Scarlet Hermit Crab
Paguristes cadenati
The red-legged hermit that truly eats algae — and bothers snails far less than most.
Difficulty: Beginner Cleanup crew Reef-safe
The scarlet (red-leg) hermit crab (Paguristes cadenati) is the blue-leg's classier cousin — bright red legs, a voracious appetite for nuisance algae (filamentous, hair and slime algae, and it'll even work on cyanobacteria), and a reputation as one of the more snail-friendly hermits. It still wants spare shells, but it picks fights with snails far less than most.
Care at a glance
| Adult size | Small–medium — around 1–1.5 in. |
|---|---|
| Minimum tank | Any established tank; roughly 1 per 10 gallons. |
| 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 | Nuisance algae — filamentous, hair and slime algae — plus cyanobacteria and detritus. |
| Temperament | Peaceful toward corals and fish, and notably less aggressive toward snails than other hermits — though not entirely risk-free. |
| Reef-safe | Yes — a genuinely reef-safe grazer, and the most snail-tolerant of the common hermits. |
| Natural range | Caribbean / Western Atlantic. |
The fish store gets these wrong
Common myths, kindly corrected — no shame for having believed them.
- "Scarlet hermits never touch snails." Less often than blue-legs, yes — but a shell-short hermit is still a shell-short hermit. Provide spares.
- "Hermits and snails can't be mixed." They can — scarlet hermits are among the most snail-tolerant; the key is enough spare shells for everyone.
Before you buy
- Spare empty marine shells in assorted sizes — even for the snail-friendly hermit.
- An established tank with algae to graze.
- Stock modestly; drip-acclimate; 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:
- Bothering snails — add more spare shells.
- Out of its shell or a discarded shell — usually a moult or shell change (normal).
- Failed moult or missing limbs — a chemistry or nutrition issue.
Work through the Scarlet Hermit Crab trouble card — the water, light, flow and pest checks that fix almost every struggling invert.
Never
- Never skip spare shells.
- Never use painted shells.
- Never dose copper with inverts in the tank.
- Never drop it straight in — drip-acclimate.
Common scarlet hermit crab questions
Are scarlet hermit crabs reef-safe?
Yes — they're voracious algae grazers (hair, filamentous and slime algae, plus cyanobacteria) and leave corals alone. They're also the most snail-tolerant of the common hermits, though you should still supply spare shells.
What's the difference between scarlet and blue-leg hermits?
Scarlets (red legs) are a little larger, eat a wider range of nuisance algae, and bother snails much less; blue-legs are tiny, cheap and excellent on hair algae but more likely to kill a snail for its shell. Many keepers run a mix.
Do scarlet hermits eat cyanobacteria?
They'll graze it along with hair, filamentous and slime algae — useful, though cyano is ultimately a nutrient and flow problem to fix at the source.
How many scarlet hermits do I need?
Modestly — roughly one per 10 gallons as a starting point, adjusted to your algae. Overstocking any cleanup animal just starves it.
Will scarlet hermits kill my snails?
Far less often than other hermits, but not never — always keep a handful of spare empty shells so a growing hermit never needs to evict a snail.
🐚 Did you know? — Scarlet Hermit Crab fun facts
Little true delights about the scarlet hermit crab — each one checked against a named source, no reef-store folklore.
- The scarlet hermit crab is a striking sight, with a bright red body set off by yellow eyestalks. Wikipedia
- Its soft, unarmoured abdomen is left unprotected, so the crab keeps it tucked safely inside a borrowed snail shell. Wikipedia
- It ranges across the Caribbean and Gulf of Mexico and can be found from the shoreline down to depths of about 80 metres. Wikipedia
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:
- Aquarium Specialty — The Friendly and Reef Safe Scarlet Reef Hermit Crab — diet, reef-safe, snail tolerance
- Foxy Saltwater Tropicals — Saltwater Hermit Crabs Care Guide — shells, stocking, care
- AlgaeBarn — All About Saltwater Tank Crabs — reef-safe behaviour
Written and fact-checked by The Coralkin Team against marine-aquarist sources — if your scarlet hermit 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 →
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