Critical Great Lakes Crustacean Disappearing

May 30, 2008 -- The freshwater Great Lakes on the Canadian-U.S. border used to be packed with a tiny, yet ecologically important, crustacean called Diporeia, but this krill-like animal has been disappearing in droves from all but one of the lakes, according to at least two teams of researchers.

The Diporeia disappearance has likely led to the physical shrinkage of alewives (a species of herring), from 10 inches in length to 8 inches or lower, since this fish relies upon the rice grain-sized crustacean as a food source. Diporeia also provides Chinook salmon and other commercially important species, such as lake whitefish, trout and walleye, with up to 80 percent of their food.

The chain of events foreshadows further population declines, scientists say. Pollution and disease could be contributing to the problem, but scientists suspect an insidious invader is primarily responsible for the deaths.

"The loss of Diporeia in lakes Michigan, Erie, Huron and Ontario coincided with the introduction and expansion of zebra and quagga mussels," collaborating researcher Thomas Nalepa told Discovery News.

Nalepa, a research biologist at the National Oceanic and Atmospheric Administration's Great Lakes Environmental Research Laboratory, explained that the mussels probably came to the lakes as stowaways on European and Asian vessels.

"A ship bound for the Great Lakes will take on freshwater ballast for stability at an overseas port before its transoceanic journey," he said. "Once in the Great Lakes, it will discharge this ballast water before it takes on cargo. When this happens, freshwater organisms that survive the journey will then be introduced into the new system."

The mussels, once in the lakes, muscle in on Diporeia's turf. They settle above the small crustacean's sediment homes and filter out algal plankton, which Diporeia must feed upon. They then leave behind copious amounts of waste -- literally defecating on the hapless crustaceans -- and transmit disease.

Freshwater mussels aren't even palatable to humans, according to Nalepa, who described them as having a "musky, ammonia taste."

He added, "I tried cooking some zebra mussels after they were first introduced, and they stunk up the whole house."