VENANGO TOWNSHIP -- Noodling" for mussels with Darran Crabtree on French Creek means never seeing a stream bed in quite the same way again.

This particular streambed rests below valuable and well-used regional fishing waters. Among the millions of rocks are millions of mussels. And though they are just as inanimate-looking as the rocks, these workhorses of the state's most ecologically diverse waterway are living filters that help keep habitat clean.

"They're fascinating," says Crabtree, a Meadville-based biologist with the Nature Conservancy, who went wading for mollusks on a recent rainy morning. "A lot of what they do we take for granted."

Crabtree reaches into the water and gently extracts a Kidneyshell from the substrate.

"Many things live on mussels besides algae," he says. "What we have here is a caddisfly. Mussels accumulate organic matter and don't digest it all, so there's usually a little hot spot ... a concentration ... of food near its siphon that caddisflies and other [insects] collect on and benefit from."

A healthy bed of mussels supports a lot of other creatures, he says.

"The stream bottom is three-dimensional, with layer after layer of life deep within."

Crabtree pulls out another mussel, this one resembling a large, smooth, elongated stone.

"Rabbitsfoot," he says," one of my all-time favorites. It has these pretty chevron markings and, unlike a lot of other mussels, isn't completely buried. You'll usually find it on its side, more exposed."

Rabbitsfoot is one of many quirkily-named species on French Creek. Others include Snuffbox and Pocketbook, names probably assigned by early settlers who depended on mussels for fishbait, material for buttons and jewelry, and occasionally even food. Although Pennsylvania has liberal creel limits for mussels -- 50 a day, year-round -- that soon could change in what Crabtree calls an effort to discourage over-harvest by collectors and amateur malacologists.

The necked-down, fast-moving Venango Riffle where we wade has one of the richest mussel supplies in the Northeast. Of its 22 species, 13, including Northern Riffleshell and Clubshell, are considered rare. But while they are one-in-a-million on other rivers, here they abound, as testament to mussel-power in maintaining the creek's health.

Collectively, mussels are actively processing the entire volume of the 117-mile creek from its headwaters in western New York to where it meets the Allegheny River in Franklin.

"They ingest some toxins," says Crabtree, "and they partition the indigestible materials, like agricultural and municipal runoff, coat them with mucous, and redeposit them in the substrate."

Although mussels do most of their work while staying in place -- some for as long as 100 years -- they can extend a hidden foot to escape danger. And they have unique ways of expanding their range.

"When they're doing their reproductive display, some mussels produce living tissue that looks just like an undulating shiner," says Crabtree. "When a bass comes over to nip it, he gets a face full of mussel larvae.

The larvae attach to the bass's gills and go for a ride. Days or weeks later, they hop off and start a new colony.

"This time of year is their last hurrah for getting larvae onto fish," Crabtree says. "Once temperatures drop below a certain degree, mussels bury themselves in the substrate and pretty much go to sleep until spring."

That French Creek is so pristine -- supporting up to 100 species of fish, from darters to walleyes to muskies -- is "a happy accident," Crabtree says, owing to the fact that no single land use has dominated the 330,000-acre watershed, half of which is forested.

"It doesn't have the intense industry, road crop agriculture or acid mine drainage that has affected other streams."

Keeping it that way is the challenge now facing conservationists and local landowners, he says.

Land trust officials from across the country, in Pittsburgh for their annual rally last week, were taken on a field trip to French Creek Thursday. The French Creek Valley Conservancy, the Nature Conservancy and the Western Pennsylvania Conservancy hosted the trip to demonstrate the watershed's unique value to those in the business of buying -- or convincing landowners to donate -- conservation easements.

"If people would sell their land outright to a developer or whoever else, they probably could get more money," says French Creek Valley Conservancy project director Tim Hecei. "But we're appealing to people who have a genuine interest in protecting their land."

Even property with little development potential can have great importance in terms of storing groundwater in the creek's flood plain, Crabtree says. "It helps maintains the flow regime; it keeps the dynamics in the river more natural."

In addition to arranging easements, the conservancies are educating property owners about best land use practices, including the herbicide-free removal of invasive plants, and the importance of creating vegetated buffers that absorb pollutants before they reach the stream.

Efforts so far encompass the main stem of French Creek and its 10 major tributaries, which include both coldwater and warmwater fisheries -- a combination that helps drive the creek's rich aquatic variety.

"It could be argued," says Crabtree, "that Tidioute, around Tionesta on the Allegheny, rivals French Creek in the number of species and number of animals. But for its size, French Creek is certainly more diverse. ... It's really a gem of a waterway."