Crabs are decapods, belonging to the Order Decapoda, a vast and diverse group of crustaceans that includes familiar animals like shrimp, lobsters, and crayfish. The term Decapoda translates from Greek as “ten feet,” referring to the defining arrangement of their thoracic appendages. Understanding this biological order requires looking closely at the specific anatomical features that all members share.
Defining the Decapod Order
The Order Decapoda is defined by the presence of five pairs of walking legs, scientifically known as pereiopods, which are attached to the animal’s thorax. These ten legs are the basis for the group’s name and are used for locomotion, whether walking across the seafloor or swimming through the water column. The first pair of pereiopods is often modified into large, grasping claws called chelipeds, which are used for defense, feeding, and courtship rituals.
Anatomically, decapods exhibit a fusion of their head and thoracic segments, forming a single, protected body region called the cephalothorax. This structure is covered by a hard, saddle-like shell called the carapace, which protects the gills and internal organs beneath. The decapod body plan also includes three pairs of appendages located just in front of the pereiopods, known as maxillipeds, which function primarily as mouthparts to manipulate food.
The True Crabs (Infraorder Brachyura)
Within the Order Decapoda, crabs are organized into the Infraorder Brachyura, which translates to “short tail” and encompasses the animals known as “true crabs.” This classification highlights the distinguishing feature of crabs: the reduction and tucking of their abdomen, or tail, beneath the cephalothorax. The abdomen is typically a small, hard flap held tightly against the underside of the body, giving the crab its characteristically broad and flattened body shape.
This evolutionary process is sometimes referred to as “carcinisation,” where an elongated crustacean body plan evolves into a crab-like form. The reduced abdomen in Brachyura contrasts sharply with the long, muscular, and segmented abdomen found in other decapods, such as shrimp and lobsters, which use their tails for powerful swimming motions. This specific arrangement of the ten legs, combined with the reduction of the abdomen, defines a true crab and separates it from “false crabs” like hermit crabs and king crabs, which belong to a different infraorder (Anomura).
Decapoda’s Place in the Arthropod Phylum
The Decapoda order occupies a position within the broader biological classification of invertebrates. Decapods are members of the Class Malacostraca, which is the largest and most diverse class within the Subphylum Crustacea. Malacostracans share a common body plan that involves a fixed number of segments: five in the head, eight in the thorax, and six in the abdomen.
Decapods belong to the Phylum Arthropoda, the largest phylum in the animal kingdom, characterized by a hardened external skeleton (exoskeleton), a segmented body, and jointed appendages. Crustaceans, including decapods, are distinct from other arthropods like insects because they possess two pairs of antennae instead of one. This taxonomic placement illustrates that crabs are specialized, ten-legged Malacostracans.