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Tricia Anderson, Natural Resource Technician, checks the growth of juvenile mussels. Photo by Melissa A. Clark/USFWS.

Tricia Anderson, Natural Resource Technician, checks the growth of juvenile mussels. Photo by Melissa A. Clark/USFWS.

Eureka! Mussel discovery made in a tub

Tucked away just off the shore of Lake Pepin in Minnesota, the Center for Aquatic Mollusk Programs (CAMP) facility operated by the Minnesota Department of Natural Resources is staffed and equipped for an overarching mission: conserving native mussels. Tubs, tanks and tubes fill the research facility with the sound of running water. A variety of fish gaze at you from shelves, while tubs filled with tiny specks are washed with the constant flow of water. If you look closely, those specks are small mussels, which will live at the Center until they are large enough to be released into the wild. Larger circular tubs hold either adult mussels or fish. It’s in one of those tubs that the enigma of the spectaclecase mussel host species was finally solved.

Mussels have a surprising life cycle, during which mussel larvae attach to the gills of their host fish. This fish serves as a mussel nursery, protecting mussel larvae until they transform into juveniles that are ready to strike out on their own. Researchers have been looking for the host species for the endangered spectaclecase mussel for decades, long before it was added to the endangered species list in 2012. More than 90 species of mussels are listed as federally threatened or endangered and 29 species have gone extinct. Threats facing mussels include dams, pollution, habitat loss, invasive species and loss of host species.

The extensive search for the spectaclecase host species has included more than 50 species of fish. Biologists even questioned if the host was some other type of animal and tested amphibians and crayfish to no avail.

“We had already tested common species that could be easily held in captivity,” said Bernard Sietman CAMP mussel biologist. “We suspected the host was either hard to catch or hard to keep alive in captivity.”

Ironically, a clue in finding the host was one of the threats facing mussels: a dam. Spectaclecase populations above St. Croix Falls Dam were disappearing, while populations below were doing well, suggesting that the host was now absent above the dam. Mooneye and goldeye fish both looked like promising candidates because they were extirpated above St. Croix Falls Dam. Unfortunately, both fish were difficult to keep alive in the lab. Mooneye and goldeye were exposed to spectaclecase mussel larvae and placed in the circular tubs, where they swam along the edges, scratching their bulbous eyes and dying from the subsequent infection.

A goldeye photo courtesy of Konrad Schmidt/Minnesota Department of Natural Resources.

A goldeye photo courtesy of Konrad Schmidt/Minnesota Department of Natural Resources.

“It was a problem we had to overcome,” said Bernard. “So we got larger tanks, improved filtration, and consulted experts, including our partners at the Minnesota Zoo. Ben Meinrich, a zoo aquarist, suggested installing a bubble curtain so fish could more easily detect the tank walls. It worked; long enough at least for spectaclecase larvae to fully transform into juveniles.”

In June 2017, Sietman and the CAMP team had laboratory proof that spectaclecase larvae successfully transform into juvenile mussels on a goldeye. A few weeks later, mooneye was also confirmed as a host in the laboratory. But to confirm these fish were hosts in nature, Sietman and the CAMP crew worked with longtime collaborator Mark Hove at the University of Minnesota. The team captured mooneye near spectaclecase colonies on the St. Croix River, held them in tanks at the CAMP facility, and within a few days, recovered juvenile spectaclecase mussels - eureka!

Although the mystery of the spectaclecase host species is finally solved, research continues to move this species toward recovery. The hundreds of juvenile spectaclecase mussels were sent to expert mussel biologists at Genoa National Fish Hatchery and Missouri State University to find the best conditions to raise these tiny mussels until they ready to be released into the wild. Protecting mussels means protecting ecosystem engineers that keep our water systems healthy for their host fish, other aquatic organisms, and us.

“Mussels are nature's water filter,” said Nathan Eckert, Mussel Biologist from Genoa National Fish Hatchery. “Some populations need more help to recover in a timely fashion. Many of these species have benefited from additional research, such as host fish requirements, that have been paid for largely by funding made available because of Federal or State Endangered status.”

The U.S. Fish and Wildlife Service is one source of funds through the State Wildlife Grant Program. CAMP uses these grant funds to establish protocols to propagate endangered and threatened mussels, identify suitable host fish and identify reintroduction sites. These grants further state priority conservation research, monitoring and management.

By Melissa A. Clark
Regional Office - External Affairs

Spectaclecase mussels photo courtesy of Bernard Sietman/Minnesota Department of Natural Resources.

Spectaclecase mussels photo courtesy of Bernard Sietman/Minnesota Department of Natural Resources.

 

Last updated: March 8, 2018