Crayfish are freshwater crustaceans, often called crawfish or crawdads, structured with a hard external skeleton and a segmented body. These animals share a close biological relationship with marine lobsters and shrimp, belonging to the same order. The count of true walking legs is ten, a number that places them within a distinct grouping of crustaceans.
The Direct Answer: Decapods and the Ten Legs
The crayfish belongs to the biological order Decapoda, a name derived from the Greek words for “ten feet.” This classification is based on the presence of five pairs of true walking legs, collectively known as pereiopods, which are attached to the thoracic section of the body. These ten limbs are the primary means of locomotion for the crayfish, allowing them to walk and crawl. The pereiopods are jointed appendages numbered one through five, starting from the front of the cephalothorax, the fused head and thorax region. The second and third pairs of walking legs often possess small claws, which assist the crayfish in handling food, while the final two pairs, the fourth and fifth, provide balance and movement.
Specialized Function of the Claws (Chelipeds)
The first pair of pereiopods is highly specialized into large, powerful claws called chelipeds. These pincers are the most noticeable appendages and serve multiple functions beyond simple walking. The chelipeds are used as defense weapons, helping the crayfish ward off predators and compete with other individuals. Their robust structure allows them to capture prey and tear food into manageable pieces before it is passed to the mouthparts. The size and presence of a claw on a leg are noted by the term chela, which is why the claw-bearing legs are called chelipeds.
Appendages That Are Not Legs
The ten pereiopods are the only true walking legs, but the crayfish possesses many other appendages that are sometimes mistakenly counted as legs. The most prominent of these are the swimmerets, also known as pleopods, which are smaller, paired appendages located on the underside of the abdomen. There are five pairs of swimmerets, and their primary roles are related to swimming and circulating water. These abdominal appendages are particularly important for female crayfish, as they are used to attach and carry fertilized eggs, aerating them by moving water over the clutch. Additionally, the crayfish has three pairs of maxillipeds, which are small, leg-like structures located near the mouth. These maxillipeds are primarily feeding appendages, used to hold and manipulate food toward the mandibles for chewing, and they are not involved in walking.