Crab

Crab is a common name for a diverse group of decapod crustaceans found in oceans, rivers, and on land. In ordinary usage, crabs include true crabs such as those in the family Cancridae and the many species in Portunidae, the swimming crabs. The term also applies to a variety of other species with crab-like forms, making crabs among the most widespread crustaceans.

Anatomy: Most crabs have a broad, flattened carapace that covers the cephalothorax, and a short abdomen tucked under the body. The first pair of legs usually bears large claws (chelae). In swimming crabs, the hind legs are flattened into paddles that aid propulsion in the water.

Habitat and behavior: Crabs inhabit a wide range of environments, including rocky shores, sandy beaches, estuaries, coral reefs, and some freshwater systems. Many are omnivorous scavengers, feeding on algae, mollusks, crustaceans, detritus, and small fish. They can be nocturnal and show diverse mating and burrowing behaviors. Terrestrial crabs spend substantial time on land but return to the sea to reproduce.

Life cycle: Most marine crabs reproduce by releasing eggs that females carry attached to the abdomen; the eggs hatch into free-swimming larval stages (zoea or nauplius) before metamorphosing into juvenile crabs. Some species exhibit direct development.

Human use and conservation: Crabs are an important food resource worldwide, with species such as the blue crab, Dungeness crab, and king crab harvested commercially. They are also kept in aquaculture in some regions. Fisheries face pressures from overfishing, habitat loss, and climate change, leading to management measures like size limits and seasonal closures.