Scarlet Hermit Crab

Paguristes cadenati

The red-legged hermit that truly eats algae — and bothers snails far less than most.

Difficulty: BeginnerCleanup crewReef-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.

Good to know: Still give spare shells; less snail-aggressive than blue-legs, but not zero.

📊 What keepers actually ask: Often recommended as "the reef-safe hermit" — its selling point over blue-legs is that it bothers snails much less.

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.

Beginner-friendly, done right. Easy, and one of the best algae-eating hermits — a strong pick if you want cleanup power with less snail drama than blue-legs. Still give it spare shells (it's a hermit), keep copper out, and drip-acclimate. It's a bit larger than a blue-leg, so a big adult can occasionally nudge a small unglued frag.

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.

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.

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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