Northern Riffleshell
by Catherine Gatenby, Ph.D.
It only looks painful. This fish lightly grasped by a northern riffleshell mussel is being inoculated with larval mussels, called glochidia. The young mussels ride the host to their next stage toward maturity. They eventually fall off and live a life on a stream bed. The host fish rarely perish as a direct result of mussel infestation.
Photo Credit: Mathew Patterson, USFWS
They sit nearly as still as a stone, river water percolating around them—and through them. The northern riffleshell (Epioblasma torulosa rangiana) is so named for its hard rippled exterior as much as it is for the habitats it uses. This American mussel was brought to the fore of science by an American businessman with diverse interests.
Based on the prodigious writing Isaac Lea did on the subject of freshwater mussels, you would think that is what he did for a living— conduct scientific inquiries into freshwater shellfish. But it was not. Lea was a Philadelphia book publisher with an ardent interest in animals with shells, and that’s not all. He published his first scientific paper at age 36, and would keep it up for another 52 years. In the end, Lea inquired into natural history, geology, and paleontology, but it was mussels where he mostly made his mark. He described hundreds of new species of snails and mussels from the heartland of the North American continent. The northern riffleshell was among them, described for science in 1838, in some of his earliest writings.
Historically, the northern riffleshell occurred in rivers and streams of the Great Lakes and the Ohio River drainages. It could be found in western Ontario to Alabama, from Kentucky to West Virginia. Today the northern riffleshell occurs in only a few short reaches of the Green River in Kentucky; the Detroit and Black rivers in Michigan; Big Darby Creek in Ohio; and French Creek, LeBoeuf Creek and the Allegheny River in Pennsylvania. It’s the latter site where the largest remaining population of the rare mussel lives at present.
The northern riffleshell was federally listed as an endangered species in 1994. A major cause for the decline of the northern riffleshell is loss of suitable habitat for both the mussel and its host fish.
Mussel conservation involves many aspects of biology and habitat, but a host fish is essential to a mussel. Simply put, a mussel cannot reproduce without its host fish. Indeed, in order to complete metamorphosis—to transform from the larvae to adult — larval freshwater mussels must attach to a host fish where they derive nutrition and possibly some immune resistance.
Scientists haven’t yet deciphered all that a mussel gains from the host fish. Nonetheless, a mussel needs a fish in order to complete full development to adulthood. Damming of rivers, channelization, and pollution have all led to the decline in host-fishes, which in turn has led to the decline of freshwater mussels. The northern riffleshell uses a host-fish, such as a sculpin or darter to complete its life cycle. She accomplishes this by attracting her unsuspecting host-fish through the use of a lure. She lies in wait on the bottom of the river, set on her dorsal side, her valves gaping open displaying her pearlescent blue mantle tissue, and a small wormlike structure that spins around. Those little darters and sculpins are such curious fellows, they think they see food. They sneak up to investigate, hoping to eat something, and the female rapidly clamps her valves shut, trapping the sculpin or darter between her valves. Only the fish’s head is trapped, its body and tail wave as it tries to escape. But to escape is in vain. After a few moments it stops trying and in about 15 minutes, the female mussel releases the fish. Though the fish might appear a bit stunned as it swims away, its head and gills are covered with attached mussel larvae that anchor into fish flesh.
Scientists believe that only the larvae attached to the gills metamorphose because the gill tissue is full of good proteins, fats, and sterols. Lest you be concerned about the health and welfare of these unsuspecting host fish, let me provide you with a little assurance. Russian scientists have evidence to suggest that fish that have been inoculated by mussel larvae are more resistant to stresss. Thus, both fish and mussels may benefit from this host-parasite relationship.
Recovering this endangered species requires establishing viable populations in 10 river drainages. Because of their complex life cycle and because the northern riffleshell’s range has been severely reduced, it could be decades if at all before the northern riffleshell recolonizes and reestablishes itself into new and improved habitat. The recovery of northern riffleshell will, therefore, require that additional self-sustaining populations be established in other rivers in stream-to-stream transfers, or stocking new waters with hatchery-reared mussels. The White Sulphur Springs National Fish Hatchery in the West Virginian town of the same name has an aquaculture program underway for rearing endangered northern riffleshell. This will allow the U.S. Fish and Wildlife Service and its partners to recover this species through stock enhancement of dwindling populations, and by reintroducing the rare mussel into new drainages where the habitat has been restored.
White Sulphur Springs National Fish Hatchery, the U.S. Fish and Wildlife Service’s Pennsylvania Ecological Services, and Ohio River Islands National Wildlife Refuge recently teamed up to release over 400 young northern riffleshell into the Allegheny River. The one-year-old mussels produced at the hatchery were large enough to tag, making it possible to come back at a later date and monitor their growth and survival in the wild. Starting in 2004, divers from the U.S. Fish and Wildlife Service, the Environmental Protection Agency, and the West Virginia Division of Natural Resources worked with the Pennsylvania Department of Transportation to remove approximately 250 adult northern riffleshell from under a new bridge project at East Brady, Pennsylvania.
The mussels were relocated to the hatchery, a temporary refuge. While held at the hatchery, female northern riffleshell became gravid, providing hatchery biologists with an opportunity they were hoping for—to propagate northern riffleshell in captivity and develop an aquaculture program aimed at recovering this endangered species.
Catherine Gatenby, Ph.D. is the manager of White Sulphur Springs National Fish Hatchery in WV.
Editor’s note: This article originally published in the Fall/Winter 2011 edition of Eddies: Reflections on Fisheries Conservation.
