Lobsters and crabs are often grouped together due to their similar appearance as armored, ten-legged sea creatures. Both possess a hard, external skeleton and navigate the ocean floor using jointed legs, leading many to believe they are simply variations of the same animal. While they share an evolutionary history, this perception is a significant oversimplification of their distinct biological identities. We will explore the precise scientific classifications and physical features that definitively distinguish a lobster from a crab.
Shared Biological Ancestry
Lobsters and crabs share a fundamental biological kinship as members of the Phylum Arthropoda, Subphylum Crustacea, and Class Malacostraca (which also contains shrimp and crayfish). The most specific level of shared classification is the Order Decapoda, which translates to “ten feet.” This name reflects the presence of five pairs of thoracic legs used for walking and feeding. In both animals, the first one or two pairs of these appendages are typically modified into claws, called chelae, used for defense and manipulating food.
The Definitive Taxonomic Answer
Despite their shared membership in the Order Decapoda, the differences between lobsters and crabs are formally established at the subsequent taxonomic level, the Infraorder. All true crabs belong to the Infraorder Brachyura, a name derived from the Greek words meaning “short tail.” Lobsters, conversely, are separated into different infraorders entirely, reflecting a deep evolutionary divergence. Clawed lobsters belong to the Infraorder Astacidea, while spiny lobsters and slipper lobsters are classified in the Infraorder Achelata.
The evolution of the crab body form, known as carcinization, is a key reason for the separation of Brachyura. This process involved an increasingly robust body shape and a significant reduction of the abdominal segment. This evolutionary path led to the compact, wide body recognizable as a crab, distinguishing them from the more elongated form retained by lobsters.
Key Physical Distinctions
Abdomen and Carapace
The most immediate and observable difference is the structure of the abdomen, commonly referred to as the tail. The lobster possesses a long, muscular, extended abdomen that ends in a tail fan (telson), used powerfully for rapid, backward swimming. Crabs, in contrast, have a significantly reduced abdomen that is tightly tucked and folded underneath their main body shell, or carapace. This hidden abdomen is a defining trait of the true crabs.
Another major distinction lies in the overall shape of the carapace, the hard shell covering the head and thorax. Crabs typically exhibit a broad, flattened, or rounded carapace, providing a wide, shield-like structure. Lobsters maintain a more cylindrical, elongated body shape that is narrower in profile. This difference dictates their movement, with crabs often scuttling sideways and lobsters walking forward more linearly.
Claws and Appendages
The claws, or chelae, also show functional specialization. Clawed lobsters (Astacidea) often feature massive, specialized claws, with one developed as a heavy crusher and the other as a sharper cutter. While crab claws vary widely, they are generally smaller relative to the animal’s body and are often used for display or dominance. Furthermore, Achelata lobsters, such as spiny lobsters, lack large, prominent claws altogether, relying instead on long, thick antennae for defense and navigation.