Purple cat’s paw pounces back
December 11, 2017
These shells (female, top right and male) are from the endangered purple cat's paw. Photo by USFWS.
Not long ago, purple cat’s paw pearlymussels were thought to be functionally extinct. When the species was listed as endangered in 1990, only a few individuals could be found, and they were too old to reproduce. Then, in 1994, a small population of the species was discovered in Killbuck Creek in Ohio.
A captive propagation and reintroduction program is among the ongoing efforts to save the endangered purple cat’s paw pearlymussel. Advances in captive propagation techniques led to an intensive effort in 2006 to find female cat’s paw to start a propagation program. However, biologists found that safeguarding the species would be a challenge. The wild population had declined significantly since its discovery, and only nine old males were found after extensive surveying efforts. In 2007, three additional males were found, but no females. The searching continued every year, yielding only a handful of old males until 2012 when Mother Nature provided a little help.
In 2012, much of the Midwest experienced drought. Ironically, this rainfall shortage provided exceptional survey conditions because Killbuck Creek is normally turbid with high flows that limit visibility and access. This survey yielded some encouraging findings: 15 males and 10 females from various age classes, including some younger individuals. Surveyors placed these mussels into in-stream holding cages so they could be collected the following spring when females would likely be carrying mature larval mussels, called glochidia. If any of the females were found to be gravid (pregnant) in the spring, they would be taken to mussel propagation facilities where the process would begin.
Freshwater mussels have an unusual and complex method of reproduction. Female mussels release glochidia directly into the water, and the glochidia must attach to the gills or fins of a specific host fish species to complete development. After attaching, glochidia transform into microscopic-sized juveniles within a few weeks and then drop off the fish, leaving the fish unharmed. The juveniles settle in the stream bottom where they filter feed and grow. In a captive propagation facility, these juveniles are kept for several years in tanks or raceways until they have grown large enough to tag and be released into streams.
In the spring of 2013, six of the 10 females from the cages were gravid with mature larval mussels. These females were divided among three mussel facilities. The facilities inoculated potential host fish with the cat’s paw larvae. Success was limited with these initial efforts, but ultimately 13 juvenile cat’s paw were produced.
The search for additional females in Killbuck Creek continued with additional females and males being found each year. In 2014, glochidia were again extracted from gravid females and, in addition to inoculating fish, in vitro culture methods were employed. In vitro culture uses a medium in a petri dish, rather than fish, to fulfill the nutritional needs of the glochidia. The in vitro process proved to be very successful, with more than 2,700 juveniles currently surviving from this method.
The captively propagated cat’s paw have grown large enough this year for reintroduction into streams in the historical range of the species. Streams targeted for reintroduction were chosen based on their high water quality, site protections and robust fish and mussel communities. All targeted reintroduction streams also currently support other federally listed mussel species. Five streams in four states (Kentucky, Ohio, Tennessee and West Virginia) were selected for cat’s paw reintroductions.
This fall biologists launched eight pilot studies in these five streams. At each study site, 50 juvenile cat’s paw were tagged and released. Biologists will monitor these mussels over the next year to determine their survival rate and growth. Sites showing high survival and growth rates will then receive additional, larger numbers of juvenile cat’s paw in an effort to establish reproducing populations of the species to help recover the species.
Meanwhile, captive propagation will continue in the hopes of producing many more juvenile cat’s paw for future stocking. The tremendous effort put forth to save the purple cat’s paw is a prime example of how state, federal and private partners, working together for a common goal, can accomplish the extraordinary.
Service Biologist Angela Boyer uses a passive integrated transponder (PIT) tag reader to locate released mussels. Photo by USFWS.