Crabs and lobsters are familiar inhabitants of marine environments, recognized by their hard exoskeletons and jointed limbs. While they share a general appearance, their scientific classifications reveal distinct differences, providing a precise way to understand their relationships within the animal kingdom.
Their Place in the Animal Kingdom
Both crabs and lobsters belong to the phylum Arthropoda, a vast group characterized by jointed appendages and external skeletons. They are further classified under the subphylum Crustacea, which includes a wide array of aquatic animals like shrimp and barnacles. More specifically, crabs and lobsters are members of the order Decapoda, meaning “ten-footed,” referring to their five pairs of legs.
The scientific distinctions between true crabs and true lobsters emerge at the infraorder level. True crabs are classified in the infraorder Brachyura, a name derived from Greek words meaning “short tail,” which aptly describes their tucked-under abdomen. Conversely, true lobsters fall under the infraorder Astacidea, which also includes crayfish. Another infraorder, Achelata, encompasses spiny lobsters and slipper lobsters, which are not considered “true” lobsters due to key morphological differences.
Key Characteristics and Distinctions
Physical characteristics help differentiate true crabs from true lobsters. True crabs typically have a broad, flattened carapace, which is the hard upper shell covering their head and thorax. Their abdomen is usually short and folded tightly underneath their body. True crabs possess five pairs of legs, with the front pair often modified into strong claws, also known as chelae.
True lobsters exhibit an elongated, cylindrical body shape with a prominent, extended abdomen or “tail.” With the first three pairs bearing claws, the first pair being notably larger and more robust. Unlike crabs, their abdomen is not tucked underneath but extends backward, ending in a broad tail fan used for propulsion. These distinct body plans reflect their differing lifestyles and evolutionary paths within the Decapoda order.
Beyond the Basics: Diversity in Names
The common names for many crustaceans can be misleading, as they may include “crab” or “lobster” without belonging to the scientifically defined “true” groups. For instance, hermit crabs are not true crabs; they belong to the infraorder Anomura. Hermit crabs are characterized by a soft, asymmetrical abdomen that they protect by inhabiting discarded mollusk shells. Similarly, king crabs are also part of Anomura and are more closely related to hermit crabs than to true crabs. They are distinguished by their typically large size and hard, stone-like shells.
Other examples of misnamed crustaceans include porcelain crabs and squat lobsters. Porcelain crabs superficially resemble true crabs with their flattened bodies, but they also belong to the infraorder Anomura and are more closely related to squat lobsters. Squat lobsters look like lobsters but are also classified within Anomura and are closer relatives of hermit crabs and porcelain crabs.