Musseling their way out of endangerment
All this time, the mussels were lurking in the shallow waters of Ohio’s Killbuck Creek.
For decades, scientists suspected that the endangered purple cat’s paw pearlymussel was nearly extinct. Thanks to the U.S. Fish and Wildlife Service and a team of partners, the mussel may soon be in the throes of a comeback.
Surveyors successfully located 10 females in Killbuck Creek in 2011. Six of the mussels were carrying their offspring, which is called glochidia. The glochidia act like parasites and leave the female mussel to hook themselves to the fins or gills of fish. In doing this, the larvae consume nutrients and grow to be juvenile mussels.
Keeping all six of the mussels together in one location would be too risky, so they were split evenly among the Ohio Columbus Zoo, the Center for Mollusk Conservation in Kentucky, and the Service’s White Sulphur Springs National Fish Hatchery in West Virginia.
Rachel Mair, a fish biologist with the Service, was granted permission to take the mussels to the lab to extract the glochidia, in hopes of breeding them in captivity herself. Using host fish, Mair was able to advance the purple cat’s paw reproduction rate. In 2013, Mair announced that 13 of her larvae successfully metamorphosed at the White Sulphur Springs National Fish Hatchery. This was a big milestone for the species, nearly doubling its known population.
But biologists needed bigger results, more like hundreds or even thousands of mussels each year.
A novel method called in vitro culture helped them do just that. A portion of the pregnant mussels was sent to Monte McGregor, a malacologist at the Kentucky Department of Fish and Wildlife Resources. McGregor extracted the glochidia by injecting sterile water into the mussels using a hypodermic needle. That flushed them out of the mussel and into a petri dish filled with rabbit blood serum.
He likens the nutrient solution to cherry kool-aid.
McGregor kept an eye on the petri dishes, waiting for the larvae to open. The open larvae are placed into a new test tube and transported to a small incubator, where temperatures and carbon dioxide levels are kept low to simulate fish blood. A few days pass, a few nutrients are added, and after 17 days, they begin to crawl. McGregor said to think of it like a caterpillar. The larva has to turn into a juvenile mussel and then miniature mussel.
Tyler Hern, another Service fish biologist, noted that this method “drastically reduced the timeline for saving the purple cat’s paw from extinction,” because McGregor was able to bypass an unreliable middle man: the host fish. The challenges with host fish can be crippling: different mussels use different fish and some fish don’t do well in captivity or are in small populations themselves. Not to mention, at each step — from which larvae make it on the host to which survive and drop off — biologists lose larvae.
Hern said that compared to traditional methods, “the in vitro culture allowed partners to transform a much higher percentage of the few larvae that were available.” Without this method, building up the population could be long and unpredictable. If only a small percentage survived long enough to leave the fish host, it could be a year before the opportunity arose again, Hern said.
Thanks to efforts across the facilities in Kentucky, Ohio and West Virginia, thousands of purple cat’s paw pearlymussels have been reared in captivity. In October 2017, the team released 400 cultured juveniles across eight different sites. About 50 were dispensed into each site: two sites in the Licking River, Kentucky; two sites in the Green River, Kentucky; two sites in the Duck River, Tennessee; one site in the Ohio River, West Virginia; and one site in the Walhonding River in Ohio.
When biologists checked in on the West Virginia location, they were pleased to see that the mussels had nearly doubled in size over the year. The team plans to add another 1,500 to 2,000 purple cat’s paws at three sites in 2019.
When a species’ population is literally on the brink of going extinct, “it is crucial that every opportunity, no matter how small, is maximized for recovering that species,” Hern said. The recovery of the purple cat’s paw pearlymussel using both methods holds powerful potential for other endangered mussels.
But there are probably less than a dozen people using the in vitro method.
“All mussel species benefit from the further development of in vitro culture techniques, but especially the rarest ones,” said Hern.
Service staff, including biologists Angela Boyer and Leroy Koch, provided funding, searched for adults in the wild and transported adults from Killbuck Creek to the recovery team.
“Teamwork from multiple people from various agencies have been crucial to the success on our end,” McGregor said.
