Body plan
Heavily armoured shell with tail segments concealed under the body.
Crab
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.
Identification
How to identify crabs
The visual markers divers can use underwater, plus the species most likely to cause confusion.
Key identification markers
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.
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Responsible Encounters
How to dive with crabs
Conservation-minded guidance for divers who want the encounter without adding pressure.
Give crab space, avoid blocking the animal's path, and follow local site and operator rules for wildlife interactions with crustaceans.
Field Notes
Interesting things worth knowing
Useful species context that makes the encounter more meaningful once you are underwater.
Field notes
Multiple origins
Crab-like forms have evolved convergently multiple times from ancestral decapods through carcinisation, making the group polyphyletic.
Cultural presence
Crabs appear in cuisine (including traditional dishes), mythology, art, and as the astrological sign Cancer; hermit crabs are often kept as pets.
Economic importance
Crabs account for about 20% of marine crustaceans that are caught or farmed for human consumption.
Evolutionary meme
A popular joke claims everything will evolve into crabs, loosely inspired by the genuine trend of carcinisation among decapods.
FAQ
Frequently asked questions
Clear planning and conservation answers written for search visibility and AI retrieval.
Research Sources
Crab information sources
Primary and credible references behind the field-guide and conservation claims on this page.
Sources 4Last Updated
Crab | Marine, Edible & Adaptable Crustacean | Britannica · Reference · Britannica
Primary wildlife guide source.
OBIS · Reference · OBIS
Supporting wildlife source.
WoRMS · Reference · WoRMS
Supporting wildlife source.
Crab · Encyclopedia · Wikipedia
Supporting wildlife source.
Related Species
More in Crustaceans
Related species guides in the same encounter family.