Researchers recently made an interesting discovery in the Sainte-Croix River: a group of endangered native mussels – some still alive – they believe to be over a century old.
The find was incredible, say the scientists, because the host fish that mussels need to reproduce has been prevented from moving upstream since the construction in 1907 of the St. Croix Falls Dam near St. Croix Falls, Wisconsin.
In August, biologists from the Wisconsin Department of Natural Resources, the University of Minnesota and the National Park Service excavated part of the St. Croix River above the dam.
They were looking for spectacle mussels, a species of native molluscs named for their elongated shape, like a spectacle case. They excavated an area where the mussels had already been found over 30 years ago.
“There you go, we found them over there, in those rock piles up there above the dam,” said Lisie Kitchel, conservation biologist with the Wisconsin Bureau of Natural Heritage Conservation. “They must be very old mussels because their host fish is no longer above this dam. So they must have been there – or mussels in them – before the host fish was lost. “
Some of the mussels were dead, but about eight were still alive, she said.
Typically, biologists can determine a mussel’s age by studying the growth rings on their shells, such as a tree. However, these mussels were so old and their shells so eroded that scientists couldn’t see the lines, Kitchel said. They plan to examine cross-sections of dead mussel shells this winter to determine their exact age.
Biologists believe they must be over 100 years old, given the dam’s construction schedule over a century ago.
Like other mussels, the spectacle case expels larvae, called glochidia, which must attach to the fins or gills of a host fish in order to continue to develop into juvenile mussels.
Since the dam was built, the fish that host the Spectacled Mussels, Golden Eyes and Moon Eyes, cannot travel up the river that far.
Biologists have returned the live mussels to their riverine habitat. It is doubtful that they still produce glochidia in their advanced age, Kitchel said.
“Once this population disappears, there is no way for them to reproduce,” she said. “We can propagate more spectacle cases and put them up there. But without the host fish, they’re not going to continue to breed.”
Biologists worked to restore native mussel populations to rivers in the Upper Midwest, including the St. Croix and Mississippi, where many species of molluscs were extinct.
Native mussels are filter feeders that can siphon 10 to 15 gallons of water per day, helping to remove pollutants from rivers and lakes, Kitchel said. They also excrete nutrients that are an important food source for other organisms.
Spectacled mussels are on federal and state lists of threatened species in Minnesota and Wisconsin. They face many threats, including population loss from dams, pollution, excess sediment that obscures the water, and competition from invasive species.
Kitchel said the finding of mussels was a sign of the health of the Holy Cross River, one of the first rivers in the United States to be designated for protection under the Federal Wild and Scenic Rivers Act in 1968.
“There haven’t been a lot of things that have destroyed the habitat or altered the habitat,” she said. “The quality of the water is really good in Sainte-Croix.”
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