Between a Rock and a Hard Place

Consider the plight of the crayfish crossing the road. It lumbers along, waving tiny pincers threateningly as your shadow falls across it. You stoop and grasp the cool, damp carapace between thumb and index finger. It twists, squirms, tries to fend you off. You deposit it gently in the ditch it seemed to be headed for. It scuttles for cover with nary a backward glance.

Crayfish are victims of paradox. These small lobster-like creatures are familiar to most country folks. They’re a staple of Cajun cuisine. Fishermen use them for bait.
Yet breathtakingly little is known about them, even by biologists.

Life histories have been worked out for less than 10% of the world’s 500 or so species and subspecies. As a group, crayfish are believed to be one of the most imperiled fauna in North America, second only to freshwater mussels. Yet only four are federally listed as Endangered Species: two cave species from Arkansas, the
Nashville (TN) crayfish, and California’s Shasta crayfish.

“There is a major void in our information,” said Bob Butler, a riparian lands restoration biologist with the U.S. Fish and Wildlife Service’s Asheville office. “There just aren’t very many people studying crayfish. They’re so poorly known that more than half of them don’t even have common names.

“On the other hand, new species are being discovered all the time. Between 1988 and 1996 alone, 45 newly recognized species were formally described. If someone goes into a remote drainage and starts collecting, chances are he’ll turn up some that haven’t been found previously.”

The Fish and Wildlife Service has funded crayfish status studies for the past 20 years.

“We think that crayfish — in general — are more tolerant of habitat alteration than was first supposed. But that’s a very general statement,” Butler warned. “While the level of imperilment may not be as bad as we once thought, there are bound to be some species out there that are not tolerant, and that may be in trouble. We’re trying to sort that out now.”

Crawdad Central

Crayfish are native to all continents except Africa and Antarctica (and recent Antarctic fossil finds indicate that, nearly 300 million years ago, they inhabited that region as well). But there are two global centers of crayfish diversity: the Southeastern U.S., and Victoria, Australia. Two of the three crayfish families — the astacids, cambarids and parastacids — and a majority of the world’s species and subspecies occur on the North American continent.

Crayfish — known in the vernacular as “crawdads,” “crawlybottoms,” ditchbugs,” and “river lobsters” — are crustaceans, like their well-known relatives, the shrimp, lobsters and crabs. Decapods, they have five pairs of segmented legs, the first set of which are greatly enlarged and end in elongated pincers (or chelipeds). Like other crustaceans, crayfish breathe with gills and have two sets of antennae. Though most crustaceans are marine, crayfish are important components of freshwater ecosystems.

While some of Australia’s parastacids are lobster-sized — weighing as much as 12 pounds and measuring as much as two feet in length — North American crayfish are small, seldom exceeding six inches. Nearly 80% of the world’s crayfish taxa — 393 species and subspecies — are found in North America.

West Coast drainages are home to a few members of the astacid family, but most North American crayfish are cambarids — and 95 percent of the cambarids live in the Southeast. Indeed, 68% of US crayfish species are limited to “an area defined as east and south of — and including — the state of Kentucky,” said crayfish expert Christopher Taylor.

Why is the Southeast so rich in crayfish species? “Because it was never flooded and never glaciated,” Butler said. Additionally, its varied physiographic provinces and habitats — high- and low-gradient streams, cave systems, and watercourses with a variety of substrates — provided crayfish with innumerable niches in which to settle and evolve.

Crayfish can be loosely grouped in three main categories, according to habitat: surface water dwellers, who live in rivers, creeks, lakes, ponds ditches and swamps; burrowers, who dig complex tunnels down to the water table; and cave (troglobitic) species.

Like many other cave-dwelling animals, troglobitic crayfish have adapted to life in the dark, becoming pale, with reduced eyes and slower reproductive and metabolic rates. While most crayfish live only a few years, cave-dwelling species may survive for decades, perhaps as long as a century.

If cave species are longer-lived, they are also more sensitive to pollution and other disturbances. Habitat specialists, they have “very highly adapted sensory systems,” Butler said.

Their antennae are longer, and they rely on chemoreceptors, and the senses of touch and smell to make a living in the dark. Highly adapted sensory systems are more easily disrupted by pollutants.

A recently-discovered cave system on the Cumberland Plateau near Chattanooga has turned up 24 subspecies of cave-dwelling animals, including crayfish. Environmentalists are concerned that a proposed wastewater treatment plant in the vicinity of the cave will degrade that environment before it has been fully explored.

Interestingly, while cave-dwelling crayfish are found in most major karst regions on the continent, including the Ozark and Cumberland Plateaus, they are notably absent from caves in the Southern Appalachian Ridge and Valley province. Why they’re not there “is a mystery,” Butler said.

Narrow Range

About 50 species of crayfish inhabit the Southern Appalachians; newly discovered species may increase those numbers.

“It’s a very dynamic field,” Butler said. About 20 of the 50 species in our region are believed to be “in trouble,” though their status is subject to change. That’s because, while many species are narrowly endemic — some are known from a single stream — once researchers start looking for them, they sometimes turn out to be more abundant than previously supposed.

That’s the case for the Hiwassee Crayfish (Cambarus hiwasseensis), once listed as a federal species of special concern. In 1999, after it turned up in many new sites, the NC Natural Heritage Program moved C. hiwasseensis from its Rare Animal to its Watch List.

Ditto for the French Broad Crayfish (C. reburrus), after it was discovered to be “fairly common in appropriate habitats.” Two other crayfish species were moved from the Rare to the Watch List that year because their range and life histories were too poorly understood to justify listing as rare “at this time.”

On the other hand, a new species —the Broad River Stream Crayfish (C. lenati) — appeared on the Rare Animal List for the first time in 2001, as a “newly described species endemic to North Carolina.”

Narrow endemism is a serious consideration when it comes to crayfish. “More than 65 taxa are currently known from a single locality or a single river drainage in a 1- to 2-county area,” Taylor and his co-authors noted in a 1996 summary of the conservation status of U.S. and Canadian crayfish species published by the American Fisheries Society.

While crayfish are threatened by habitat destruction or degradation, pollution, and the introduction of non-native species, “small natural range is also implicated as a factor underpinning potential or realized imperilment.,” Taylor and his colleagues said. Their survey documented 11 crayfish species known from single localities; another 20 from 5 or fewer sites; and 83 more from seven or fewer locations.

“Taxa restricted in range to an area of 100 square miles or less are particularly vulnerable to habitat destruction or degradation in the form of stream channelization, dredging, siltation, acid mine drainage and impoundments,” they wrote. “The overarching reality is that a species with a small range is more vulnerable to extirpation and should be considered for conservation measures.”

“The list of ‘potentially imperiled’ crayfish is long because so many are so narrowly distributed — simply because they are rare,” Butler said. “But to be considered ‘imperiled,’ there has to be a threat on top of that.” The FWS has been contracting with experts to conduct status surveys (to determine population and distribution) of potentially imperiled species.

“We’ve done a little work on crayfish,” he said, “but we need more information on a lot of species.”

Alien Invaders

If narrow range is a problem for many of our native crayfish, so is the introduction of exotic crayfish species through aquiculture or bait bucket releases. Crayfish are a primary bait for smallmouth bass. At the end of a fishing trip, anglers often dump their remaining bait over the side of the boat.

The result: species such as the rusty crayfish (Orconectes rusticus), commonly raised and sold for bait, have dramatically extended their ranges. “Some of the bait species are more aggressive than the native crayfishes they displace, and are able to out-compete them for shelter,” Butler said. “That makes the native species more vulnerable to predation.”

Introduction of non-native crayfish “has contributed to population declines of native crayfishes and represents an ongoing threat,” Taylor reported. “Both the rusty crayfish and the signal crayfish (Pacifactus leniusculus) are native to the United States but displace native species when they are transplanted into river basins outside their native ranges.”

The larger body size and aggressiveness of the rusty crayfish has helped it displace native species from New Mexico to Maine. Hybridization between it and native species “has also been suggested as a displacement mechanism.”

Displacement can occur quickly. Within 24 years of the 1933 introduction of a non-native species into Mountain Lake, VA, the lake’s native crayfishes had been eliminated. The endangered Shasta crayfish has been displaced from much of its range by an introduced species. Despite this kind of evidence, few states have adequate or enforceable laws to prevent introduction of non-natives. States that have enacted laws already have well-established non-native populations.

And federal legislation governing the introduction of non-native species from foreign countries is “practically non-existent,” Taylor said. “This is particularly troubling given the fact that aquacultural facilities and markets for foreign species are being developed in the U.S. without an adequate understanding of the potential effects of non-natives on native species. If we are to protect our native crayfishes, especially the endemics with small ranges, proactive and effective legislation must be implemented at both state and federal levels.”

Natural History

While they remain basically unprotected, crayfish are important to the functioning of aquatic ecosystems — so important that some ecologists consider them keystone species. Opportunistic omnivores, they feed on a wide range of plant and animal material, living and dead: aquatic and terrestrial plants, plant detritus, insects, snails and small aquatic vertebrates. Their bodies are even equipped with a filter-feeding mechanism.

In turn, they are preyed upon by everything from alligators to insects. Fish, frogs and salamanders, wading birds and kingfishers, snakes and many mammals eat crayfish. To avoid predation, most take shelter during the day — under rocky substrate, in vegetation or woody debris. Many are highly territorial, aggressively guarding their shelter sites.

Research indicates that several species may occupy different habitats within a single body of water, and that a single species may occupy different habitats at various life stages.

Life histories differ from species to species. Many mate in late fall and winter (some in early spring). The male deposits sperm in the female’s sperm receptacle, then stoppers it with a mucous plug. The sperm remain viable until fertilization, after which the female glues the eggs to her swimmerets (paddle-like appendages attached to her abdomen).

She sequesters herself in a safe spot while the eggs develop, constantly aerating them with the fanning motion of her swimmerets. Because the eggs resemble berry clusters, a female in this condition is referred to as “in berry.”

After three or four weeks, the eggs hatch, in late spring or summer. The young stay with their mother for several weeks. Juveniles pass through three hatchling stages, looking more and more like their parents with each molt. Cambarid species, who generally do not live more than 5 years, may reach sexual maturity within the first year.

Male cambarids, once they reach sexual maturity, switch back and forth between two distinct forms. Form I males are reproductively active; Form II males aren’t. The two forms are told apart by the shape of their first set of gonopods (modified swimmerets used for sperm transfer during copulation).

Generally, males spend the summer months in Form II, then molt into Form I at season’s end. While young crayfish must molt often as they grow, adults may only molt twice a year. Molting periods are dangerous for crayfish; shedding their exoskeletons leaves them more vulnerable than usual to predators and pollutants.

Despite their reclusive ways during the daylight hours, crayfish are surprisingly colorful. They come in a rainbow of colors — brilliant blues, greens, oranges, reds and yellows, “sometimes in amazing combinations,” Butler says. Their shells may be patterned, mottled, or sport bold stripes and bands. Others are bleached and pale.

Body types vary too, from delicate, shrimplike cave species who never see the light of day, to robust burrowers whose mudball towers mark tunnel entrances on low-country lawns. While only two of our native crayfish species are thought to be extinct, 48 percent are considered imperiled. Burrowers and cave species are particularly difficult (and expensive) to find, capture and study.