Crayfish are freshwater crustaceans belonging to the order Decapoda, meaning “ten-footed.” This refers to the five pairs of large thoracic appendages that dominate the animal’s body plan. These ten limbs are attached to the fused head and thorax region, known as the cephalothorax, and are primarily dedicated to movement, defense, and capturing food. The crayfish’s entire body is covered by a hard exoskeleton which is shed periodically for growth.
The Ten Limbs Used for Movement and Defense
The crayfish’s ten major limbs are called pereiopods, or walking legs, and are attached to the thorax segment of the cephalothorax. The first pair is modified into large, powerful claws known as chelipeds. These chelipeds are used for self-defense, territorial disputes, and the initial capture and cutting of prey.
The remaining four pairs of pereiopods are used primarily for locomotion across the substrate. The second and third pairs often possess small pincers at their tips, called chelae. These chelae provide dexterity, allowing the crayfish to manipulate, sort, and pass food toward its mouthparts. The fourth and fifth pairs are simpler walking legs that lack chelae, serving mainly to propel the animal forward and maintain balance.
Specialized Appendages for Feeding and Senses
The crayfish possesses numerous specialized appendages on its head and thorax for sensing the environment and processing food. The two pairs of sensory organs protruding from the head are the antennules and the antennae. The shorter, branched antennules maintain balance and detect chemicals in the water for taste and smell.
The longer, whip-like antennae are the primary organs for touch and vibration, allowing the crayfish to navigate its environment, especially in murky water or at night. Beneath these sensory organs are three pairs of appendages called maxillipeds, or “jaw feet,” which are part of the feeding apparatus. These maxillipeds hold and shred the food item before passing smaller pieces to the mandibles, which are powerful jaws that crush the material.
Abdominal Appendages and the Tail
The abdomen is highly muscular and features its own set of smaller, paired appendages that serve distinct functions. These five pairs of appendages are known as pleopods, or swimmerets, and are attached to the underside of the abdominal segments. While they can aid in slow swimming, their primary roles involve creating water currents over the gills for respiration and circulating water to keep the abdomen clean.
In female crayfish, the swimmerets also carry and aerate the developing eggs after fertilization, a phase often described as being “in berry.” The final segment of the abdomen features a modified structure called the tail fan. This fan is composed of a central telson and a pair of lateral uropods, which together allow the crayfish to perform a powerful, rapid backward thrust, or “tail-flip,” to escape danger.