Crab

Emerald crab

Mithraculus sculptus

Also sold as Mithrax crab

Group: Crab
Care level: Beginner
With corals: Reef safe with caution

What does the emerald crab do in a reef tank?

Nuisance algae including bubble algae, and leftover meaty food

With corals: With caution

How many should you keep?

Start with at most 1 per 100 L (22 US gal)

One US gallon is 3.785 litres and one UK gallon is 4.546 litres, so a "per gallon" rule depends on whose gallon it means.

What can go wrong?

If hungry may eat corals, other inverts, coralline or small fish; some never touch bubble algae

Which tankmates are risky?

Fish

Fish that may eat or harass it.

None of the fish in this guide is known to eat or harass it.

Other invertebrates

Invertebrates it may harm, or that may harm it.

None known.

These checks follow the reasons fish and invertebrates clash: same species or genus, similar shape, mouth size, temperament, food, burrows, tank size and the order they are added. Individual animals differ; every source agrees on that.

Sources

Practical Fishkeeping (UK), essential cleaners for the reef tank (date not shown)Read 21 October 2026, read in full.
Reef Central CUC advice (hermits, emerald crabs)Read 21 October 2026, search summary only.

Last checked 21 October 2026. How we check

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