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Captive Breeding Brings Hope for the Purple Cat's Paw
By Angela Boyer
Photo Credit: USFWS
North America boasts the most diverse assemblage of freshwater mussel species in the world. But this group of animals is now among the most imperiled due to intensive harvest to satiate a wave of pearl fever in the mid-1800s, the button market in the early 1900s, and loss and degradation of their stream and river habitat.
One of the nation's rarest freshwater mussels, the purple cat's paw pearlymussel (Epioblasma obliquata obliquata), was once widespread in the southern Ohio River and its larger tributaries before they were dammed. These changes in river flow led to a slow and steady decline of the species over many years. On the brink of extinction, with just a handful on known individuals surviving in the wild, the species gained Endangered Species Act protection in 1990.
Renewed hope for survival came four years later, when a small population was discovered in Ohio's Killbuck Creek. However, water quality in the creek degraded to such an extent that drastic measures were necessary to ensure the purple cat's paw mussel's survival. The U.S. Fish and Wildlife Service (Service) initiated a search for surviving individuals in 2006, and efforts to collect purple cat's paw for captive propagation began.
Photo Credit: USFWS
Biologists learned that safeguarding the species in captivity would be a challenge. After an extensive search for the endangered mussel, biologists determined the status of the only known remaining population in Killbuck Creek was even more dire than previously thought. Days of searching yielded only a handful of individuals—all mature males. The absence of females suggested the species may be functionally extinct in the wild – with no reproduction taking place – placing even more pressure on captive breeding to keep the species alive.
Hope was restored once more in 2011, when the shell of a recently expired female was discovered, indicating recent reproduction in the creek and the possibility that younger males and females still survived.
In an ironic twist of fate, the drought the following year caused low-flow conditions in Killbuck Creek, which provided excellent survey conditions for biologists who found 25 healthy purple cat's paw mussels, including 10 females.
The mussels were placed into in-stream holding cages so that they could easily be retrieved the following spring, when fertile females would likely be gravid, or carrying mature larval mussels called glochidia.
Freshwater mussels have an unusual and complex method of reproduction. In nature, mussels rely on host fishes to provide essential nutrients and protection for developing larvae. Female mussels release glochidia directly into the water, where they affix themselves to the gills or fins of a specific host fish species. After attaching, glochidia transform into microscopic-sized juveniles within a few weeks and then drop off the fish, ready to fend for themselves. While the fish are oblivious to their role, they are nonetheless essential to completing the transition from a larval to juvenile mussel.
In March 2013, biologists removed the females from the cages and found that six of them were gravid with larvae. The gravid females were divided among three facilities for propagation, including White Sulphur Springs National Fish Hatchery in West Virginia, the Columbus Zoo and Aquarium Freshwater Mussel Conservation and Research Center in Ohio, and the Center for Mollusk Conservation in Kentucky.
Photo Credit: USFWS
Each mussel facility extracted larvae and introduced them onto host fish. The purple cat's paw mussel is known to use some darter and sculpin species as host fish. In addition to these fish, the Kentucky facility also attempted in vitro propagation to substitute growth medium in Petri dishes. Malacologists at the Kentucky facility have had success with the in vitro method for other species, including the endangered snuffbox mussel.
Again, biologists waited eagerly for results while the glochidia matured on the fish. Several weeks later, the first ever captively propogated cat's paw juveniles were born at White Sulphur Springs, when juvenile mussels successfully developed and fell off the fish. While the success rate was not nearly as high as biologists had hoped, the event marked a significant milestone in the species' recovery effort. Currently, there are 13 surviving juveniles and all of these appear to be healthy and growing.
In April 2014, the process was repeated with females that are currently in a cage in Killbuck Creek. Biologists are hopeful that the captive breeding program will have continued success. Females will be left in the creek for a longer period this year to maximize the time allowed for the larvae to mature.
Angela Boyer, an endangered species biologist in the Service's Columbus, Ohio Ecological Services Field Office can be reached at firstname.lastname@example.org or 614-416-8993, ext. 22.
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