The blue crab (Callinectes sapidus) is a recognizable marine crustacean found along the Atlantic and Gulf coasts of North America, known for its distinctive appearance. Identifying this species relies on observing its unique shell shape, specialized limbs, and bright coloration.
Carapace Structure and Size
The most prominent feature of the blue crab is its hard, protective upper shell, known as the carapace. This shell is wide, flattened, and significantly broader than it is long, reaching up to 9 inches across from point to point.
Along the front-outer edges of the carapace are nine marginal teeth on each side. The ninth tooth forms a strong, sharp spine that protrudes laterally, giving the crab its characteristic diamond-like silhouette. The carapace color is typically olive green or greenish-brown, providing camouflage on the muddy and sandy bottoms.
Specialized Appendages
The blue crab possesses five pairs of legs, each serving a distinct purpose. The first pair are the chelipeds, which are the large, robust claws used for defense, feeding, and grappling with prey. One claw is often a “crusher” for breaking hard shells, while the other is a “cutter” for tearing meat.
The three pairs of legs following the claws are walking legs, used primarily for movement across the substrate. The final, fifth pair of appendages are highly specialized swimming legs, or dactyls. These rear legs are flattened and shaped like paddles, allowing the crab to efficiently scull through the water and bury itself quickly in soft sediment.
Primary Coloration and Markings
The name of the blue crab comes from the intense blue coloration found on its legs and claws. This bright blue is most prominent on the inner surfaces and joints of the chelipeds. The rest of the exoskeleton, the carapace, generally maintains its camouflage color of olive or bluish-green.
The crab’s vibrant blue hue is due to a mix of pigments in the shell. The underside, or abdomen, which is folded beneath the body, is typically a paler off-white or cream color. This distinct color pattern helps with identification.
Visual Differences Between Sexes
Identifying the sex of a blue crab is straightforward due to clear visual differences. The most reliable way to tell them apart is by observing the shape of the abdominal flap, or apron, tucked underneath the body. In adult males, this apron is long, narrow, and sharply pointed, often described as resembling an inverted T.
The female apron changes shape as she matures. An immature female has a triangular flap, while a fully mature female possesses a broad, rounded, or dome-shaped apron. Furthermore, males have entirely bright blue claws, whereas mature females often display red or orange accents, particularly on the tips of their claws.