Rainbow Crab

About

The rainbow crab is not an aquarium animal, and almost everything that goes wrong with it follows from being sold as one. Wikipedia opens by calling Cardisoma armatum 'a species of terrestrial crab' and records that 'while juvenile and adult crabs spend most of their time on dry land, the females must return to the ocean to release their eggs', closing the section with the sentence that matters most on this page: 'This land crab cannot remain submerged for long periods.' Fishkeeping World says the same thing from the keeper's end — 'The Rainbow Crab needs to be able to submerge in water but will drown if they can't get out of the water' — and Shrimp and Snail Breeder opens its housing section by repeating that this 'is not a fully freshwater species. Adult Rainbow crabs are semi-terrestrial (land) crabs. They need water only to moist surface to refresh their gills.' It has gills, but modified ones, and it keeps them wet by carrying water rather than by living in it. Put one in a fish tank with a few centimetres of gravel above the waterline and it will eventually tire and go under. It is a big, spectacular, bad-tempered animal: a violet-blue carapace, orange-red legs and cream claws when young, faded and yellowish with age, 7–12 cm across the shell in captivity by Shrimp and Snail Breeder's reckoning, while Wikipedia gives a carapace reaching 20 cm across, 'although captive individuals rarely reach this size'. Its aggression is legendary enough to have given it a trade name — Wikipedia notes adults are sometimes labelled 'soapdish' crabs because on the way to pet shops 'they are often packed in soap dishes to prevent them from killing each other'.

Care

What this animal needs is a heated, humid paludarium, and the volume figure in our data is the enclosure, not a water volume. Both care sources ask for at least 90 litres per crab. Our gates add 80 cm of floor and 40 cm of height, so a low 20-gallon footprint will not do: deep substrate, a climbable water edge and air space above them do not fit under 40 cm. Shrimp and Snail Breeder puts the split at '10 – 30 % water so that the crabs can keep their gills wet and molt properly' and Fishkeeping World describes 'the correct mix of water and land'; either way the majority of the floor is land, with a slope or ramp of rocks and pebbles at every water edge so the crab can always climb out. Give it 7–12 cm of moist sand or a coco-fibre and sand mix, deep enough that a pencil pushed in and pulled out leaves a tunnel that does not collapse — burrowing is not decoration for a gecarcinid, it is how it regulates its own moisture and where it goes to molt. Keep the air at 24–29 °C and humidity at 75% or higher; keep the water clean, fresh or lightly brackish at around 5–10%, hard, and change it. The lid has to be tight: these are strong climbers and, in Fishkeeping World's words, you should 'make sure that the habitat you've lovingly set up isn't high enough for them to climb out'. Keep one. Fishkeeping World is blunt — 'They'll fight to the death with their own, and really should be kept alone in a tank' — and Shrimp and Snail Breeder allows at most a male and a female in a much larger enclosure. This entry still gates at one, because that pair allowance depends on doubling the enclosure and the planner has no way to check that it was doubled. Everything else in the tank is at risk too: 'they may attack other crabs, fish, snails, and shrimp'. This entry is marked as unsafe with live plants, and the reason should be said out loud rather than left as a flag on a chart: Shrimp and Snail Breeder answers the question with 'No, Rainbow crabs are not plant safe. They will eat, cut, and uproot everything that is possible in your tank', adding that 'if live plants are kept in their tank, they may eat these as well', and Wikipedia's wild-diet line puts fruit and vegetation first. Plant the land side with things you are willing to lose, or with tough foliage in pots, and expect redecorating. One last thing worth saying plainly: this crab cannot be bred at home. Females carry hundreds of thousands of eggs to the sea and the larvae need weeks in salt water, so every rainbow crab in a shop was taken from a beach in West Africa.

Diet

Omnivore whose wild diet is mainly fruit, vegetation and carrion, with animal prey taken when catchable. Feed on the land side: fruit, greens, blanched vegetables and dried leaves as a base, sinking crab or shrimp pellets, and protein about once a week. Calcium is essential for molting.

Telling males from females

There is no reliable way to sex this species by eye. That is the honest answer, not a gap in our data.

Also in these lists

Similar species

Same category and the closest water parameters — if this species isn't quite it, start here.

Sold in Lithuania

These Lithuanian shops listed this species in their public catalogue (checked August 8, 2026). Stock moves — ask before you travel.

These links are unpaid: we get nothing for them and ask for nothing.

Frequently asked questions

What tank size does a Rainbow Crab need?

A Rainbow Crab needs an aquarium of at least 90 litres (23.8 US gallons) with a footprint at least 80 cm (31.5 in) long — for this species, swimming length matters as much as raw volume.

What water parameters does a Rainbow Crab need?

Rainbow Crab does best at 22–30 °C (71.6–86 °F), pH 6.8–8.2 and a general hardness of 12–22 °dGH.

Is the Rainbow Crab suitable for beginners?

Rainbow Crab is a demanding species best attempted with some experience — it needs stable, closely watched conditions to thrive.

Sources

• Wikipedia — Cardisoma armatum
• Shrimp and Snail Breeder — Rainbow Crab: Detailed Guide, Care, Diet, and Breeding
• Fishkeeping World — Complete Guide to Rainbow Crab Care & Species Profile