Lobsters and crabs often spark curiosity due to their similar appearances. While both are armored inhabitants of the ocean and share common ancestry as crustaceans, they represent distinct groups of animals.
Key Physical Distinctions
Lobsters and crabs exhibit notable differences in their overall body structure. A lobster typically has an elongated, somewhat cylindrical, and relatively narrow body. In contrast, a crab’s body is broad, flattened, and often more rounded, forming a compact, shield-like carapace.
One distinguishing feature is their tail, or abdomen. A lobster possesses a prominent, muscular tail that extends backward, ending in a fan-like telson, used for powerful backward propulsion to escape predators. Conversely, a crab’s abdomen is typically small and tucked neatly underneath its cephalothorax, often going unnoticed.
Their claws, known as chelae, also show variations. Lobsters commonly have two distinct types: a larger “crusher” claw for breaking shells and a smaller, sharper “pincer” or “cutter” claw for tearing softer food. Crab claws, while varying by species, are generally more uniform for grasping or tearing.
Differences in body structure also dictate their primary modes of locomotion. Lobsters typically walk forward or backward on the seafloor using their legs, and can swim backward with powerful flicks of their muscular tails. Crabs are known for their sideways scuttling, a movement enabled by their jointed legs and broad bodies.
Their Place in the Biological Family
Both lobsters and crabs belong to the subphylum Crustacea, part of the phylum Arthropoda. As crustaceans, they have a hard external skeleton (exoskeleton) and jointed appendages. They also share the classification of decapods, meaning they possess ten legs, with the front pair often modified into claws.
Despite these shared characteristics, lobsters and crabs diverge at a more specific taxonomic level. True lobsters, such as the American lobster, are primarily found within the infraorder Astacidea, which includes the family Nephropidae. Spiny lobsters and slipper lobsters, which lack the large claws of true lobsters, belong to a different infraorder called Achelata. In contrast, true crabs are classified under the infraorder Brachyura, a name derived from Greek words meaning “short tail,” referencing their tucked abdomen.
This classification indicates that while they share a distant common ancestor, lobsters and crabs have evolved along distinct evolutionary paths. The compact, broad body plan of crabs, often referred to as “carcinization,” represents a significant evolutionary development that occurred independently in various decapod lineages. This evolutionary divergence led to their unique forms and specialized adaptations seen today.