# Crab — Facts, Threats & Conservation

*Infraorder Brachyura*

> A crab is a ten-legged crustacean of the infraorder Brachyura, with a hard exoskeleton, a short broad body, a pair of front pincers and a sideways walk. About 7,000 species live in the oceans, in fresh water and on land worldwide.

**IUCN status:** Most species Not Evaluated (IUCN)  ·  **WARN range:** Oceans worldwide, Coastlines and estuaries, Freshwater rivers and lakes, Tropical land and forests, Deep sea

## Quick facts
| Fact | Value |
| --- | --- |
| Common name | Crab (true crab) |
| Group | Infraorder Brachyura |
| Type | Decapod crustacean |
| Number of species | About 7,000 |
| Legs | 10 (front pair are pincers) |
| Body covering | Hard exoskeleton (carapace) |
| Growth | By moulting the shell |
| Movement | Typically walks sideways |
| Habitats | Oceans, fresh water, land |
| Diet | Mostly omnivorous scavengers |

## Scientific classification
- **Kingdom:** Animalia
- **Phylum:** Arthropoda
- **Subphylum:** Crustacea
- **Class:** Malacostraca
- **Order:** Decapoda
- **Infraorder:** Brachyura (true crabs)
- **Species:** ~7,000 described

## Conservation status
- **Status:** Crabs are a vast group of roughly 7,000 species, and the IUCN Red List has assessed only a fraction of them. Most crab species are therefore listed as Not Evaluated, meaning their conservation status is unknown rather than secure. Many widespread marine and coastal crabs are common, while some freshwater and narrow-range island species face real threats from habitat loss and pollution.
- **Population:** Unknown; varies enormously by species, with many common and abundant
- **Trend:** Unknown overall; declining for some range-restricted freshwater and island species
- **Assessed:** Varies by species (most not assessed)
- **CITES:** Not broadly listed; true crabs are generally not on CITES appendices
- Because most crab species have never been formally assessed, statements about crabs in general should be treated qualitatively rather than as precise figures.

## Key facts: Crab
- Crabs are decapod crustaceans of the infraorder Brachyura, with about 7,000 known species.
- They have ten legs in total; the front pair are modified into pincers (chelae) for feeding and defence.
- A hard exoskeleton protects the body, and crabs must moult it periodically to grow.
- Their sideways walk comes from the way their legs bend at the joints.
- Crabs live in oceans, fresh water and on land, from shallow shores to the deep sea.
- Most crab species have never been formally assessed by the IUCN, so their conservation status is largely unknown.

## What makes a crab a crab?
Crabs belong to the infraorder Brachyura, a name meaning "short tail" that points to their defining feature. Unlike lobsters or shrimp, a true crab's abdomen is small and folded tightly underneath the body, hidden beneath a broad, flattened shell called the carapace. Like all decapods, crabs have ten legs. The rearmost four pairs are used mainly for walking, while the front pair is enlarged into chelae, or pincers, used to grasp food, fight, dig and signal to other crabs. The hard outer exoskeleton is made largely of chitin reinforced with minerals, giving crabs their armoured appearance. Because this shell cannot stretch, a crab grows by moulting: it sheds the old casing and emerges soft and vulnerable, then takes in water to swell before the new shell hardens. Crabs sense the world through two stalked compound eyes, a pair of antennae and fine sensory hairs. Many look-alikes, including hermit crabs, porcelain crabs and horseshoe crabs, are not true Brachyura at all, despite sharing the name.

## Why do crabs walk sideways?
The sideways scuttle is one of the most familiar things about crabs, and it comes down to anatomy. A crab's legs are jointed so that they bend outward to the sides rather than forward and back, the way ours do. Pushing off in a sideways direction lets a crab move quickly and efficiently across sand, rock and seabed without its legs tangling, while keeping its broad body low and stable. Walking sideways also presents the narrower edge of the body to the direction of travel, which can help when squeezing between rocks or into crevices. That said, the rule is not absolute. Some crabs can walk forwards or backwards when needed, and certain swimming crabs have flattened, paddle-like rear legs that let them propel themselves through open water. Spider crabs and others amble more slowly in any direction. The sideways gait is best understood as the most natural and rapid movement for a typical broad-bodied crab, rather than the only one available to it.

## Where do crabs live and what do they eat?
Crabs are extraordinarily adaptable and have colonised an enormous range of habitats. The majority are marine, found from intertidal rock pools and sandy beaches down to coral reefs and the cold, dark deep sea. Others thrive in brackish estuaries and mangroves, while true freshwater crabs live in rivers, streams and lakes across the tropics and subtropics. Some species are largely terrestrial: land crabs and robber crabs roam tropical forests and shorelines, returning to water mainly to breed, and famous mass migrations see millions move between forest and sea. Most crabs are omnivorous opportunists. They scavenge dead plants and animals, graze on algae, sift detritus and hunt smaller creatures such as molluscs, worms and other crustaceans, using their pincers to tear and manipulate food. This broad diet makes crabs important recyclers, cleaning up organic matter and turning over sediment. In turn they are eaten by fish, birds, octopuses, mammals and people, giving them a central place in many food webs.

## True crab vs hermit crab
| Feature | True crab (Brachyura) | Hermit crab |
| --- | --- | --- |
| Group | Infraorder Brachyura | Superfamily Paguroidea |
| Abdomen | Short, hard, folded under body | Soft, curled, often unprotected |
| Shell | Own hard carapace | Borrows empty snail shells |
| A true crab? | Yes | No |
| Movement | Usually sideways | Drags borrowed shell along |

## What WARN does
World Animal Rescue Network does not run field projects specifically for crabs, which live far beyond WARN's five partner countries and are mostly outside its rescue remit. This guide is part of WARN's free educational work, helping people understand the wildlife that shares our planet. The threats that affect crabs, such as habitat loss, coastal pollution and overharvesting, are the same pressures that harm the animals WARN does protect, so learning about one corner of the natural world supports care for all of it.

If this guide deepened your wonder at the natural world, a small gift helps keep WARN's educational work free and its rescue work going.

## Frequently asked questions: Crab
### Are crabs insects or fish?
Neither. Crabs are crustaceans, a group of arthropods that also includes lobsters, shrimp and crayfish. Like insects they have a hard exoskeleton and jointed legs, but they are not insects, and they are certainly not fish. Crabs belong to the infraorder Brachyura within the order Decapoda, meaning "ten-footed", which describes their five pairs of legs.

### How many legs does a crab have?
A true crab has ten legs in total, which is why crabs are classed as decapods. The front pair is modified into pincers, called chelae, used for feeding, fighting and defence rather than walking. The remaining four pairs are walking legs. In some swimming crabs the rearmost pair is flattened into paddles, helping the crab move through open water.

### Why do crabs walk sideways?
Crabs walk sideways because their legs are jointed to bend outward to the sides rather than forward and back. Moving sideways lets a broad-bodied crab travel quickly and efficiently while staying low and stable, and helps it slip between rocks. Many crabs can also move forwards or backwards when they need to, so the sideways gait is the easiest, not the only, option.

### How do crabs grow if they have a hard shell?
Crabs grow by moulting. Because the hard exoskeleton cannot stretch, a crab periodically sheds its old shell and emerges with a soft new one underneath. It then takes in water to swell its body to a larger size before the fresh shell hardens around it. During this brief soft-shell stage the crab is very vulnerable and usually hides until its armour sets.

### Are hermit crabs and horseshoe crabs real crabs?
Not in the strict sense. True crabs belong to the infraorder Brachyura, with a short tail folded beneath a broad shell. Hermit crabs and porcelain crabs are crustaceans in a different group and are not true crabs, even though they look similar. Horseshoe crabs are not crustaceans at all and are more closely related to spiders and scorpions than to crabs.

### Are crabs endangered?
It varies, and for most species it is simply unknown. With around 7,000 crab species, the majority have never been formally assessed by the IUCN, so their conservation status is largely unrecorded. Some freshwater and island crabs with tiny ranges are threatened by habitat loss, while many widespread coastal species remain common. Pollution, climate change and overharvesting are shared pressures on crab populations.

## Sources
- [Crab — Wikipedia](https://en.wikipedia.org/wiki/Crab)
- [Brachyura — Wikipedia](https://en.wikipedia.org/wiki/Brachyura)
- [Crab — Encyclopaedia Britannica](https://www.britannica.com/animal/crab)
- [Decapoda — Wikipedia](https://en.wikipedia.org/wiki/Decapoda)
- [IUCN Red List (search)](https://www.iucnredlist.org/)
- [CITES](https://cites.org/)

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Full guide: https://worldanimalrescuenetwork.org/wildlife-guides/crab