The Hermit Crab

Getting Started with Hermit Crabs

Hermit crabs are not fish. They don't live in water full-time, they don't need a fully submerged tank, and they certainly don't fit in a little plastic cage from the craft store. I learned this the hard way after killing my first two crabs within three months by following online care sheets that were outdated or just wrong. The actual requirements are specific and non-negotiable if you want them alive longer than a few weeks. You need a glass tank, at least 10 gallons for one crab, but 20 gallons is where it actually makes sense. The footprint matters more than the height because these things climb and explore horizontally. A standard aquarium with a tight-fitting lid is fine, but you need ventilation. Mesh lids work but they dry out the substrate fast. I ended up using a combination—a solid glass top with small gaps on the sides for airflow—and that's been stable for years. The heating side is where most people mess up. You need a ceramic heat emitter, not a light. These crabs are tropical and they need 75 to 80 degrees Fahrenheit on the warm side and 70 to 75 on the cool side. A thermometer and hygrometer are mandatory. I started with cheap digital ones from Amazon and replaced them every four months because they drifted out of calibration. The Inkbird thermostat I use now has been accurate for over a year and it shuts the heater off if things get too hot, which saved me once when the bulb stuck on.

Humidity is the other big one. You need 70 to 80 percent humidity at all times. If it drops below 60, their gills dry out and they suffocate. It sounds dramatic but it's literally how they breathe. I keep a shallow water dish on one side and mist the tank twice a day. The substrate holds moisture and releases it slowly, which helps stabilize things between mistings.

Substrate and Hiding Spots

The substrate is not optional decoration. They need at least six inches of it—more if you can do it—because they burrow. I use a mix of coarse sand and play sand, roughly a 50-50 blend, slightly dampened so it holds shape when squeezed. Coconut fiber works too but it molds faster and smells worse when it gets wet. If you've ever opened a tank lid and been hit with the smell of rotten substrate, you know what I mean. It happens fast. You need hiding spots on both the warm and cool sides. Half logs, cork bark, anything they can curl into. They're vulnerable when they molt and they need darkness and cover. I had a blue hermit crab disappear for eleven days and when I finally moved the cork bark, he was buried three inches down in a Y-shaped tunnel, soft-shell and vulnerable. He came out fine two weeks later after I left him alone the whole time. Most people would have dug him up out of curiosity and killed him.

Feeding and Nutrition

These are scavengers and they eat basically everything. Protein is critical, especially when they're growing or molting. I offer dried shrimp, fish flakes, cuttlebone for calcium, and occasional pieces of fruit or vegetable. Never use anything with seasoning or salt. Their kidneys can't handle it and I watched one crab bloat and die after someone in a forum told me a pinch of table salt was fine. It wasn't fine. They also need fresh water and salt water dishes. I mix marine salt into a separate bowl using the kind made for reef tanks, not the kind for humans. The concentration should be about 1.020 to 1.022 specific gravity. They dip into both to regulate their internal chemistry. Check the water daily and replace it. Stagnant water breeds bacteria and I once lost a crab to what I think was a bacterial infection from a water bowl that hadn't been changed in a week.

Molting: What Actually Happens

Molting is the hardest part to deal with because you have to do almost nothing. When your crab starts molting, it will dig deep and dig fast. Do not pull it out. Do not move the substrate. Do not check on it constantly. The molt itself can take anywhere from two weeks to three months depending on the size of the crab and the temperature in the tank. Warmer tanks tend to speed it up slightly. During this time the crab is extremely soft and defenseless. Its old exoskeleton splits and it pulls itself out of it. Then it sits there, swollen and pale, while the new shell hardens. It will not eat. It will not move. People think it's dead and dump it or dig it up. I've had to stop people from doing this more than once on forums where they'd post pictures of a motionless crab and ask what was wrong. Nothing was wrong. It was molting. After the molt, the old shell stays in the tank. Leave it there. Some crabs eat their old exoskeleton to recycle the calcium. Others don't. Either way is normal. I've seen both.

Handling and Social Considerations

You can handle hermit crabs but you need to wet your hands first. Dry skin irritates them and they grab onto anything, including your fingers, and it's hard to gently remove them without causing stress. Pick them up from behind, support their abdomen, and don't hold them for more than a few minutes at a time. Their legs are fragile and a fall from even waist height can break them. They're social in the sense that they do better in groups. I've kept single crabs before and they were fine but they seemed less active and less interested in food. A pair or a small group changes the dynamic noticeably. They compete for shells, they bicker over territory, and they seem more engaged overall. Just make sure you have enough shells for everyone plus a few spares. Shell competition is real and I've seen a larger crab harass a smaller one for weeks until the smaller one died from stress and lack of eating.

Common Problems and How to Fix Them

If your crab is always out of its shell and moving slowly, check the humidity first. That's almost always the issue. If it's gasping at the water surface, check the temperature. If it's losing legs, that could be aggression from tankmates or a calcium deficiency. Missing legs sometimes regenerate over multiple molts but not always. Parasitic mites are another thing. They look like tiny moving dots on the crab's body and gills. You can spot them with a magnifying glass. Treat with imidacloprid-based products made for crustaceans, not general insecticides. I found that treating the whole tank substrate too often caused more problems than it solved, so I only treat when I actually see mites and I spot-treat the affected crab's area rather than blanketing everything. Bubble mold in the substrate is common if the bedding gets too wet and stays that way. It looks like white foam and smells bad. Remove the affected layer, replace it, and adjust your misting schedule. This happened to me in winter when I was over-misting to combat dry indoor air and I lost about an inch of the top substrate layer to mold before I caught it.

What to Buy and Where to Get It

Essential supplies: glass tank, ceramic heat emitter, thermostat, UTH mat heater as backup, thermometer and hygrometer, marine salt, dechlorinated water, coarse sand, play sand, coconut fiber (optional), cuttlebone, dried shrimp, fish flakes, hiding spots, shallow dishes for water. A spray bottle for misting and a small net for water changes are useful but not critical. Avoid the little habitat kits from big-box pet stores. The heaters they include are underpowered, the lids don't seal properly, and the substrate depth they show is impossible to achieve with their tanks. I bought one once and spent more money redoing it than I would have just buying proper gear from the start. Save yourself the money and the dead crabs. If you're looking for a care sheet to keep handy, the Caribbean Crustacean Society has a solid online guide. The Hermit Crab Initiative also runs a Facebook group where people post pictures and diagnose issues. I've gotten help from both when I wasn't sure about something. Not all advice on those forums is good, but the ones who know what they're talking about are genuinely helpful.