Hermit Crab Care Guide

Land hermit crabs (Coenobita species) are tropical shoreline crustaceans that protect their soft abdomen with borrowed snail shells. They are sold as cheap, short-lived souvenirs, but they can live for decades in a humid, deep-substrate setup. Nearly all are taken from the wild, because they need the sea to breed. They suit a patient keeper who wants a nocturnal display animal rather than one to handle, and who has thought about the ethics of buying wild-caught animals.

Care requirements

Need	What to provide
Enclosure	Glass tank with a well-sealed lid to hold moisture. Substrate about 6 in (15 cm) deep for digging and molting, plus plants, moss and leaf litter (Smithsonian’s National Zoo)
Temperature	Hobbyist care sheets commonly suggest about 75–80°F (24–27°C). No authoritative figure was found. The natural habitat is tropical coastal forest and marsh (Smithsonian’s National Zoo)
Humidity	High humidity is vital because land hermit crabs breathe with modified gills. Hobbyist sheets usually suggest 70–80% relative humidity (Smithsonian’s National Zoo)
Lighting	Natural day–night light cycle, with plants, moss and hides for cover. Keep the tank out of direct sun, which overheats a closed tank. Cover mimics the zoo’s planted habitat (Smithsonian’s National Zoo)
Diet	Omnivorous scavengers that eat whatever they find. Offer a varied diet that includes leaf litter, and leave shed exoskeletons in the tank to be eaten (Smithsonian’s National Zoo)
Lifespan	Can reach 40 years, but most pet crabs die far sooner from poor care (Smithsonian’s National Zoo)
Social needs	Usually kept in small groups. Wild crabs gather and swap shells, but no authoritative minimum group size is published (Smithsonian’s National Zoo)
Legal notes	No common restrictions; check local rules. Almost all pet hermit crabs are wild-caught (Smithsonian’s National Zoo)

Wild-caught: what you are buying

The Smithsonian’s National Zoo is blunt. Land hermit crabs “do not breed in human care”. Adults release their larvae into the sea, and the young pass through several aquatic stages before migrating ashore. So almost every crab in a pet shop, gift kiosk or beach store was collected from the wild, and the zoo calls the trade “unsustainable”. Wild populations also lose habitat when mangroves and coastal forest are developed.

If you still want hermit crabs, take on crabs someone else is giving up rather than buying new ones, and set them up to live the full span. A Smithsonian intern writing about the zoo’s own habitat noted that these crabs “can live to be 40 years old and grow to the size of a coconut”, and that most pets don’t because of poor husbandry.

Humidity, water and substrate

Land hermit crabs have reduced gills inside a moist gill chamber. They also keep water inside their shell, and wet their gills and shell interior in pools of seawater (Smithsonian’s National Zoo). Dry air slowly suffocates them. The zoo provides:

• Both freshwater and saltwater pools. The crabs choose where to wade. Make each dish deep enough to submerge in but easy to climb out of, for example with a ramp or stones. Hobbyist guidance is to dechlorinate the fresh water, and to make the salt water with a marine aquarium salt mix rather than table salt.
• A re-insulated lid to trap moisture. A screen or mesh top lets humidity escape fast. Measure humidity with a hygrometer at crab level.
• Substrate about 6 in (15 cm) deep, because the crabs spend “a good deal of time digging and molting”. Hobbyist setups commonly use damp sand mixed with coconut fibre, which holds moisture and keeps burrows from collapsing.
• Plants, moss and leaf litter, which raise humidity and double as food.

Molting: hands off

Hermit crabs grow by molting. They build up water pressure to split the old exoskeleton and then form a new, larger one (Smithsonian’s National Zoo). Some bury themselves in the substrate to molt. Others store water in their shell and molt inside it. The Smithsonian says the whole process can take 45 to 120 days in some species.

• Don’t dig up a buried crab. A crab that has vanished for weeks is usually molting, not dead. Disturbing it while its new exoskeleton is soft can injure or kill it.
• Leave the old exoskeleton. The crab often eats it, possibly to reclaim calcium and minerals.
• Recognise a fresh molt. It shows a clean, bluish colour.
• Keep tankmates from bothering a molting crab. Keepers commonly isolate a surface molter behind a barrier.

Shells

Without a shell, a land hermit crab’s soft abdomen is unprotected, and a crab “cannot survive for long without its shell” (Smithsonian’s National Zoo). As it grows it must find a larger one. A crab assesses a shell by checking that the opening is clear, that it can withdraw fully inside, and that the shell lets it right itself when tipped over. Keep several clean, natural, empty shells in a range of sizes slightly larger than each crab’s current one. Wild crabs even queue to swap shells in turn, so a shortage leads to squabbling.

Gotchas

• The beach-shop starter kit. A small plastic box with gravel, a sponge and painted shells can’t hold humidity or depth. Crabs kept this way rarely survive long (Smithsonian’s National Zoo).
• Fresh water only. Land hermit crabs use seawater pools to wet their gills and shell interiors. Provide both fresh and salt water (Smithsonian’s National Zoo).
• Shallow substrate. Without enough depth to bury, crabs try to molt exposed, where they are vulnerable.
• Too few spare shells. A crab that outgrows its shell with nothing to move into is stressed and may fight others for theirs.
• Assuming a missing crab has died. A buried crab may be molting for weeks or months. Digging it up can kill it.

Avoid

• Painted or varnished shells, and shells with holes or chipped openings
• Mesh or open lids that let humidity escape
• Gravel, which can’t be burrowed into
• Table salt in place of marine aquarium salt
• Untreated tap water
• Handling or digging up crabs that are molting
• Releasing pet crabs on a beach or anywhere outside their native range (Smithsonian’s National Zoo)