2018, the species was found at a few different locations in the harbour last year. In 2025, researchers collected 81 individuals in both light traps and nets.

The species prefers warm and shallow environments, and is known to avoid direct sunlight by clustering on rocky lake bottoms or in sheltered areas near hard structures, including piers, boats, and docks. At night, the shrimp move toward the water's surface in swarms that resemble reddish clouds and move quickly when disturbed.

The combination of their behavioural traits and nocturnal activity makes them difficult to detect, according to researchers.

Although their full effects on the ecosystem are unknown, scientists have established that the species can grow and multiply rapidly, with some swarms consisting of more than 500 shrimp per cubic metre.

They also consume large quantities of zooplankton, which means the shrimp's presence could result in reduced food available for young, native fish and threaten other species.

Females are usually up to 1.7 centimetres long, while males are up to one centimetre long, and can be identified by their translucent bodies with bright red to reddish-orange colouring.

To prevent their spread, the provincial government recommends inspecting your boat, trailer, and equipment after each use, and removing all plants, animals, and mud before moving to a new body of water.