What kinds of animals are crabs?

Crabs are decapod crustaceans that include true crabs (Brachyura) and related groups within Anomura, such as hermit crabs and allies.

How big do crabs get?

Sizes range from pea crabs only a few millimetres wide to the Japanese spider crab with a leg span up to 4 m (13 ft).

Many crabs are marine omnivores; others are specialist herbivores or carnivores, and some are parasitic; some species live in freshwater or non-marine habitats.

Crabs commonly hide in rocky crevices.

Are crabs important to people?

Crabs make up about 20% of the marine crustaceans caught or farmed for human consumption and feature in cuisine, mythology, art, and as pets.

What does ‘carcinisation’ mean?

Carcinisation is the evolutionary process by which varied decapods have independently evolved a crab-like body plan.

Are hermit crabs true crabs?

Hermit crabs belong to groups within Anomura and are related to, but distinct from, the true crabs of Brachyura.

Crustaceans

Crab

Q: Do crabs always live in the sea?

While many crabs are marine, a substantial number of species are adapted to freshwater or other non-marine habitats.

Crabs are decapod crustaceans with armored shells, concealed tails, and a tendency to run sideways and hide in rocky crevices.

Brachyura (true crabs)Anomura (hermit crabs and allies)

Last Updated Mar 9, 2026 · 4 sources

Photo byChandler Cruttenden

Species Guide

What to know about crabs

A clean field guide focused on habitat, identification, behavior, and conservation context without burying the useful parts.

Crabs are decapod crustaceans that include the true crabs (Brachyura) and various related groups within the Anomura (such as hermit crabs and allies). They are characterised by a heavily armoured shell, tail segments concealed beneath the body, the ability to run sideways, and a habit of hiding in rocky crevices. Crabs do not form a single natural group; they have evolved the crab-like body plan multiple times from ancestral decapods through a process called carcinisation, making the group polyphyletic. Size varies widely across species, from pea crabs a few millimetres wide to the Japanese spider crab with a leg span up to 4 m (13 ft). Many crabs are free-living marine omnivores, while others are specialist herbivores, carnivores, or parasitic; a substantial number of species are adapted to freshwater or other non-marine habitats. Crabs are significant in human culture and cuisine, are kept as pets (hermit crabs), and appear in mythology and art.

Many crabs are capable of running sideways and commonly hide in rocky crevices.

Diet varies by species: many are free-living marine omnivores; others are specialist herbivores, carnivores, or parasitic; some inhabit freshwater or non-marine environments.

The strongest linked planning options currently surface around destinations such as Anilao + Batangas (Verde Island Passage), Coron, Caye Caulker and countries such as Philippines, Portugal, Indonesia for divers building trips around crabs.

Identification

How to identify crabs

The visual markers divers can use underwater, plus the species most likely to cause confusion.

Key identification markers

Body plan

Heavily armoured shell with tail segments concealed under the body.

Movement

Noted ability to run sideways.

Behavior

Tendency to hide in rocky crevices.

Size range

Species range from a few millimetres (pea crab) to leg spans up to 4 m (Japanese spider crab).

Range and Movement

How crabs move through the world

The broad range, seasonal movement, or migratory behavior that shapes where divers encounter this species.

Many crabs are capable of running sideways and commonly hide in rocky crevices.

Diet

What crabs tend to eat

Useful feeding context that often explains habitat, movement, and encounter style.

Diet varies by species: many are free-living marine omnivores; others are specialist herbivores, carnivores, or parasitic; some inhabit freshwater or non-marine environments.

Responsible Encounters