Freshwater mussels have a particularly unusual system of reproduction. Males release their sperm into the water with the hope that a nearby female will siphon them up to fertilize her eggs. Once fertilization occurs, the female produces larvae called glochidia, which she then releases back into the water. At this point the mother is out of the equation. The glochidia (if they're lucky) find a nearby fish, attach themselves to the gills or fins, and spend a few weeks parasitically absorbing nutrients and oxygen from their hosts. A few weeks later it's back into the open water again, as the now-microscopic-size juvenile mussels start the rest of their lives, which could last as long as 50 years.

That complex system depends on a number of factors, including the appropriate number of parent mussels, the presence of suitable host fish and flowing clean water. For many decades now the purple cat's-paw pearly mussel (Epioblasma obliquata obliquata) hasn't had any of that. Once common in the Ohio River and nearby tributaries, the mussel nearly disappeared after the river system was dammed. (Overcollection for the button market in the 1900s, which made use of their shells’ purple lining, also played an earlier role in their decline.) By the time the mussel was added to the Endangered Species List in 1990 only a few individuals were known to exist. The subspecies was actually labelled functionally extinct.

Even so, the U.S. Fish and Wildlife Service (FWS) published a recovery plan (pdf) for the mussel in 1992. As they wrote at the time, saving the purple cat’s-paw pearly mussel would do more than prevent its extinction. It would also help play a role in preserving important habitats for other species as well as the watershed for the entire region. This, in turn, would benefit local commercial fisheries. A healthy mussel population, meanwhile, would help keep the water cleaner in the future.

Although things looked bleak at first, hope emerged just a few years later. A small breeding population was located in Killbuck Creek, Ohio, in 1994. Biologists had reason to believe that maybe the mussel wasn't going to go extinct after all.

It wasn't going to be easy, however. Killbuck Creek itself isn't very healthy, with heavily degraded conditions due to agricultural runoff and pollution from the petroleum industry. That one last population of mussels crashed soon after discovery. Surveys in 2006 and 2007 found a total of 12 males and zero females.

And there things stood, until a 2012 drought lowered the speed of Killbuck Creek, making it easier for biologists to survey the area. This time they struck gold: 15 males and 10 females. The biologists placed all 25 in special in-stream cages so the mussels could be monitored and protected—and, hopefully, breed.

They did just that. During the 2013 spring breeding season biologists brought the 10 females to the Columbus Zoo and Aquarium's Freshwater Mussel Conservation and Research Center. Six of the female mussels, it turned out, were carrying glochidia.

Keeping all of those mussels in one location was too risky, so the six females were split up among three facilities: Columbus Zoo, the FWS’s White Sulphur Springs National Fish Hatchery in West Virginia and the Center for Mollusk Conservation in Kentucky. The glochidia were extracted and placed into containers that contained the necessary host fish. A few weeks later they had success: 13 of the hundreds of transplanted glochidia at White Sulphur Springs grew, fell off the fish and are now healthy juveniles. This was much lower than expected, but it was considered a good first step.

The whole process is currently underway again, reports Angela Boyer, an FWS endangered species biologist. This year the females were left in their creek cages a few weeks longer in hopes that the larvae would have more time to mature, which could increase their chances of survival. Although it's too early to say for sure, she says "at this time it appears we are having similar results to the 2013 propagation efforts." Another dozen juveniles, if we're so lucky, would effectively double the known population the species. A few dozen and we could actually be talking about the beginning of a species recovery, although it will still take at least another decade for any juveniles to mature and start reproducing themselves.

The breeding efforts also got a boost this month in the form of a new grant from the FWS's State Wildlife Grant Program. The $397,000 grant (plus another $167,000 in nonfederal matching funds) supports a multistate effort to augment imperiled mussel populations for several species, including the purple cat's-paw. "Funding from the 2014 grant will help support the ongoing and future propagation and reintroduction efforts that we are working on for this species," Boyer says. "More specifically, it will help to support the search efforts for additional females for propagation, the cost associated with the propagation—transportation, housing, etcetera—as well as restoration and investigations into potential reintroduction sites within the species’s historic range." FWS has already put work into habitat restoration and has used landowner incentive funds to build livestock fencing and off-stream water supplies for farmers to restore 10 kilometers of Killbuck Creek.

For now, though, the purple cat’s-paw pearly mussel remains the rarest mussel in North America. Luckily, it has a good team looking out for it. "It is hard to believe that I am one of the few to work with this species," Boyer says. "It truly reminds me that I have a very amazing job. The sense of responsibility is huge."

Photos: U.S. Fish and Wildlife Service