Crayfish

Crayfish

History and habitat

Appearance

Breeding habits

Resources

Crayfish are freshwater crustaceans of the order Decapoda, which includes crabs, shrimps, lobsters, and hermit crabs. Crayfish are nocturnally active, live in shallow freshwater habitats, and feed on aquatic plants and animals, as well as dead organic matter. Their natural predators include fish, otters, turtles, and wading birds. Crayfish are particularly vulnerable to predation during their periodic molts, when their hard exoskeleton is shed to permit body growth. Aggressive behavior among crayfish often occurs over access to resources such as habitat, food, or mates. Crayfish are used by humans as live fish bait, and are also a popular culinary delicacy.

History and habitat

Crayfish evolved from marine ancestors dating back some 280 million years. There are 604 species of crayfish worldwide, which are classified into three families: the Astacidae, the Cambridae (found only in the Northern Hemisphere), and the Parastacidae (indigenous to the Southern Hemisphere). A few species have adapted to tropical habitats, but most live in temperate regions. None occur in Africa or the Indian subcontinent, although one species is found in Madagascar. Crayfish live in freshwaters that do not freeze to the bottom, hiding beneath rocks, logs, sand, mud, and vegetation. Some species dig burrows, constructing little chimneys from moist soil excavated from their tunnel and carried to the surface. Some terrestrial species spend their whole life below ground in burrows, emerging only to find a mate. Other species live both in their tunnels as well as venturing into open water. Many species live mostly in open water, retreating to their burrows during pregnancy and for protection from predators and cold weather.

Appearance

Crayfish are usually colored in earth tones of muted greens and browns. The body has three primary sections: the cephalothorax (the fused head and thorax), which is entirely encased by a single shell; a six-segmented abdomen; and a five-sectioned, fan-shaped tail, the telson. Five pairs of strong, jointed, armored legs (pereiopods) on the cephalothorax are used for walking and digging. The first pair of legs, known as the chelipeds, end in large pincers (chelae), which are used for defense and food gathering. Two pairs of small antennae (the antennae and antennules) are specialized chemical detectors used in foraging and finding a mate. The antennae project on either side of the tip of the rostrum, which is a beaklike projection at the front of the head. A third and longer pair of antennae are tactile, or touch receptors. Two compound eyes provide excellent vision, except in some cave dwellers that live in perpetual dark and are virtually blind.

KEY TERMS

Antennule— Small antenna on the front section of the head.

Carapace— Shell covering the cephalothorax.

Cephalothorax— The head and thorax (upper part of the body) combined.

Chela— Pinchers on first pair of legs used for defense and food gathering.

Chelipeds— First pair of pereiopods ending with large pinchers.

Mandibles— Jaws.

Maxillipeds— Small leg-like appendages beneath the cephalothorax which aid in feeding.

Pereiopods— Ten jointed, armor-plated legs attached to the cephalothorax.

Pleopods— Small, specialized appendages below the abdomen which aid in swimming.

Rostrum— Beak-like projection at the front of the head.

Telson— Fan-shaped tail composed of five segments.

Below the rostrum are two pairs of mandibles (the jaws) and three pairs of maxillipeds, which are small appendages that direct food to the mouth. The second pair of maxillipeds facilitates gill ventilation by swishing water through the banks of gills located at the base of each pereiopod on the sides of the carapace in the gill chambers. The strong, long, muscular abdomen has ten tiny appendages (the pleopods), which aid in swimming movements. When threatened, the crayfish propels itself backward quickly with strong flips of the telson, located at the tip of the abdomen.

Breeding habits

Crayfish usually mate in the fall. Females excrete pheromones that are detected by the antennules of males. The openings of the sex organs are located on the front end of the abdomen just below the thorax. In the male, the first two pairs of abdominal pleopods are used as organs of sperm transfer. Using his first set of pleopods, the male deposits sperm into a sac on the female’s abdomen. The female stays well hidden as ovulation draws near, lavishly grooming her abdominal pleopods, which will eventually secure her eggs and hatchlings to her abdomen. Strenuous abdominal contractions signal the onset of egg extrusion. Cupping her abdomen, the female crayfish collects her eggs as they are laid (they can number 400 or more), securing them with her pleopods, fastidiously cleaning them with thoracic appendages, and discarding any diseased eggs.

Young crayfish hatchlings emerge from the eggs in the spring, and closely resemble adult crayfish in form (although much smaller). The hatchlings cling tightly to their mother’s pleopods, eventually taking short foraging forays and scurrying back for protection at the slightest disturbance. During this time, the mother remains relatively inactive and is extremely aggressive in protecting her young from other, non-maternal crayfish, which would cannibalize the young. The mother and young communicate chemically via pheromones; however, the young cannot differentiate between their own mother and another maternal female.

See also Crustacea; Lobsters.

Resources

BOOKS

Holdich, David M., ed. Biology of Freshwater Crayfish. Oxford, UK: Blackwell Science Ltd., 2002.

OTHER

Brigham Young University. “Crayfish Home Page.” 1996-2006. <http://crayfish.byu.edu/> (accessed October 13, 2006).

Marie L. Thompson