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New Insights on Tumor-Like Anomaly Infestation in Lake Michigan

By Chad Boutin

Something is attacking the plankton in Lake Michigan.

For nearly two years, scientists have observed strange growths protruding through the shells of flea-sized crustaceans known as copepods in the lake. Whatever the mysterious culprit is, it must be like something right out of a Stephen King novel for these tiny, shrimp-like creatures. Just imagine waking up one morning to find that something had burrowed a hole through your skin, causing your innards to balloon out into a cyst the size of your head.

And as unpleasant as this must be for the copepods, finding the reason behind it has caused a considerable amount of consternation for the scientists as well. Over eighteen months has passed since researchers discovered the cysts growing through the creatures' shells; the cause, however, has remained elusive. But researcher Tom Bridgeman has now zeroed in on a possible perpetrator: a microscopic parasite called an ellobiopsid that feeds on the copepods.

A healthy female copepod from Lake Michigan. Its body is just over 1mm long. A male copepod taken from the lake with a cyst growing from its carapace.

"You might compare these parasites to the sea lamprey," says Bridgeman, a doctoral candidate who works at GLERL in Ann Arbor, Michigan. "The ellobiopsid parasites puncture copepod shells and absorb their bodily fluids. It's similar to what a lamprey does to a fish, but on a smaller scale."

Another researcher, Mohamad Omair, first noticed the cysts in October 1998 when he was instructing other scientists in plankton identification. In one section of Lake Michigan, 60% of the copepods had the cysts on their bodies.

As a primary food source for the young of most fish, including salmon, trout, and yellow perch, the copepods are a critical link in the food chain. Omair had been sampling them for over 25 years, and had never seen anything like it. He knew something was wrong.

Scientists put forth theory after theory as to what that something was. But gathering enough data to support any single theory proved difficult. "The trouble is that it's such a spotty problem," says Henry Vanderploeg, a research ecologist at GLERL. "In some locations, more than half of the copepods were affected; in some places, very few were. We don't know the precise nature of the problem and that makes it tough to design an experiment."

To compound the difficulty, ellobiopsids are not the only beasts which find the copepods tasty. "We have observed several other parasites in copepod tissues," says Gretchen Messick, a scientist at NOAA's Center for Coastal Environmental Health and Biomolecular Research in Oxford, Maryland. Scientists trying to identify their perpetrator now had a full lineup of suspects against the station wall.

But the ellobiopsids' assumed method of attack made Bridgeman suspicious. Ellobiopsids are thought to attack copepods through their carapaces, so he decided to find out what would happen if something pierced a copepod shell. He took a tiny needle and stuck a few copepods. The results were remarkable. "The pierced copepods developed cysts which appear very similar to the ones we saw in the samples from Lake Michigan," said Bridgeman.

While the discovery is a promising step toward solving the mystery, it does not answer all the scientists' questions. Nor does it rule out other possible causes.

"When we first began investigating the cysts, some people thought they were cancerous," says Vanderploeg. "People were rightfully concerned about waterborne contaminants causing cancer in lake life. This could still be a factor. There might be a chemical in the environment which is preventing copepod shells from fully developing, which could make them more susceptible to parasites."

Bridgeman says it will take at least a year to determine whether a definite relationship exists between the cysts and the parasites. "The parasite is still poorly understood, unfortunately, and we have our work cut out for us. The presence of parasites could be the indicator of an even greater problem with water quality in the lakes. And any negative effect these parasites have on the copepods could adversely affect fish populations."

[8/14/00]

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