Crabs in Reef Tanks: Useful, Chaotic, Occasionally Murderous
Marine crabs have a reputation problem. Some are brilliant CUC members. Others eat corals, kill snails, and cause mayhem. Knowing which is which makes all the difference.
Emerald Crabs
Famous for eating bubble algae — those annoying green bubbles that spread across rocks.
The good: Genuinely eat bubble algae, also graze film algae and detritus, attractive green colour.
The risk: As they grow (up to 5cm), they can become opportunistic. There are credible reports of emerald crabs nipping at zoanthids, soft corals, and LPS — especially when underfed. Keep them well-fed with supplementary nori and pellets to reduce this risk.
Useful for a specific problem. Once bubble algae is controlled, monitor closely and consider rehoming if they show interest in corals.
Dwarf Hermit Crabs
Blue Leg Hermits — The standard reef tank hermit. Tiny, active, eat algae and detritus. The issue: they murder snails for shells. Keep a handful of empty shells scattered around the tank to dramatically reduce casualties.
Scarlet Hermits — Bright red, slightly larger, generally considered less aggressive. Many reefers prefer these over blue legs.
Halloween Hermits — Orange and black striped. Larger than dwarfs. Fine in medium to large tanks.
Crabs to Avoid
Decorator Crabs — tear pieces off corals for camouflage. Arrow Crabs — eat bristle worms but also feather dusters and small shrimp. Any large crab species — they get aggressive and predatory.
Managing Hermit-Snail Conflict
- Empty shell supply — the single most effective measure
- Trochus snails — shell shape is wrong for most hermits, reducing targeting
- Don't overpopulate hermits
- Feed the CUC — well-fed hermits are less likely to hunt
For hermits: 1 per 8–10 litres. For emerald crabs: 1 per 100 litres max — they're territorial.
Mixed CUC packs often include hermits at good value.
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