Emerald Crab
Mithraculus sculptus
The go-to fix for bubble algae — a hardy little crab that eats what snails won't.
Difficulty: BeginnerCleanup crewReef-safe
The emerald crab (Mithraculus sculptus) is the reef hobby's go-to fix for bubble algae (Valonia) — the stubborn green bubbles most snails and fish won't touch. A well-chosen emerald punctures them at the base and eats the contents. The honest catch is right there in the research: given the choice, emerald crabs prefer easy food (pellets, mysis) to bubble algae, and a hungry or oversized crab will start picking at corals, small snails and even sleeping fish at night. It's reef-safe with caution — and the caution is “keep it a little hungry.”
Care at a glance
| Adult size | ≈2 in across the carapace. |
|---|---|
| Minimum tank | 10+ gallons with plenty of rockwork; usually kept one per tank. |
| Temperature | 75–82°F (24–28°C). |
| pH | 8.1–8.4. |
| Specific gravity | 1.023–1.026 (reef-standard ≈35 ppt). |
| Other targets | Standard reef calcium and alkalinity support healthy moulting; like all inverts it has zero tolerance for copper. |
| Diet | Omnivore — eats bubble algae (Valonia) and other nuisance algae, plus meaty scraps and detritus. It will take pellets and mysis readily — which is exactly why an overfed crab ignores the algae. |
| Temperament | Nocturnal and can be territorial — usually one per tank. Reef-safe with caution (see below). |
| Reef-safe | With caution — a hungry or large emerald may pick at LPS, zoanthids, small snails or sleeping fish at night. |
| Natural range | Caribbean / Western Atlantic. |
The fish store gets these wrong
Common myths, kindly corrected — no shame for having believed them.
- “An emerald crab guarantees bubble algae is gone.” Not guaranteed — studies show they prefer pellets and mysis and only reliably work on Valonia when other food is scarce, and bigger crabs eat more than small ones.
- “Emerald crabs are fully reef-safe.” Reef-safe with caution — a hungry or oversized crab will pick at LPS, zoanthids, small snails and even sleeping fish at night. Keep it a little hungry, and watch it.
- “Add several to clear algae faster.” They can be territorial — usually one per tank. More crabs mean more mouths to overfeed and more coral risk, not faster results.
Before you buy
- Buy it for a real bubble-algae problem — a medium/large crab, usually one per tank.
- Don't overfeed the tank, or it'll ignore the algae for pellets.
- Watch it around LPS, zoanthids and small snails — remove it if it turns predatory.
- Standard invert care: cycled tank, slow drip-acclimation, never any copper.
Signs something's wrong
A struggling invert is usually telling you about water quality, acclimation, or (for shrimp and crabs) a fresh moult — here's what to watch for:
- Picking at corals, zoanthids or small snails — it's hungry or too big; remove it (this is the main reason emeralds get pulled).
- Ignoring bubble algae entirely — usually overfed on other food, or too small a crab for the job.
- Failed moult / missing limbs — a chemistry or nutrition problem; stabilise parameters.
Never
- Never dose copper-based medications with the crab in the tank — it's lethal.
- Never assume it's harmless to corals — a hungry emerald turns predatory; keep watch.
- Never overfeed the tank if you want it eating bubble algae.
- Never crowd several into one tank — they're territorial; one is usually right.
Common emerald crab questions
Do emerald crabs really eat bubble algae?
Often, yes — the emerald crab is the hobby's most reliable eater of Valonia bubble algae, puncturing the bubbles at the base. But it's not guaranteed: research shows they prefer pellets and mysis and mainly work on bubble algae when other food is scarce, and larger crabs eat more of it than small ones.
Are emerald crabs reef-safe?
Reef-safe with caution. A well-fed crab mostly grazes algae and detritus, but a hungry or oversized emerald will pick at LPS corals, zoanthids, small snails and even sleeping fish at night. Keep it a little hungry (so it eats algae), and watch it — remove it if it turns on your corals.
How do I get my emerald crab to eat bubble algae?
Don't overfeed the tank. Given easy food, emeralds choose pellets and mysis over Valonia, so a slightly hungry crab is far more likely to work on the algae. A medium-to-large crab is also more effective than a small one.
How many emerald crabs can I keep?
Usually just one per tank — they can be territorial with each other, and more crabs mean more mouths to overfeed and more risk to corals, not faster algae control.
Are emerald crabs hard to keep alive?
No — they're hardy and undemanding as long as you treat them as inverts: add them to a cycled tank, drip-acclimate slowly, and never use copper medications. The real management is their appetite, not their hardiness.
Sources & further reading
This guide was written and fact-checked against these authorities. It's honest, general guidance — not a diagnosis, and not a substitute for an aquatic vet or your own test kit:
- Cambridge Core (J. Marine Biological Association UK) — Efficiency of emerald crabs controlling bubble alga — peer-reviewed: food preference (pellets/mysis over algae), size effect
- AquariumStoreDepot — Emerald Crab Care Guide — reef-safe-with-caution, keep fed, tank size, nocturnal
- Shrimp and Snail Breeder — Emerald Crab Detailed Guide — care, diet, behaviour
Written and fact-checked by The Coralkin Team against marine-aquarist sources — if your emerald crab is struggling, it's almost always water quality, acclimation or a moult — check those first and ask an experienced reefer. We never guess a care number.
🔬 Review status: team-written and fact-checked against cited sources · not yet reviewed by a marine vet. How we research & fact-check →