Crabs, with their hard exoskeletons and distinctive sideways scuttle, often spark curiosity about their anatomy, particularly their legs. A common question is how many legs these fascinating crustaceans possess and whether their prominent pincers are included in that count. Understanding the structure and function of crab appendages reveals the diverse adaptations that allow them to thrive in various environments.
The Standard Crab Leg Count
True crabs, belonging to the infraorder Brachyura, typically have ten appendages, organized into five pairs. These include a pair of large pincers, known as chelipeds, and four pairs of walking legs. Chelipeds are modified legs and count towards the total of ten. This characteristic number classifies crabs, along with lobsters and shrimp, under the order Decapoda, which translates to “ten-footed.”
Their robust exoskeleton supports these segmented limbs, providing flexibility and articulation crucial for locomotion. This allows true crabs to move efficiently across diverse terrains, from rocky shores to sandy seafloors.
Specialized Functions of Crab Legs
Each pair of a crab’s ten legs serves specific purposes, highlighting their remarkable adaptations. The chelipeds, or pincers, are highly versatile tools used for defense against predators, capturing and manipulating food, and even for communication and mating displays. In some species, one pincer may be significantly larger than the other, serving different roles such as crushing prey or signaling to other crabs.
The four pairs of walking legs are primarily used for locomotion, enabling crabs to walk, run, and climb. While many crabs are known for their sideways movement, some can also move forward or backward. For certain aquatic species, the rearmost pair of walking legs are flattened into paddle-like swimmerets, adapted for swimming.
Crabs That Break the Mold
While the ten-leg count is standard for true crabs, some crustaceans commonly called “crabs” do not fit this rule. Hermit crabs, for instance, are decapods and possess ten legs, but not all are readily visible. Their soft, curved abdomen is housed within a scavenged mollusk shell, and their last two pairs of legs are often smaller and adapted to help grip the inside of the shell, making them less apparent. Hermit crabs are not true crabs; they belong to the infraorder Anomura, which also includes king crabs.
King crabs also present an exception to the typical true crab anatomy, commonly having only eight visible legs, including their two chelipeds. Despite their name and crab-like appearance, king crabs are not true crabs; they are more closely related to hermit crabs within the Anomura infraorder. This evolutionary phenomenon, where different crustaceans evolve similar crab-like body forms, is known as carcinization. Their specialized fifth pair of legs is often tucked underneath their carapace and used for specific tasks such as cleaning embryos or sperm transfer.