Mussel Mystery on the St. Croix

Katie Steiger-Meister is a Public Affairs Specialist with the U.S. Fish and Wildlife Service Midwest Region. The article was originally published on the USFWS’s website.

Something has silently grown for the past ten years at an abandoned research site in the St. Croix River. Three empty cages in murky river water can pique the imagination. Was it a secret government project to grow freshwater spy sharks? Nope. Try to imagine something smaller, quieter, and a lot less scary. Like a mussel.

In early July, members of the inter-agency Mussel Coordination Team decided to return to an old cage site on the St. Croix River where a decade before they tried to grow baby winged mapleleaf mussels. The team is comprised of federal, state and nonprofit partners committed to working together on native mussel propagation, management and invasive species control. Visualize a group of field biologists in wetsuits and dive gear underwater, slowly moving over the riverbed. First one, then dozens, of mussels are collected. A spark of hope as the first one is inspected at the surface.

An endangered winged mapleleaf mussel!

The truth unfolds as more are brought up from the riverbed. The site is holding over 400 winged mapleleaf mussels.

‘Beginning to lose hope’

“I’m still in shock at the numbers and quality of the subadults,” said former Service employee Gary Wege who was part of the team that originally placed the cages in the water, and who continues to volunteer with the Mussel Coordination Team. He was on-hand at the cage site and viewed the mussels when they were being examined.

“We were beginning to lose hope in our recovery efforts with this species,” said Genoa National Fish Hatchery manager, Doug Aloisi. “Early and significant progress has been made on another federally endangered species, the Higgins eye pearly mussel, with over 45,000 sub-adult mussels being stocked back into the region. But the winged mapleleaf is harder to propagate and has a more difficult life history.”

Mussels are innocuous creatures, but they do have one sci-fi creep factor to them. When they are extremely small, they need a host. But unlike gory sci-fi parasitic host scenarios, the gentle mussel only hitches a ride on a host; most commonly a fish species. What mussel biologists have learned is that the host fish of choice for the endangered winged mapleleaf mussel is the channel catfish.

Happy homecoming

In 2005, six mussel propagation cages were placed in the St. Croix River. The cages contained 100 channel catfish, each infested with larval winged mapleleaf mussels known as glochidia. As larval mussels develop, they detach from the gills of their host fish and move on to their next stage of life in the river. The cages were therefore designed to have two compartments, with the catfish in the top portion and the detached mussels growing in the bottom portion.

In 2005 and 2006, the bottom cages were examined for juvenile winged mapleleaf mussels, but only 24 sub-adult mussels were found. To add insult to injury, the mussels were covered with unfriendly zebra mussels, an invasive species that has found its way into the Midwest. The cage site was subsequently abandoned in search of a better mussel rearing location in the river.

For nearly ten years three of the cages sat empty in the St. Croix River. When they were revisited this past July and found to be surrounded by hundreds of thriving winged mapleleaf mussels, old assumptions were revisited. Researchers are now beginning to suspect that high water conditions in the St. Croix River may have carried the young mussels out of the cages and dispersed them around the cage site.

As the Mussel Coordination Team works to understand how this success can be replicated, hundreds of endangered winged mapleleaf mussels continue to flourish beneath the surface of the St. Croix River. Sometimes nature, with a little help from the U.S. Fish and Wildlife Service and our partners, finds a way forward.