A tiny invader would have slipped into Lake Erie unnoticed if not for biologist Patrick Hudson.

He checked for the parasitic copepod in September and found it on the second fish he sampled.

Neoergasilus japonicus. Photo: Patrick Hudson, U.S. Geological Survey

The species, Neoergasilus japonicus, has been in Michigan’s Saginaw Bay and Lake Huron since 1994. It hooks onto fish fins and eats organic material off their scales.

The animal is as long as a dime is wide and doesn’t draw much attention, Hudson said. “As long as it doesn’t impact some sort of fishery in the Great Lakes or cause some compounding of water intake, like zebra mussels, it’s probably of no interest for most people.”

The species has no known economic or environmental effects on the Great Lakes, said Abigail Fusaro, research associate at the Great lakes Environmental Research Laboratory in Ann Arbor, Mich.

Although the copepods latch on to many types of Great Lakes fish  – like pumpkinseed sunfish, yellow perch, rock bass, bluegill and carp – scientists are still unsure how they affect the fish.

“We don’t know what impact [the species] has, whether it’s actually affecting the biology or behavior of the fish, or whether it’s innocuous and just tagging along for a ride,” Fusaro said.

Although he hasn’t seen the species affect the ecosystems of Lake Huron and Saginaw Bay, Hudson said other species of parasitic copepods hurt fish in hatcheries.

“No one notices them until they get into a hatchery situation where everything is confined and then they could cause some mortality,” Hudson said.

Although N. japonicus isn’t known to benefit or harm Great Lakes ecosystems, Hudson and Fusaro are interested in how it spread into Lake Erie.

It hails from Thailand, and Hudson said after its introduction in Hungary, it spread across Europe in about 20 years. Alabama is the first place it was found in the U.S.

Because of their size, Hudson said the copepods probably didn’t swim from the Saginaw Bay to Lake Erie. He also doubts the fish they attach to would swim around the thumb through Lake St. Clair and into Lake Erie.

Rather, he thinks it’s been distributed by people tossing goldfish, which are often kept or used as bait.

Because they’re hard to see, small invaders like parasites, viruses and bacteria are often introduced by humans dumping pet fish or fish stocking.

“If the disease hasn’t reached a level of symptoms, you don’t see any outward signs of ill health, so that could be missed,” Fusaro said.

Other diseases and parasites, like largemouth bass virus, whirling disease and Asian tapeworm were probably spread through the bait, aquarium, or aquaculture trade.

Fusaro hopes outreach and education will keep people from accidentally spreading invasive species.

“Education and outreach has done a lot to communicate to the public how to properly dispose of unwanted fish,” Fusaro said. “We’re trying to minimize that factor of introduction.”