Cardisoma armatum, commonly referred to as the Rainbow Crab, or sometimes even Halloween Crab (Gecarcinus species are more commonly given this name though) are a fairly commonly available Invertebrate sold in the UK, mostly in shops that mainly sell Fish. It is often sold with wrong, or misleading information, and many may suffer, or cause suffering to Fish, that they should not be kept with, because of this.

These Crabs can get quite large, up to 20cm across the carapace, though this would be unusual for captive specimens, they usually stay smaller in captivity. They will eat, or kill most Fish, and other seemingly possible tankmates that they can catch, though their usual diet in captivity should consist of mainly fruit and vegetable matter.

Captive breeding of this species is not possible, certainly not for the average keeper, in specialised labs it may be possible though. Like many Crabs, the young are planktonic, the eggs are released into the sea where the young develop before returning to land, where they spend the majority of their time. And so, every Rainbow Crab that is kept would be wild caught.

In the wild they are found in West Africa, in the lower Volta river system, and also coastal West Africa, where they occupy mud flats, constructing burrows. The ones in captivity are imported mostly from Nigeria.

Sexing is fairly easy, female Crabs has a larger flap on their underside than males, which is used to carry eggs. Males are also usually more colourful than females.

Keeping this species is fairly easy if all their needs are met. Since they are Land Crabs they should have a larger land area than water. Though, they should still have an option to submerge themselves fully in a, heated, fully filtered, water area.

An area of water, approx 1/3 the length of the tank, around 6-8 inches deep would be ideal. A small submersible heater, heated to around 24c, an also a small submersible power filter and airstone powered by an external air pump can all be used, and are all recommended, the aquarium heater especially, since this species is tropical.

The water can be freshwater, or slightly brackish (SG 1.004-1.008)

To mix brackish water sea salt is needed, not commercial aquarium salt. Brands suitable include instant ocean, and red sea salt. It should be mixed in a bucket, left to completely dissolve, and checked with an hydrometer, and diluted if necessary until the desired SG is reached.

Brackish water isn’t essential though, they can also be kept in freshwater, with a ph above 7. Liquid calcium, usually meant for Apple Snails can be added to the water (in quantities that should be stated on the bottle) aragonite coral sand can be used to raise the ph if necessary.

Partial water changes should be carried out periodically, the filter should be cleaned every week or so, the filter sponge should be cleaned using water from the Crabs tank, not the tap.

The land area should have a mixed substrate of fine gravel, sand, coir and moss. It should be deep enough for the Crabs to construct burrow’s, around 8 inches or so deep would be ok. Small pieces of bogwood, mopani wood etc.. can be added to the land area, also fake plants can be used. Most live plants will be uprooted and destroyed fairly quickly.

It’s a good idea to heat the land area with a heat bulb, usually sold for Reptiles. The humidity can stay around 50-60%, though a higher humidity would also be fine for them. The temperature of the land area should be around 25-28c.

These Crabs are opportunists, and will eat most foods offered. Fruits, including banana, apple and orange are all readily taken, very often they are taken into the Crabs burrows to eat later, or into the water area of the tank. Checks should be made to remove any decomposed fruits, vegetables, and especially meaty foods that are offered.

They are territorial, and may not tolerate other Crabs in their territory, so very large tanks are often needed if attempting to keep more than one. A tank around 3.5ft x 1.5ft should be ok for two Crabs. They will need to be checked often for any fighting, as this may lead to only one Crab being left in the tank, or the other Crab badly injured.

These Crabs should always be kept with a land area, keeping them in aquariums with little or no land will be very stressful for the Crabs, they would likely die shortly after being introduced to such a set-up.

No other animals should be kept with these, smaller Crabs will be killed and eaten, and so will Fish. Any Fish larger than the Crabs are likely to see them as a possible meal, so no combinations would likely work.

Woodlice can be attempted as part of a clean-up crew for the tank, though these will even be eaten, I have in the past seen my Crabs pick at live Woodlice before eating them.

They can be active throughout the day and night, though are most likely to be active at night.