Crabs and lobsters are recognizable residents of the ocean floor. Their physical similarities are not coincidental; both belong to the Phylum Arthropoda (joint-legged invertebrates) and the Class Malacostraca, which also includes shrimp and crayfish. Their closest relationship is defined by the Order Decapoda.
The Decapoda Classification
The Order Decapoda is the foundational taxonomic group for both crabs and lobsters. The name “Decapoda” comes from the Ancient Greek words déka (“ten”) and poús (“foot” or “leg”), referring to the five pairs of walking legs (pereiopods) attached to the thoracic segments. This large order contains nearly 15,000 extant species, which are further divided into distinct infraorders.
True crabs (Brachyura) are characterized by their broad shape. True lobsters are primarily found in the infraorder Astacidea, which includes clawed lobsters (Family Nephropidae). Other types, such as spiny and slipper lobsters, belong to the infraorder Achelata, illustrating the diversity within the “lobster” designation. This hierarchical system shows that while both are decapods, their evolutionary paths diverged to create the distinct body forms observed today.
Shared Physical Characteristics
All decapods possess defining physical features that place them in the same order. A primary shared characteristic is the rigid, chitinous exoskeleton, a hard outer shell that provides protection and structural support. Since this shell does not grow with the animal, crabs and lobsters must periodically shed it in a process known as molting to increase in size.
The body structure is unified by the fusion of the head (cephalon) and the thorax into a single unit called the cephalothorax. This shield-like structure is covered dorsally by the carapace, which extends laterally to enclose the gills in a protective branchial chamber. The gills require a constant flow of water for respiration.
The defining ten appendages are arranged as five pairs of legs on the thorax. The first pair is almost universally modified into large claws, known as chelipeds, which are used for defense, feeding, and sexual signaling. In front of the walking legs are three pairs of specialized appendages called maxillipeds, which function primarily as feeding tools to manipulate food toward the mouth.
Key Morphological Differences
The most striking difference between true crabs (Brachyura) and true lobsters (Astacidea) is the structure of their abdomens and overall body shape. Lobsters exhibit an elongated, cylindrical body form with a prominent, muscular abdomen (the tail) that extends straight behind the cephalothorax. This powerful abdomen is used for rapid, backward swimming and quick escape.
In contrast, the crab body form is compact, characterized by a broad, flattened, or rounded carapace. The crab’s abdomen is greatly reduced and permanently tucked underneath the cephalothorax. This tucked abdomen is involved only in reproduction, where the female uses it to hold eggs.
The chelipeds also show a general distinction: lobsters often display asymmetry, possessing one heavy crushing claw and one sharper, pinching claw used for tearing. While crab claws vary widely, their body design facilitates their characteristic sideways scuttling motion, allowing them to wedge into crevices. The lobster’s long body allows it to move forward using its walking legs, though it can also move backward with its powerful tail flip.