Big Darby Creek Watershed

Mussels

Life Cycle of a Mussel
© The Ohio State University

Freshwater mussels belong to the order Unionoidea. Almost one-third (approximately 300 species) of all known freshwater mussels occur in the United States, making it the most diverse nation on Earth for this group. The Darby Creek watershed supports a diverse mussel fauna composed of 38 species. By comparison, Europe has only 10 known mussel species, Africa 56, India 54, and China 38 (Bogan 1993).

The federally endangered Clubshell, Northern Riffleshell, and the state endangered Snuffbox, Elephant-ear and Rabbitsfoot are present. It has been reported that for its size, the Big Darby Creek watershed has the greatest diversity of freshwater mussels in North America, and perhaps on earth.

Vulnerability
Because mussels respond to changes in water quality, they can also tell us something about the health of the environment on which we both depend. Although water quality has improved in some areas, pollution, especially non-point source pollution, causes the greatest threat to native mussels. Gradual mussel die-offs or sudden mussel kills are reliable indicators of water pollution problems and other environmental health concerns.

As a whole, organisms that depend on freshwater ecosystems are in grave condition. The 1997 Species Report Card, released by The Nature Conservancy in cooperation with the state Natural Heritage Network, found that 67 percent of U.S. freshwater mussels are vulnerable to extinction or are already extinct; more than 1 in 10 mussels may have become extinct during this century alone.

Biology
Most freshwater mussels live burrowed in sand and gravel at the bottom of rivers and streams. Unlike most animals, which must travel in search of food, their food drifts to them, mainly tiny plants and animals called plankton suspended in the water. By drawing water inside their shells through a siphon, their gills filter out food and take in oxygen.

Mussels usually do not move much, but a muscular “foot” helps them burrow and allows limited travel if disturbed by floods or drought. The foot also helps anchor them against strong currents. Their hard, calcium-based shells consist of two halves joined by a hinge. Unique names like “fawn’s foot,” “purple pimple-back,” and “white heelsplitter” refer to the wide range of shell size, color, shape, and texture found among mussel shells.

Mussels are long-lived, with many species living more than 10 years and some reported to live more than 100 years. Mussels are an important food source for many animals, including muskrats, minks, otters, fishes, and some birds

Life Cycle
Freshwater mussels need a lot of luck to successfully reproduce. Their unusual life cycle begins when eggs held inside the female are fertilized by sperm drawn inside her while siphoning water. For most species, if a male of her kind is not nearby upstream, she cannot reproduce. For a few, this is not a problem because they seem able to fertilize themselves.

Once fertilized, the eggs develop into a larval stage, called glochidia, inside the female. When they mature they are released into the water. With little time to waste, they must attach themselves to a host fish or perish. Here they live off the host’s tissue for a few weeks before the larvae transform into young mussels and are ready to drop off the fish and begin a life in the stream bottom.

To increase the chances of their young making contact with a fish host, some females display specially adapted tissues that look like fish prey, and try to lure fish to swim near them. When they sense a fish nearby, the female releases her young into the water, ready to attach onto the fish.

Conservation Status
Freshwater mussels are one of the most endangered groups of animals in North America. Surveys conducted over the past few decades have documented significant declines in mussel populations. Although water quality has improved in some areas, pollution, especially non-point source pollution, is a great threat to native mussels. Also sedimentation continues to take a serious toll. Habitat losses through channelization, clearing of riparian and streambank vegetation, dredging, and dam construction still persist. Mussels are also impacted by the loss of fish hosts from fish kills or dams that prevent fish migration. There is also competition from exotic species such as the zebra mussel. Poachers also impact mussels by violating harvest regulations set by conservation agencies.