CLEVELAND - The latest threat to the fragile health of Lake Erie doesn't come from chemical spills, algae blooms or raw sewage.
It comes from a gray, 6-inch fish.
This is a fish with an appetite - for the eggs of other fish and for mussels that clean the water.
It's a fish that reproduces faster than a high-tech photocopier - capable of spawning every 20 days and overwhelming similar species that provide key links in the food chain.
It's a fish that wasn't supposed to be here - but hitched a ride on a transatlantic freighter from the Black or Caspian Sea.
Meet the round goby, an aggressive, bottom-feeding fish with buglike eyes and thick lips that make it look as though it's smiling. The goby is fast becoming the dominant species in the shallow waters of this and other shoreline cities on Lake Erie.
And it's making scientists nervous.
"We've had a basic, fundamental change in the ecosystem," said biologist David Jude of the University of Michigan's Center for Great Lakes and Aquatic Sciences. "There's tremendous numbers of them developing."
The goby presents a nagging concern for humans as well. The hardy fish love to dine on zebra mussels, dime-size mollusks that tend to accumulate cancer-causing PCBs as they filter lake water. Scientists worry that, as gobies are devoured by sport fish, which are then caught and eaten, poison could move along the food chain to humans.
"They're a new kid on the block, and there has been some concern," said Roger Knight, supervisor of the state Wildlife Division's Fish Research Unit in nearby Sandusky. "There could be a link up the food chain. There's that threat. We don't like to be alarmist. We just don't know at this point."
The invasion of the Eurasian goby is a problem the Great Lakes didn't need. They are under continuous assault, not just from overfishing and pollution, but also by exotic species that travel here on oceanic ships. Their introduction - from the lamprey to the Spiny Water Flea to the Chinese Mitten Crab - has disrupted and damaged aquatic life across the lakes, according to the Great Lakes Sea Grant Network, a group of educators and researchers.
Lake Erie is the shallowest and warmest of the lakes, the smallest in volume, but still the 12th-biggest lake on Earth. It has fought a long and often losing battle for its health.
Discovered by European explorers, by the 1900s Erie was lined with steel mills and factories, swallowing the toxins that spewed from slaughterhouses and manufacturers in Buffalo, Toledo, Cleveland and Detroit. Pollution worsened with time.
By the 1960s, the lake was the dump site for 420,000 tons of solid and liquid wastes from Ohio's Cuyahoga River alone each year. By the 1970s, Erie was dead.
Stricter environmental laws and intense cleansing efforts brought it back, though scientists say its health remains tenuous. For instance, last summer saw the unexpected return of a poisonous algae microcystis, or blue-green algae, which can kill fish and sicken people.
Jude, the research scientist, found what was believed to be the first Eurasian goby in the Western Hemisphere while conducting a 1990 research project on the St. Clair River near Detroit. That was a tube-nosed goby, the round goby's cousin. Soon afterward, a round goby was discovered in Lake St. Clair, between Lake Huron and Lake Erie. Gobies soon stole into southern Lake Michigan, and last year were found in Ontario and Superior.
That meant that within five years gobies had invaded all five lakes - an incredibly fast distribution, scientists say. Most alien species move much more slowly, taking up to 25 years to emerge in all the lakes.
But then, they generally swim into the lakes. The goby was a stowaway.
Authorities believe the fish first journeyed here in the ballast water that ships take on for stability. That water - and the fish and larvae in it - is discharged when the vessels reach port.
Some species take years to be noticed. The goby's impact has been more immediate.
The fish that has felt it most is the mottled sculpin, a small brown fish that shares some characteristics with the goby. Both dwell among near-shore rocks and crevices, laying eggs in fissures and defending their nests. Both feed at night.
But the goby is more aggressive, chasing the sculpin from its breeding grounds. It's also a faster and sharper hunter. In head-to-head competition for food and territory, the sculpin loses out.
Consequently, where the goby has intruded, a steep decline in mottled sculpins has followed. Some experts fear the sculpin could become extinct in parts of the lakes - eliminating a major source of food for brown trout, bass and walleye.