Crabs are crustaceans found across diverse environments globally. They are characterized by hard exoskeletons and ten jointed legs, including a prominent pair of pincers. Crabs exhibit a wide array of forms, sizes, and behaviors, adapting to various ecological niches. Understanding the different kinds of crabs involves recognizing specific anatomical features that define distinct groups.
Understanding True Crabs
True crabs belong to the infraorder Brachyura, meaning “short-tailed.” A defining characteristic is their short, flattened abdomen, typically tucked entirely underneath the cephalothorax, the fused head and thorax region. This tucked abdomen gives them their broad, compact body shape.
True crabs possess five pairs of legs. The first pair is modified into large pincers (chelae or chelipeds), used for defense, feeding, and manipulating objects. The remaining four pairs are walking legs, enabling their distinctive sideways movement.
Diversity Among True Crabs
The infraorder Brachyura encompasses approximately 7,000 known species, displaying remarkable diversity in habitat and form. True crabs inhabit all the world’s oceans, freshwater bodies, and even terrestrial environments. This wide distribution highlights their adaptability to various conditions and resource availability.
Marine true crabs represent a significant portion of this diversity. For instance, the Blue Crab ( Callinectes sapidus ) is a well-known swimming crab with paddle-shaped rear legs that assist in agile movement through water. Other marine species, like decorator crabs (superfamily Majoidea), exhibit unique camouflage behaviors. They attach pieces of seaweed, sponges, or other marine debris to hooked bristles on their shells, blending into their surroundings to avoid predators.
True crabs also thrive in freshwater and terrestrial settings. Over 800 species are found in freshwater environments. Terrestrial true crabs, such as the Atlantic Ghost Crab (Ocypode quadrata), are adapted to life on land, often found on sandy beaches. These swift-footed crabs can move quickly, up to 10 miles per hour, and have specialized gills that allow them to absorb water from damp sand, reducing their need to return to the ocean frequently.
Fiddler crabs (Uca spp.) are another example of semi-terrestrial true crabs often found in coastal marsh habitats. Male fiddler crabs are recognized by one significantly enlarged pincer, which they wave to attract mates and deter rivals. This large claw is not primarily used for feeding; instead, they use their smaller pincer, or the female’s two small pincers, to sift through sand for food particles.
Beyond True Crabs: Other Crab-like Animals
While many creatures are commonly called “crabs,” not all of them belong to the true crab infraorder Brachyura. Several other crustacean groups have evolved crab-like appearances through a process called carcinization, where non-crab-like animals develop features similar to true crabs. These “false crabs” are classified under different infraorders, primarily Anomura.
Hermit crabs, for example, are prominent members of the Anomura infraorder. Unlike true crabs with their hardened, tucked abdomens, hermit crabs possess a soft, elongated, and often spirally curved abdomen. This vulnerable abdomen necessitates their use of scavenged mollusk shells for protection. Hermit crabs typically have three pairs of visible walking legs, with a reduced fourth pair used to grip the inside of their shell.
King crabs, despite their name, are also not true crabs; they are more closely related to hermit crabs within the Anomura group. They often have only three pairs of visible walking legs, with the fourth pair being small and tucked away. Porcelain crabs, another Anomuran group, superficially resemble true crabs but possess only three pairs of walking legs and have long antennae that originate from the front outside of their eyestalks, unlike the short antennae between the eyes found in true crabs.
Mole crabs, also known as sand crabs, are small, burrowing anomurans found in sandy surf zones. These creatures have elongated, oval bodies adapted for digging into the sand, contrasting with the broad, flattened bodies of true crabs. The anatomical distinctions in leg count, abdomen structure, and antennae placement are key features that differentiate these crab-like animals from their true crab relatives.