Crabs are invertebrates that belong to the class Crustacea, a group of arthropods that includes shrimp and lobsters. These marine and terrestrial creatures are classified in the order Decapoda, a name that directly relates to their total number of appendages.
The Decapod Count
The definitive count of a crab’s major appendages is ten, arranged in five pairs. All ten appendages, technically known as pereopods, are attached to the thoracic region of the body. The first pair of pereopods is highly modified into prominent claws, formally called chelipeds.
The remaining four pairs of pereopods are used for locomotion and are commonly referred to as legs. Therefore, the total count is consistently two chelipeds (claws) and eight legs. This numerical breakdown is fundamental to their body plan, focusing on the number of pairs of limbs attached to the thorax.
Specialized Roles of the Chelipeds
The chelipeds are complex tools used for a variety of functions beyond simple grasping. These large claws are primarily used for defense against predators and for aggressive interactions with rival crabs. Powerful muscles allow them to deliver crushing force, necessary for opening the shells of prey like mollusks.
In many species, the claws also play a significant role in mating rituals and social signaling. Male fiddler crabs, for example, possess one extremely enlarged cheliped that they wave to attract females and deter competing males. The claws are also essential for manipulating food, tearing it into smaller pieces before consumption.
Functions of the Pereopods and Dactyls
The four pairs of posterior appendages, the pereopods, are mainly responsible for the crab’s characteristic sideways movement. The first three pairs are typically elongated and robust, providing the traction and power needed for walking and climbing across various substrates. These walking legs allow the crab to move with speed and agility across the seafloor, rocks, or sand.
The final, fifth pair of pereopods often exhibits the greatest specialization, particularly in aquatic species. In these swimming crabs, the last segment of the leg, called the dactyl, is flattened into a paddle-like shape. This adaptation transforms the limb into an efficient swimmeret, enabling the crab to move quickly through the water column. The pointed tips of the walking leg dactyls provide a firm grip on surfaces, while specialized dactyls on swimming legs can also be chemosensitive, helping in detecting food.