I doubt you would have any negative issues with them as long as you have plenty of space, areas for them to hide and ample water so that smaller fish won't be subject to damage from the crayfish (see diet below). I also doubt that you will see any major impact on the string algae volume by having them unless you had a large population of crayfish, but they would be a beneficial partner in the overall system. The main problem that I can see would be, if they really don't care for your pond as their pad, they will move out in search of a habitat more suitable to their tastes. You would need a natural, deep, mud bottom for them to stay alive over the winter. They would need to burrow down into the mud in order to survive the cold without freezing. In the wild, they usually burrow into muddy banks at the water's edge, so they may require just moist, but not submerged mud. That would be difficult to offer to them in a pond with a liner. Otherwise you would have to recapture them and move them to an indoor aquarium to overwinter there. That may be highly entertaining for the family.
Here is an article from a Penn State Univ. student which may assist in your discussion:
Scientific name: Cambarus sp.
Common name: Crayfish
(Information in this Species Page was compiled by Alicia Fitzgerald in Biology 220W, Spring 2006, at Penn State New Kensington)
Crayfish (with special reference to genus Cambarus) are extremely distinctive freshwater crustaceans. Their large anterior-most pairs of legs have powerful claws which are efficient tools for defense, food gathering, and object manipulation. Their four pairs of walking legs assist them in rapid locomotion across the bottom substrates of their aquatic habitats. Surprisingly, they are not really capable of swimming although they can use powerful thrusts of their tails to rapidly propel themselves (backwards) through the water. This latter movement is particularly effective when the crayfish is attempting to elude a predator.
Distribution
Species of crayfish are widely distributed throughout the world and are found abundantly in most of the continental United States. They live in ponds, streams, rivers, and lakes most typically under submerged rocks and logs. Some species are also capable of digging burrows frequently topped with tall, distinctive “turrets” that are often located some distance from the water’s edge. On our Nature Trail, crayfish are found in the flowing waters of our stream and in the surrounding isolated wetlands.
Environmental Tolerance Limits
Crayfish are extremely hardy animals that can tolerate wide ranges of water temperatures and salinities. They can even survive the drying up and loss of their streams and ponds. They respond to these extreme habitat disruptions by aestivation in burrows or other refugia or by migration to still intact water sources. Crayfish are very intolerant of pollution and other human-generated fouling of their environment. A rich crayfish population, then, is a very positive index of habitat quality. Crayfish are more abundant in streams that have acidic water. This abundance may be due more to the acid-generated absence of fish which prey on crayfish than to a direct, positive influence of the acid on the crayfish itself.
Diet
Crayfish activity centers around food. When food sources are abundant, a crayfish tends to forage (i.e. leave its refuge under a rock or log) a very small percentage of the time. When food is scarce, though, crayfish will spend a considerable amount of time foraging. Crayfish consume both plant and animal materials and will, depending upon seasonal and local availabilities, consume a great variety of types of foods. Almost all aquatic plants, mollusks, insect larvae, mature insects, tadpoles, amphibian eggs, and small fish are common foods. Also taken opportunistically are small rodents and even small birds. Young crayfish must consume 1 to 4% of their body weight each day and tend to concentrate upon animal food sources. Adult crayfish, on the other hand, only require an intake of 0.3 to 1% of their body weight each day and tend to primarily take plant materials for their food. If living food sources are not available, crayfish will, at need, consume carrion. Crayfish tend to forage for food at night
Moulting
Crayfish, like all arthropods, have an encasing exoskeleton made out of the structural polysaccharide “chitin.” For a crayfish to grow, it must shed its exoskeleton and then re-grow a new and larger one. This shedding and re-growth process is called “moulting.” When a crayfish moults it is very vulnerable to both injury and predation and, so, must spend the two or three days it takes to re-grow its exoskeleton relatively inactive in its refugia. Young crayfish moult 6 to 10 times during their first year while older crayfish moult 3 to 5 times during their second (and, typically, final) year of life.
Mating and Reproduction
Crayfish mate in the early spring and females carry the fertilized, developing eggs inside their bodies for 4 to 6 weeks. These developing eggs are then transferred to the outside of the female’s body and glued via an adhesive called “glair” to the female’s tail. The eggs then hatch by the end of spring. Only 20 to 40% of the eggs, though, actually produce young. Failure of these eggs is often due to a low percentage rate of initial fertilization and to a frequent failure of the glair, external adhesive.
Predators, Parasites, Symbiosis and Diseases
Crayfish are eaten by many species of animals including raccoons, red foxes, muskrats, northern water snakes, eastern painted turtles, and many types of birds. Crayfish are also frequently beset by parasites and diseases which affect their gills, eyes, exoskeletons, and intestines. Many of these infections and infestations do little apparent harm to the individual crayfish unless the animal is stressed or debilitated in some way (these contributing stresses are often in the form of polluted, or in other ways low quality water). Crayfish also appear to have a mutualistic symbiosis with an aquatic annelid called “Cambarincola” which, apparently, helps to clean debris out of the crayfish’s gills thus improving the crayfish’s respiratory efficiency and fitness.
Ecological Significance
Crayfish are an important component of our stream ecosystem. They are significant links in the complex aquatic and terrestrial food webs in our ecosystem and, by their feeding, burrowing, and foraging activities, help to maintain a high level of water quality in our stream to the great benefit of so many of our Nature Trail species.
