Opossum Shrimp

• It is a glacial relict: it was stranded in inland lakes as the ice retreated, and its populations have been isolated for thousands of years.
• Its introduction into North American lakes to fatten the salmon had the opposite effect: it ate the zooplankton the salmon depended on and brought about the collapse of the fishery and of the bird populations that lived off it.
• It is a glacial relict: it was left isolated in cold lakes as the glaciers retreated, cut off from its marine relatives.
• What was called Mysis relicta for decades turned out to be four distinct species, separated in 2005: M. relicta in northern Europe, M. diluviana in North America, the circumpolar M. segerstralei and M. salemaai in the Baltic.
• It is called an opossum shrimp for the female's brood pouch, in which she carries the embryos for several weeks.
• Its introduction into North American lakes between 1949 and the 1980s to feed salmonids went badly wrong: as it shelters on the bottom by day, the fish could not reach it, while it stripped out the zooplankton they lived on. On Flathead Lake, anglers caught 45 tonnes of kokanee salmon in 1985 and none at all in 1987.
• The collapse ran on up the chain: less zooplankton, fewer forage fish, and with them went the eagles, otters, coyotes and bears.

Habitat & environment

Deep, cold waters of glacial lakes and cold seas of the northern hemisphere

Sources consulted: Audzijonyte y Väinölä (2005); Lasenby, Northcote y Fürst (1986); Ellis et al. (2011), PNAS