Shrimp are crustaceans, belonging to the large subphylum Crustacea. This classification places them firmly within the Phylum Arthropoda, the group of invertebrate animals with jointed legs and external skeletons. This taxonomic placement is based on shared anatomical features that define the crustacean lineage.
Defining the Crustacean Group
The subphylum Crustacea includes over 50,000 known species, all sharing distinguishing characteristics that separate them from other arthropods like insects or arachnids. A primary feature is the rigid exoskeleton, or carapace, which protects the soft body beneath. This structure must be shed periodically for the animal to grow. Crustaceans typically have a body segmented into three regions: the head, thorax, and abdomen, though these are often fused into a cephalothorax.
A defining characteristic is the presence of two pairs of antennae on the head, which serve as sensory organs for touch and taste. These appendages are distinct from the single pair found on insects. Crustaceans also possess specialized, jointed appendages that are primitively branched, allowing for diverse functions like feeding, movement, and sensory input.
Because most crustaceans are aquatic, they rely on gills for respiration, typically housed in branchial chambers under the protective carapace. This combination of a segmented body, a hard exoskeleton that requires molting, specialized biramous (two-branched) appendages, and two pairs of antennae establishes the crustacean identity.
The Specific Classification of Shrimp
Shrimp fit within the crustacean group, tracing a lineage from the Phylum Arthropoda down to the Infraorder Caridea, where most “true shrimp” reside. They belong to the Class Malacostraca, which is the largest class of crustaceans. Within this class, shrimp are members of the Order Decapoda, a name that literally means “ten-footed” from the Greek words deka and pous.
The Decapoda order encompasses many species, including crabs and lobsters. It is defined by having three pairs of appendages modified as mouthparts (maxillipeds), followed by five pairs of walking legs (pereiopods). In shrimp, the elongated body structure is adapted for swimming, which is why they are often categorized as Natantia, or “swimmers,” in older classifications.
Modern “true shrimp” are placed in the Infraorder Caridea, which contains over 3,000 species found in marine and freshwater environments globally. Caridean shrimp possess an elongated abdomen, or tail, which bears specialized appendages called pleopods (swimmerets) used for propulsion. The abdomen ends in a tail fan, composed of the telson and uropods, which allows for rapid, backwards escape movements.
Common Relatives and Confusing Species
Shrimp share the Decapoda order with close biological relatives, including crabs, lobsters, and crayfish. These animals all share the fundamental ten-footed body plan and the presence of a carapace covering the cephalothorax.
Beyond the Decapods, shrimp are related to a diverse array of other crustaceans in different classes. This broader group includes barnacles (sessile filter feeders) and copepods (tiny planktonic organisms). Krill, an abundant marine crustacean, is also a close relative, belonging to the Order Euphausiacea.
The distinction between true shrimp and other organisms often comes down to misleading common names. For example, the mantis shrimp is not a true shrimp, as it belongs to the separate Order Stomatopoda. The difference between shrimp and insects, such as cockroaches, is that while both are Arthropods, insects belong to the Class Hexapoda and possess only one pair of antennae and three pairs of legs.