FaunaHub

Crab

MarineCrustaceanInvertebrate

A brightly coloured Sally lightfoot crab on dark coastal rock.

Sally lightfoot crab (Grapsus grapsus) on a rocky shore.

Image: James St. John, CC BY 2.0, via Wikimedia Commons.

Overview

Crabs are crustaceans belonging mainly to the infraorder Brachyura, the "true crabs", with thousands of species worldwide. They have a broad, hard shell (carapace), a pair of clawed front legs (chelae or pincers), and four pairs of walking legs. This page is a group-level overview. While many crabs are marine, others live in fresh water or spend much of their lives on land.

Habitat & Range

Crabs occupy a remarkable range of habitats: rocky shores, sandy beaches, coral reefs, the open seabed, estuaries, mangroves, rivers, and even forests far from water in the case of some land crabs. Many live in the intertidal zone, sheltering in crevices or burrows. Habitat and depth vary greatly by species and region.

Diet

Most crabs are omnivores or scavengers with a varied diet that may include algae, detritus, molluscs, worms, small fish, and the remains of dead animals. Some are more specialised predators or filter feeders. By scavenging and recycling organic material, crabs play an important role in many coastal and marine ecosystems. Diet varies by species.

Behavior

Crabs are well known for walking sideways, a movement suited to the arrangement of their legs, though some can move forwards and backwards too. They use their pincers to feed, fight, and signal, and many species display or wave their claws during courtship or territorial disputes. Like other crustaceans, crabs grow by moulting their hard shell. Behaviour ranges from solitary to mass gatherings during breeding.

Human Interaction & Conservation

Crabs are ecologically important and are also harvested in large quantities for food in many parts of the world. Some coastal crab populations are managed through fisheries regulation. Conservation status varies by species and for many crabs is not well documented, so it should not be assumed; consult current marine-science and fisheries sources for specific species.

Appearance & Recognition

True crabs have a short, broad body protected by a hard carapace, with a small abdomen tucked underneath. The front pair of legs ends in pincers, often unequal in size, and the remaining four pairs are used for walking. Eyes sit on short stalks. Colour, shape, and size vary enormously — from tiny pea crabs to large spider crabs with long legs — and some species decorate or camouflage their shells.

Similar Animals

Crabs are crustaceans, the same broad group as the lobster covered separately on FaunaHub, as well as shrimp and crayfish. Hermit crabs and king crabs are not "true crabs" but close relatives within the wider crab-like crustaceans. Their jointed legs and hard exoskeleton place them among the arthropods.

More photos of the crab

A crab resting on rock near the water line.

A shore crab among coastal rocks.

Image: Richard Cheung, CC0, via Wikimedia Commons.

Frequently Asked Questions — Crab

Why do crabs walk sideways?
The joints of a crab's legs are arranged so that sideways movement is the most efficient way to walk, especially for the broad-bodied true crabs. Some crabs can also move forwards and backwards, but sideways walking is characteristic of the group.
Do all crabs live in the sea?
No. Many crabs are marine, but others live in fresh water or on land. Some land crabs spend most of their lives away from water and only return to the sea to breed. This page is a group-level overview covering that diversity.
How do crabs grow if they have a hard shell?
Crabs grow by moulting — periodically shedding their hard exoskeleton and forming a new, larger one. The animal is soft and vulnerable for a short time after moulting until the new shell hardens.

Sources and further reading

Authoritative wildlife references used for general educational context. Conservation status should always be verified against current IUCN Red List data. External links open in a new tab.