Alien Alert: Shrimpy invader raises big concerns

In November, an unusual swarm of tiny critters caught the attention of a crewmember on a National Oceanic and Atmospheric Administration boat docked in a Lake Michigan channel. He asked Steven Pothoven of NOAA’s Great Lakes environmental field station at Muskegon, Mich., what the critters were.

“I could see they weren’t fish, so I netted some,” the biologist recalls. Under magnification, the half-inch-long animals appeared to be crustaceans known as mysid shrimp. But “they couldn’t be the native mysid,” Pothoven realized, because those are cold- and deep-water denizens, not shoreline dwellers.

Within about a week, scientists at another federal lab identified the shoreline crustacean as a new invader, the warm-water species Hemimysis anomala. It’s native to rivers in Eastern Europe’s Ponto-Caspian region, also the home of zebra mussels.

This week, NOAA received a report of “large concentrations” of Hemimysis that appeared to be reproducing in southeastern Lake Ontario.

From the 1970s through the 1990s, waves of notorious Ponto-Caspian species entered the Great Lakes in ships’ ballast waters. In 1998, Anthony Ricciardi and Joseph B. Rasmussen of McGill University in Montreal predicted 17 additional Ponto-Caspian species that they worried were poised to invade North America via the Great Lakes. Hemimysis is the first animal on that list to show up.

“I predict it will be a highly disruptive species,” says Ricciardi. He points out that the mysid voraciously consumes microscopic animals at the bottom of the food chain, which are dietary staples for many young fish.

David Reid, director of NOAA’s National Center for Research on Aquatic Invasive Species in Ann Arbor, Mich., says that he’s virtually certain that transatlantic cargo ships picked up Hemimysis in ballast water in Europe. Ironically, he adds, the species probably arrived on ships that had dumped ballast water before leaving Europe. However, those ships—called NOBOBs, for “no ballast on board”—still carry dozens of gallons of water at the bottom of their ballast tanks.

Since the mid-1980s, roughly 90 percent of saltwater ships entering the Great Lakes have been NOBOBs, Reid says.

Guidelines now recommend that NOBOBs flush their ballast tanks with salt water to kill freshwater stowaways before entering the Great Lakes. If they don’t “swish and spit,” Reid says, they can release European invaders as the ships pick up and release ballast water while offloading and taking on cargo in the Great Lakes.

Although Hemimysis deprives some young fish of food, it could be a new menu item for larger Great Lakes fish, Ricciardi says. However, as a new link in the Great Lakes food chain, Ricciardi worries, the fatty crustacean could boost concentrations of pollutants such as polychlorinated biphenyls in the larger fish.

Ricciardi says that H. anomala’s small size and innocent look shouldn’t fool anyone. “This is not a species to ignore.”