A small, brilliantly red-legged hermit crab prized as a reef-safe cleanup-crew workhorse. It grazes nuisance hair algae, cyanobacteria, and detritus, staying small and peaceful enough for nano reefs.
ℹ️
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Small; typically 1-1.5 in (2.5-4 cm) including shell.
Lifespan
2–4 years
Social needs
group
Native region
Western Atlantic and Caribbean reefs, including Florida, the Bahamas, and the West Indies
Origin
New World
Climate
🌴 Tropical
Water type
🌊 Marine
Family
Diogenidae
Genus
Paguristes
Part of the Hermit Crabs
Shell-dwelling crustaceans kept as scavenging cleanup crew. Marine reef hermits graze algae, cyanobacteria, and detritus across live rock and sand, staying small and mostly peaceful; provide assorted empty shells so growing crabs can upgrade homes instead of attacking snails.
From the minimum an animal needs to be kept humanely, up to the ideal setup. Bigger is almost always better — minimums are floors, not targets.
Photo coming soon
Minimum
Reef aquarium
20 gal (≈ 76 L) reef
Paguristes cadenati ('scarlet reef hermit') is a marine reef cleanup-crew hermit. Keep ~1 per 10 gal in a reef with rockwork and abundant spare shells. They are notably peaceful and rarely bother snails.
Photo coming soon
Recommended
Reef aquarium with shell library
30–55 gal reef, varied shells
A 30+ gal reef with rockwork, sand bed, and a 'shell library' of empty shells. Scarlets are excellent algae and detritus eaters; supplement with seaweed sheets.
Photo coming soon
Ideal
Mature display reef
55+ gal mature reef
A mature reef with diverse cleanup crew and abundant shells. Striking red colouration with yellow eyes shows beautifully against natural rockwork.
Life & growth stages
How this animal changes through its life — each stage often has its own care, diet and space needs.
Photo coming soon
Larva
Most marine invertebrates hatch into microscopic planktonic larvae (such as the zoea of crustaceans or the bipinnaria/veliger of echinoderms and mollusks) that drift and feed in the water column. The larva looks nothing like the adult and undergoes major reorganization.
Photo coming soon
Juvenile
After settling out of the plankton, the juvenile takes on a recognizable miniature of the adult body plan — a tiny shell, a small star, or a translucent shrimp. Crustaceans grow by molting, shedding the exoskeleton to enlarge.
Adult
Adults reach full size and reproductive maturity with the species' mature shell, shape, or coloration. Many continue to molt or grow throughout life, and some show sex differences in size or claw/appendage shape.
Habitat & enclosure
Suited to any established marine or reef tank from about 38 L (10 gal) upward, with one crab per roughly 38-75 L as a rough stocking guide so they don't compete for food. Keep typical reef parameters: temperature 24-27 C (75-80 F), pH 8.1-8.4, specific gravity 1.023-1.026, and stable alkalinity and calcium; like all inverts they are intolerant of copper and ammonia.
Provide plenty of live rock with crevices for grazing, hiding, and molting. Moderate flow and standard reef lighting are fine — these crabs are not light-dependent and simply need surfaces coated in algae and biofilm to work.
Substrate
Live rock is the most important aquascape element, giving grazing surface and molt-hiding crevices. A fine aragonite sand bed lets them sift detritus, and scattered empty shells of graduated sizes prevent shell competition.
Equipment & setup
Standard reef setup: biological filtration via live rock, a heater (and chiller if the room runs hot) for stable temperature, and a protein skimmer for nutrient export. No special lighting is needed; gentle to moderate powerhead flow keeps detritus suspended for grazing.
Diet
Primarily a herbivorous detritivore that grazes hair algae, film algae, cyanobacteria, and leftover food off rock and sand. In well-fed or low-algae tanks supplement with dried seaweed (nori), sinking pellets, or bits of meaty food so they don't starve. Offer occasional calcium-rich foods and ensure dissolved calcium is adequate to support healthy molts.
Behavior & temperament
Generally peaceful and reef-safe, making it one of the better-behaved hermits for mixed reefs. It is far less aggressive than blue-leg hermits but will still kill snails to steal a larger shell if empty shells are scarce. Compatible with most fish, corals, and other cleanup-crew inverts; keep several spare shells available to prevent shell-related squabbles.
Health
Main risks are copper exposure (lethal to all inverts), unstable salinity, and being out-competed for food in sparse tanks, leading to slow starvation. Molting is a normal, vulnerable period — the crab may lie motionless or shed an empty exoskeleton that looks like a dead crab, so never discard a 'body' until certain. Drip-acclimate slowly, as hermits are sensitive to sudden salinity and pH swings.
Tips, DIY & hacks
Always drip-acclimate over 30-60 minutes to avoid salinity shock, and quarantine or inspect for hitchhiking pests before adding. Toss in a handful of empty snail shells in several sizes so growing crabs upgrade homes without attacking your snails. They excel at clearing red-slime cyanobacteria and hair algae in nano and pico reefs.
