Emerald crab

Mithraculus sculptus · Mithracidae

Also known as Green mithrax crab

Beginner

Eats bubble algae and detritus; useful, but an underfed adult starts sampling corals.

In short: An emerald crab needs at least 40 L (11 US gal), water at 24–27 °C (75–81 °F) and pH 8.1–8.4. It is semi-aggressive, usually kept singly, and rated beginner to keep.

Updated 6 October 2026

Minimum tank: 40 L (11 US gal)
Temperature: 24–27 °C (75–81 °F)
pH: 8.1–8.4
Adult size: 5 cm (2 in)
Group size: 1+

Reef & marine husbandry

Specific gravity: 1.024–1.026
Reef safety: Reef safe with caution
With corals: Will pick at soft coral and zoanthids when hungry.
With inverts: May catch slow snails.
Minimum in a mixed reef: 40 L (11 US gal)
Minimum fish-only (FOWLR): 40 L (11 US gal)
Aggression: Opportunistic at night.
Feeding: Offer nori if bubble algae runs out.

Care requirements

Minimum tank volume: 40 L (11 US gal)
Temperature: 24–27 °C (75–81 °F)
pH: 8.1–8.4
Hardness: 8–12 dGH
Lifespan: 2–4 years
Diet: Omnivore — bubble algae, detritus, sinking pellets, nori.
Care level: Beginner

Origin & habitat

Native region: Caribbean
Water type: Saltwater
Biotope: Rocky reef crevices
Natural conditions: Stable salinity, plenty of rock.

Behaviour & stocking

Temperament: Semi-aggressive
Minimum group: 1 fish
Swims: Bottom
Good tank mates: Tangs
Clownfish
Snails
Avoid: Small sleeping fish
Delicate SPS frags

Breeding & difficulty

Breeding difficulty: Advanced
Sexing: Females have a broader abdominal flap.
Notes: Not bred at home.
Common ailments: Moult failure
Copper poisoning

Will it fit your tank?

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