Crabs are members of the decapod crustacean order, a group defined by having ten legs, which also includes lobsters and shrimp. With over 7,000 species known globally, crabs demonstrate immense diversity in size, shape, and habitat, ranging from the tiny pea crab to the giant Japanese spider crab. Scientific classification organizes these creatures based on distinct anatomical characteristics, particularly the structure of their abdomen, to determine which species are considered “true” crabs and which are merely crab-like in appearance.
Brachyura: The True Crabs
The infraorder Brachyura, meaning “short tail,” encompasses the true crabs, representing the largest and most diverse group of decapod crustaceans. Their defining anatomical characteristic is a small, flattened abdomen (pleon) that is tucked securely beneath the broad, often flattened, carapace. This highly reduced abdomen is not functional as a tail fan.
True crabs possess five pairs of thoracic appendages. The first pair is modified into prominent claws, known as chelipeds, and the remaining four pairs serve as walking legs. This robust body plan contributes to their characteristic sideways gait. Common species include the Portunidae family, or swimming crabs, such as the Blue Crab, recognized by their last pair of legs being flattened into paddle-like appendages. Other notable groups include the spider crabs (Majidae), which often decorate their shells with materials from their environment for camouflage.
Anomura: The Crab Look-Alikes
The infraorder Anomura contains several groups that have evolved a crab-like body form through a process called carcinisation, but they are not considered true crabs. This evolutionary convergence means many Anomura superficially resemble Brachyura, but close inspection reveals structural differences that place them in a separate lineage. The primary distinction lies in the abdomen, which is not fully tucked and fused beneath the body, or in the number of visible walking legs.
Most Anomura appear to have only three pairs of walking legs because the fifth and last pair is often greatly reduced in size. This tiny pair of legs is usually hidden inside the gill chamber and used for cleaning the gills. Notable examples of these “false crabs” include the King Crabs (Lithodidae), which are actually highly modified relatives of hermit crabs. Hermit Crabs (Paguroidea) are also Anomura, distinguished by their soft, often asymmetrical abdomens that they protect by inhabiting scavenged mollusk shells.
Diversity by Habitat: Marine, Freshwater, and Terrestrial Adaptations
Crabs are found in aquatic and semi-terrestrial environments, and their physiology has adapted to the specific challenges of each habitat. The largest group is marine, inhabiting environments from shallow intertidal zones to the deep sea. Marine crabs are primarily osmoconformers, meaning their internal salt concentration aligns closely with the surrounding seawater.
Crabs that have moved into freshwater environments, such as rivers and lakes, are a much rarer group facing the challenge of osmoregulation. They must actively work to maintain a higher salt concentration in their blood than the surrounding water. This requires specialized ion-transporting cells in their gills to take up salt from the dilute environment and prevent the loss of essential ions.
Terrestrial, or land, crabs have made significant physiological adjustments, having transitioned from water to land multiple times in their evolutionary history. These species, such as the coconut crab, have evolved specialized respiratory structures, sometimes referred to as branchiostegal lungs, to breathe air. However, they still retain gills and must keep them moist within a protective branchial chamber to facilitate gas exchange. Most terrestrial crabs must also return to the water, often the ocean, to release their eggs and complete their larval development.
Major Commercial and Culinary Varieties
The global seafood market values several crab species for their meat, which varies widely in flavor and texture.
The Dungeness crab, harvested along the Pacific Coast, is a highly prized true crab known for its sweet, slightly firm meat and high yield. Another economically important Brachyuran is the Blue Crab, particularly prominent along the Eastern Seaboard of the United States, offering delicate, flaky meat with a distinct briny flavor. The Snow crab, or Tanner crab, provides a milder, slightly sweet meat that shreds easily and is popular due to its long, thin legs. The Stone crab, primarily sourced from Florida, is unique because only its large claws are harvested, with the crab being returned to the water to regenerate the lost limb.
The most massive commercial varieties are the King crabs, which are Anomura. These crustaceans yield huge leg segments with dense, intensely sweet meat that commands a premium price.