Hermit crabs often spark curiosity due to their distinctive appearance and behavior, leading many to wonder if they are indeed true crabs. Despite their common name, a biological distinction separates hermit crabs from what scientists classify as “true crabs.” This difference lies in their unique anatomical features and evolutionary paths within the vast world of crustaceans.
What Makes a True Crab
True crabs belong to the infraorder Brachyura, a diverse group with defining biological traits. They possess a broad, flattened, highly calcified carapace, a hard outer shell covering their cephalothorax. Their short abdomen, often called a “tail,” is typically tucked completely underneath their thorax. True crabs have five pairs of legs, making them decapods; the first pair are powerful pincers (chelipeds) used for defense and feeding, and the remaining four pairs are used for walking. Unlike hermit crabs, true crabs grow their own complete, hard external shell, providing consistent protection as they mature.
The Unique World of Hermit Crabs
Hermit crabs are classified under the infraorder Anomura, distinct from true crabs. While also decapods, their body structure differs significantly. Their soft, asymmetrical, and often spiraled abdomen lacks the hard, calcified protection found in true crabs. This vulnerable abdomen necessitates their reliance on discarded mollusk shells for shelter and protection.
Hermit crabs have evolved specialized adaptations to accommodate this shell-dwelling lifestyle. Their last two pairs of legs are often reduced in size and modified to help them grip and hold onto the inside of a borrowed shell, leaving only three pairs of visible walking legs for locomotion. One of their claws is frequently larger than the other, serving as a plug to block the shell’s entrance when the crab retreats inside. As a hermit crab grows, it must periodically search for and swap into larger shells to accommodate its increasing size. This unique evolutionary path highlights their divergence from true crabs, despite sharing a broader crustacean ancestry.
Why the Name “Hermit Crab”
The common name “hermit crab” arises from their appearance and characteristic behavior, though it doesn’t fully reflect their biological classification. The “crab” part refers to their general crab-like features, such as claws and walking legs, which superficially resemble true crabs. The “hermit” aspect refers to their perceived solitary nature and, more accurately, their reliance on a borrowed shell for shelter. These crustaceans inhabit and carry their mobile homes for protection. While the name suggests solitude, many hermit crab species can be social, often living in groups and cooperating during shell exchange events.