Emerald crab (Mithraculus sculptus) care guide

Group
Other invertebrates
Other names
Green clinging crab, Emerald green crab, Mithrax sculptus, Maia sculptus, Mithraculus coronatus
Family
Mithracidae (formerly Majidae)
Origin
Caribbean Sea and Gulf of Mexico, from the Bahamas and southern Florida to northern Brazil, down to about 54 m
Water
saltwater
Maximum size
6 cm
Temperature
22–26°C
pH
8.1–8.4
Hardness
dKH 8–12, SG 1.020–1.025
Minimum tank
75 litres
Care level
beginner
Temperament
semi_aggressive
Reef safe
with_caution
Lifespan
2–3 years

Care

The emerald crab is a small, flat, shiny green crab from the Caribbean and Gulf of Mexico. Wikipedia gives a size of about 4 cm and LiveAquaria up to about 6 cm across the legs. The carapace is dark green, the spoon-shaped claws are green with white tips, and the paler walking legs are hairy. It is often found in back-reef habitats, among finger coral branches, and hides under sun anemones when threatened. Its family is now usually given as Mithracidae; older sources and LiveAquaria use Majidae. Set-up: an established reef tank of 75 litres or more with plenty of rock and caves. It is mostly nocturnal, hiding by day until it feels secure. Its strong legs grip so well that it can be hard to remove. LiveAquaria gives 22–26 °C, pH 8.1–8.4, dKH 8–12 and SG 1.020–1.025, and notes it does not tolerate unstable water. Clean-up crew: the emerald crab is best known for eating bubble algae (Valonia), and research suggests it controls it best when also fed mysis rather than pellets. It also eats other nuisance algae and leftover meaty food. Keep one per tank as a rule; two can fight unless the tank is large. Reef safety: if underfed it may eat coral polyps, small invertebrates or small sleeping fish, so feed it and watch it. Copper: copper-based medicines and some plant fertilisers are toxic to shrimp, snails and most other invertebrates. Never treat a tank holding them with copper, check the label of any fertiliser or medicine, and keep nets and buckets used for copper-treated tanks separate. Sources: LiveAquaria (liveaquaria.com/products/emerald-crab); Wikipedia (en.wikipedia.org/wiki/Mithraculussculptus). Checked October 2026.

Diet

An omnivorous scavenger. It eats bubble algae and other nuisance algae, plus leftover meaty food. Feed dried seaweed, chopped mysis or shrimp and quality pellets a few times a week, especially once nuisance algae is gone.

Breeding

Females have a wider, rounder abdominal flap, used to carry eggs; males have a narrow flap. Females carry eggs under the body, and the larvae are planktonic and are not raised in home tanks.

Compatibility and tank mates

Generally tolerant of tankmates when well fed, but an opportunist: hungry crabs may take coral polyps, small shrimp, snails or small fish. Avoid triggers, puffers, hogfish and large wrasses, which eat crabs. Keep only one in smaller tanks.

Last updated 8 October 2026. Community-edited care guide on Aquatic-Auctions.

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