The men were on a reconnaissance mission, but not exactly the spy type. Their target was one of the last remaining colonies of Appalachian elktoe mussels in the world. As biologists with the U.S. Fish and Wildlife Service, they are charged with protecting endangered species, and the elktoe mussel is at the top of their list in the region.
Freshwater mussels look surprisingly like the salt-water kind that might end up on a platter in a seafood restaurant. But they are terribly tricky to find. During last week’s expedition, the Tuckasegee was so clouded with silt that scouting the river bottom was nearly impossible without also scraping your nose. But John Fridell, one of the USFW biologists who is known also as “mussel man,” seemed to have a knack for the sport.
“It’s like looking for anything. After a while you develop a search image for it,” Fridell said.
Complicating matters, the Appalachian elktoe mussel looks a lot like a rock. Its preferred station in life is being partially burrowed into the river bottom, its shell slightly cracked with a small siphon poking out. The siphon serves as its life line, drawing water into its shell, through the gills and back out again, filtering out anything useful in the process.
“It brings in their food, their oxygen and the females get the sperm that way,” said Gary Peebles, also with the USFW team.
The elktoe’s method of reproducing is particularly complex and likely took eons to perfect. At the right time of year — right about now in fact — the male mussels release sperm into the water. Meanwhile, the females have socked away eggs in their gills waiting to be fertilized.
“Through her normal respiration, she picks up the sperm in the water column and it passes through her gills,” Fridell said.
The eggs develop into larvae inside the female’s shell over the winter. By spring, its time to send them on their way. But here’s where things get tricky.
While mom’s shell has gotten awfully crowded, the larvae aren’t quite ready to live on their own yet. The larvae have to take up residence on a new host, namely a fish, where they can continue to grow in safety for a couple of more months.
The female mussel shoots the larvae out of its siphon. If they’re lucky, a fish will be swimming past and the larvae will attach to the fish’s gills.
As if that’s not hard enough, only certain species of fish will do.
“If it’s not the right host, the fish can slough off the larvae,” Fridell said.
Some fresh water mussels only have one or two fish species that make suitable hosts for their larvae. For the Appalachian elk toe, there are at least 11 fish species that can fill the role. Figuring out which fish work for which mussels has to be done in the lab. Researchers take larvae out of a female and inject it into different fish species to see which one takes. It’s important to know, since the elktoe mussels’ long-term survival is dependent on a healthy population of their host fish, Fridell said.
Ensuring that the larvae latch on to the right fish takes either a lot of luck or superb coordination. Some mussel species got tired of leaving things to chance.
“Different species of mussels have developed amazing techniques to get their larvae in contact with the right species of fish,” Fridell said.
The wavy-rayed lamp mussel, for example, which lives in the Little Tennessee River, extends a flap of flesh from its shell that mimics a small fish favored by small mouth bass.
“It even has eye spots and tiny markings just like the real fish,” Fridell said.
When a small mouth bass closes in to eat the tasty morsel, the mussel sucks in the flap of flesh and squirts out a stream of larvae instead. Another fresh water mussel species forms its larvae into designer packets that look like an aquatic insect.
“The packets attach to rocks or aquatic vegetation. Fish will come to feed on it and they get a mouthful of larvae,” Fridell said.
The Appalachian elktoe doesn’t have a specialized way of getting its larvae in the mouth of the right fish, at least not that researchers know of yet.
After two or three months of living in the fish’s gills, the larvae have finally grown big enough that the fish can slough them off. Mussels aren’t very mobile, however, so they simply have to hope they are detached in a good place.
“Where ever they land, they are pretty much stuck,” Fridell said.
Of the thousands of larvae that a female mussel produces, only a few attach to the right fish, and even fewer get dumped off in a place that’s suitable for them to take up residence, Fridell said.
Then they have to hope they’re near other mussels — which can prove challenging for an endangered species — so they can reproduce.
The Tuckasegee River downstream from the Dillsboro dam is home to a colony of perhaps 200 to 300 Appalachian elktoe mussels. Fridell and others are doing their best to study and track the colony, learning as much about the lifecycle as possible to improve the chances of protecting the species.
Fridell found only one Appalachian elktoe on the mission last week, largely do to the silty water. It had small yellow number on its shell, meaning it was one that Fridell had tagged previously.