Cardisoma armatum are more commonly known as Rainbow Crabs, Patriot Crabs, Tricolor Crabs, Moon Crabs or Soapdish Crabs. Rainbow Crabs are a popular group of semi-terrestrial crabs. They are hardy and easy to care for, making them a great choice for beginner aquarists.
In this guide below, I cover everything you need to know when caring for Rainbow Crabs, including ideal tank setups, healthy diets, breeding, compatibility with other species, etc.
Before I start, I would like to repeat again that Cardisoma armatum 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.
Quick Notes about the Rainbow Crabs
||African rainbow crab, Rainbow crab, Indigo crab, Land crab, Moon Crab, Patriot crab, Tricolor crab, Harlequin crab, Halloween Land Crab and Soapdish crab|
|Scientific Name||Cardisoma armatum|
|Tank size (minimal)||20 gallons (~90 liters)|
|Size of the carapace
||7 – 12 cm (3 – 4 inches)|
|Optimal Temperature||24 – 29°C (~75°F – 85°F)|
|Aquarium type||Paludarium (aquarium that has both terrestrial and aquatic elements (freshwater)|
|Optimal PH||7.0 – 7.5|
|Optimal KH||2 – 10|
||12 – 22|
||75% and higher|
|Nitrate||Less than 20 ppm|
|Life span||up to 8 years|
|Color Form||Purple-blue carapace and orange-red legs|
Origins, Natural Habitat of the Rainbow Crabs
Rainbow Crabs are land crabs that originate from the West Coast of Africa. They can be found along the coastal regions in the river deltas and other brackish areas, building homes near sources of fresh or seawater.
Their homes are burrows of sand that grow deeper and more complex as they age. Though in nature they can build deep colonies of tunnels, in captivity male Rainbow Crabs grow territorial in the presence of other male crabs.
Description of the Rainbow Crabs
Young and newly molted Rainbow Crabs are often very colorful with a purple-blue carapace and orange-red legs and tips of chelae. With age and approach to molt, carapace turns dirty yellow with occasionally slight reddish spots dorsally.
When they are young or when they have freshly molted their colors will be more vivid. As they age their colors will become lighter and more washed out.
Rainbow Crabs can grow pretty big in aquariums. Naturally, they reach 15 cm (6 inches) long, but they are likely to be nearer 7 – 12 (3 – 4 inches) when kept at home. The total weight can range between 96 – 290 g. Therefore, it will be better to give them extra space in your tank in case they grow a little larger than expected.
Rainbow Crabs have a lifespan of about 5 – 8 years.
Note: Do not confuse with Halloween Moon Crab.
The Behavior of the Rainbow Crabs
Like most crab species, Rainbow Crabs are natural loners. They often become more aggressive as they age. They are not social and do not need to be kept in groups. In fact, they will guaranteed fight fellow members of their own species.
Rainbow Crabs are natural diggers and construct deep burrows. They need it to maintain an optimal moisture level. Although these crabs appear to be terrestrial, they actually have modified gills. They require water/high moisture to breathe.
In the wild, Rainbow Crabs have an active phase on the land surface at night and an apparent resting phase in burrows in the daytime. However, they are not very timid and usually become used to its owners. In addition, with time, they can adapt their behavior to a feeding schedule.
Warning: Rainbow Crab has very strong and sharp claws and can even cut skin. Handle it with care, use gloves or tweezers.
Rainbow Crabs Molting
The increase in size is achieved by successive molts during which Rainbow Crabs rejects its shell and all calcified parts.
During molting, Rainbow Crabs absorb a lot of water until their carapace opens up in a breaking point. After that, they use this opening to wriggle out of its old shell. If molting goes well, it usually takes from 5 – 30 minutes.
It looks similar to a dead crab but is hollow if you look real close. Do not remove the old shell. It contains a lot of calcium and other microelements, so they will eat it when they are finished. This is nourishment to your crab.
Rainbow Crabs need to be left alone during this very vulnerable time and they should not be disturbed during the entire molting process.
Note: Rainbow crabs need calcium to mineralize (harden) the shell. Calcium is vital for good shell growth. I highly recommend reading my article “How to Supplement Shrimp and Snails with Calcium”.
Depending on the age, Rainbow crabs molt from every few weeks to several months. Before that, they will stop eating their food (sometimes for weeks) before molting and become more sluggish when moving around.
Tip: Some experienced aquarists do not recommend feeding them after molting until they consume their old shell and restore the proper calcium balance. After all, this is what they do in nature.
Are Rainbow Crabs Plant Safe?
No, Rainbow crabs are not plant safe. They will eat, cut, and uproot everything that is possible in your tank.
Identifying and Sexing Rainbow Crabs
Males have bright blue backs, red legs, and white claws. Their underbelly apron will be shaped like an upside-down T. Females typically have pale blue to grey backs, pale orange legs, and white claws. Their underbelly apron will be shaped somewhat like a bell.
The males are more colorful compared to females.
In addition, the effect of gender reveals a significant difference for the width of the shell and the diameter of the left clamp, showing the superiority of the male to the female.
Feeding Rainbow Crabs
Rainbow Crabs are extremely opportunistic eaters and sometimes cannibalistic. They are omnivores who are also scavengers. They will appreciate a varied diet of fruits, berries, flowers, leaves, vegetables, pellets for fish, shrimp, turtles, or crabs, frozen krill, shrimp, bloodworms, and insects.
This means, however, that creatures smaller than the crabs can become prey. They are opportunistic predators who will attack smaller animals or consume the carrion of those that die in the tank.
If live plants are kept in their tank, they may eat these as well.
Rainbow Crabs are very adaptable when it comes to feeding. For example, according to observations, Rainbow Crabs showed leaf preference because of the flora associated with their habitat. They showed a high level of omnivorous feeding habit, as shown in the stomach content analysis indicated that they both feed on plant materials, crustaceans, fish fragments (bone and scales), sand grains, and unidentified items. The wide opportunistic feeding pattern of Cardiosoma armatum was due to their accidental predatorship. A large amount of sand grains discovered was attributed to the burrowing nature of the crabs and inherent soil habitat.
Rules of Diet in Captivity for Rainbow Crabs
- Leaves and vegetables should be their primal food. Oak leaves, Indian Almond leaves, Walnut leaves, etc. should be always available in the tank.
- Frequency: feed them once a day. Remove any leftovers. Keep in mind that Rainbow crabs can drag and store food in their burrows (hiding places). Check them from time to time to prevent any bacterial contaminations.
- Give them a protein-rich food only once a week. For example, a piece of boiled shrimp or bloodworm, chicken, liver, fish, etc.
- To improve their coloration, provide them with Hikari products (for example, Hikari Tropical Crab Cuisine (links to check the price on Amazon)). One of the main components of their food is astaxanthin. Balanced formulation offers necessary nutrients that promote proper shell development as well.
- One of the most important things is that Rainbow crabs need diversity in food.
- Do not be afraid to give them fasting days to clean their digestion.
You can read some of my related articles:
Keeping and Housing Rainbow Crabs
Once I started reading about Cardisoma armatum species, I found so much bullsh… conflicting information on the internet! Especially about keeping them. Eventually, I had to refer to rather sparse scientific articles about this species just to be sure.
Some aquarists believe that if they can keep Rainbow crabs in a freshwater tank with a piece of rock or driftwood that they can use to get out of the water. In fact, this is vice versa. Rainbow Crabs need an aquarium that has both terrestrial and aquatic elements (Paludarium).
Land vs Water Ratio
Rainbow crabs, in scientific literature, are usually referred to as “Land crabs”. Some of them have been found several kilometers away from the coast. They show significant behavioral, morphological, physiological, and/or biochemical adaptations permitting extended activities out of the water.
In a tank setting, Rainbow Crabs should be provided with freshwater or brackish water (around 5 – 10%), there is no difference, and they do just fine in both. Their environment should be 10 – 30 % water so that the crabs can keep their gills wet and molt properly.
Tip: This water should be free from chlorine and other harmful chemicals that are often found in residential tap water.
Water Bowl and Water Type
Instead of creating a complex tank setup, you can use a big water bowl. The only rule here is that it should be big and deep enough to let your crab get into the bowl and completely submerge in them.
In this case, the water change will be very easy and simple. Additionally, you will not have to use any filters to maintain cleanliness and prevent toxicity!
If you decide to keep Rainbow crabs in brackish water, do not ever use simple aquarium salt, or table salt! To prepare saltwater, I would recommend using Instant ocean marine salt (check the price on Amazon). This is a great choice. It is pretty cheap and will last very long.
Regarding freshwater, you will need something like bottled spring water. Tap water will be the last choice. Use Searchem Prime to remove chlorine, chloramine. This water conditioner will also bind to heavy metals, any ammonia, nitrites, or nitrates present for up to 48 hours. Consider Prime as your additional safety net.
The rest of the tank should be filled with a combination of moist sand or mud into which they can burrow. A substrate of pebbles, river rocks, or other material that the crab can use to climb into and out of the water.
Substrate should always be kept moist enough for a Rainbow crab to be able to dig underground and form a cave. It needs to damp enough that it holds its shape when you squeeze, but not so wet that it drips or pools water (“sandcastle consistency”).
A mixture of cocofiber and sand will be the best option for Rainbow crab care. Although they can be used by themselves. Place at least 5 cm (2 inches), ideally 7 – 12 cm (3 – 5 inches) of the substrate in your enclosure with stones, cork bark, and other items as hides.
Tip: You can test the consistency of the substrate with a pencil. Stick it all the way down and pull back up. If the tunnel does not collapse, your substrate is fine.
The substrate may also require maintenance such as spraying with fresh dechlorinated water on a regular basis in order to keep it moist enough since the moisture in it will evaporate over time.
Important: Do not ever use saltwater to maintain moisture as salt does not evaporate and can lead to a harmful salt build-up.
Large rocks can also be beneficial for the crabs to rest on when in need of a dry area. Rainbow Crabs, much like other pets, will need places to hide when they are frightened (or where they can safely molt). Therefore, providing caves or shadowy areas is good for their well-being.
A lot of people completely forget about the humidity. However, adequate humidity in the tank is vitally important to Rainbow crabs. These crabs “breathe” via gills, the proper exchange of oxygen by the crabs depends on the humidity in the air.
Therefore, if the tank air is too dry, the crabs will essentially suffocate. They need a relative humidity of around 80 percent.
Aquarists also noticed that their Rainbow crabs do not dig as much when their gills are moistured.
Note: High level of humidity is not desirable either as it will cause condensation as well as encouraging the growth of bacteria and fungus in the tank.
Interesting fact: Cardisoma armatum can tolerate a maximum loss of body water of 20%, which developed in 3-4 days without water, and resulted in a 25-30% increase in hemolymph osmolality. However, within 24 hours of rehydration, they will return to the original levels of hemolymph and osmolality.
People who wish to keep and raise Rainbow Crabs should plan on keeping them in a water-tight aquarium of at least 20 gallons (90 liters) per crab. As adults, one male and one female may be kept together in the same aquarium of 40 gallons (180 liters) or more, but two adult males would need far more space to prevent territorial fighting.
Also important in setting up their tank is ensuring that none of the elements placed in the tank reach high enough for the crabs to crawl out. Rainbow Crabs are excellent climbers and given the opportunity, they will escape their tank and explore more of their surroundings. If the tank is short, or the elements inside are too tall, it is important to tightly fasten a lid to the tank to prevent the crabs from escaping and becoming injured.
As Rainbow Crabs are natives of temperate regions they prefer a warm habitat. A tank kept between 23 – 29 C (75 – 85 F) would be within the correct range. However, heaters and any other electronic elements should be carefully hidden or placed out of reach, so that the crabs cannot interfere with them while they are exploring their environment.
Important: Keep in mind that Rainbow Crabs have very strong claws and can even damage the metal cords! Hide everything or they can short circuit across the contacts.
Important: Before putting them in your tank do not forget to acclimate them (read more about it).
Basic Tank Equipment (links to check the price on Amazon)
Mating Rainbow Crabs
Though Rainbow Crabs can be kept in male and female pairs in the same tank it would range from extremely difficult to nearly impossible to breed them in captivity.
In the wild, Rainbow Crabs mate on land based on lunar cycles and tides. They reach sexual maturity in approximately 3 – 4 years. In this species, the minimum sizes at first sexual maturity in males and females is 5.5 and 6 cm, respectively.
Courtship ritual is common (through olfactory and tactile cues). The male Rainbow Crab will approach the female Rainbow Crab with his claws held high. If she is not ready to mate, she will strike at him with her own claws.
However, if the female crab is prepared for mating, she will keep her claws lowered and allow the male crab to approach. The two crabs will then engage in an hour to an hour and half of preparatory behavior. During this time the female Rainbow Crab will seem docile and still, while the male Rainbow Crab will stroke the female crab with his legs and claws.
Breeding Rainbow Crabs
The appearance of the eggs would be preceded by molting. After mating, the female Rainbow Crab carries fertilized eggs beneath her body for 2 – 3 weeks. Fertilized eggs shape a cluster whose color changes from orange to yellow, to gray, and finally to black. After that, she will make her way to the coast and release them into shallow inshore waters.
In the wild, a female may produce 300,000 – 4,800,000 eggs per spawn, but very few larvae (Zoea) survive to become small crabs. The larvae are eaten by fish and other ocean life. The ones that survive and develop into crabs must make their way back to land, or they will drown.
Rainbow Crab Larvae Stages
Although Rainbow Crabs can tolerate brackish water and even freshwater, their larvae require salinity for successful development to metamorphosis.
The complete larval development consists of 6 stages followed by a megalopa (final stage, where larvae metamorphose into tiny crabs). Depending on the temperature, salinity, and feeding, it takes 40 – 50 days from hatching to metamorphosis.
The size of the larvae after each stage:
1st Zoea – 0.85 mm
2nd Zoea – 1.21 mm
3d Zoea – 1.61 mm
4th Zoea – 1.90 mm
5th Zoea – 2.11 mm
6th Zoea – 2.41 mm
Megalopa – 1.54 mm. Unlike larvae, the carapace of megalopa becomes broader.
After a period in the plankton, baby Rainbow Crabs return to the mainland as megalops or first crab stages where they live in burrows several feet deep at least to a level that will allow water to seep in for moisture.
Larvae Rainbow Crab Rearing Setup and Salinity
Interestingly, the larvae appear to prefer moderately brackish conditions rather than full-strength seawater, which suggests adaptation to some larval retention within the lower estuarine parts of coastal mangrove swamps adjacent to the habitats of the adults.
Scientists have found that Rainbow Crab larvae can survive in water with salinity between 15 – 35 % percent. A refugium tank created specifically for this purpose could potentially host Rainbow Crab spawn, but it would need to be well-stocked with microflora and fauna.
Diet: enriched Artemia sp. (about 10 freshly hatched nauplii/mL).
Water change: daily.
Light: 12:12 hours (Light : Dark cycle)
Temperature: 25 – 25 C
Salinity: 15 – 25 %
Additionally, it would need to be adapted continually to the changing needs of the growing crabs. For example, once larvae reach stage 4, the salinity level should not be higher than 20%.
At this stage, an essential addition would be sand, rocks, or other items that reached above the water’s surface. This would ensure that the crabs could crawl to safety once they reach maturity. Of course, a lid would also be necessary in order to prevent the crabs from escaping and potentially becoming injured.
Rainbow Crabs and Suitable Tankmates
As Rainbow Crabs can be predatory, ideally they would be placed in a tank alone or with one other Rainbow Crab of another sex. While they can coexist with fish, there is always a risk of predatory behavior.
Invertebrates like shrimp and snails are best avoided as they might get attacked.
Cardisoma armatum is a simple species to care for. Beginners will be able to keep a single Rainbow Crab with few problems since they are hardy and undemanding.
However, the biggest concern is their aggression, as they may attack other crabs, fish, snails, and shrimp.
- Isa Olalekan E, Lawal-Are AO (2015) Biodiversity of a Mangrove Swamp Ecosystem: Size Composition and Growth Pattern of Land Crabs as an Ecological Indicator. Poult Fish Wildl Sci 3: 139. doi:10.4172/2375-446X.1000139.
- Diversity, biology and exploitation of brackish water crabs in West Africa: A review. Article in International Journal of Biological and Chemical Sciences 12(5):2355. January 2019. DOI: 4314/ijbcs.v12i5.34.
- Breeding of the land crab Cardiosoma armatum (Herklots 1851) in enclosure in Benin. December 2015. DOI: 10.4314/jab.v96i1.7.
- Larval morphology and salinity tolerance of a land crab from West Africa, Cardisoma armatum (Brachyura: Grapsoidea: Gecarcinidae). Journal of Crustacean Biology, 25 (4), pp. 640-654. hdl:10013/epic.24145.
- Oyebisi R, Lawal-Are AO, Alo B (2013) Comparative Study of Persistent Toxic Metal Levels in Land Crab (Cardiosoma armatum) and Lagoon Crab (Callinectes amnicola) in Lagos Lagoon. J Mar Biol Oceanogr 2:1. doi:10.4172/2324-8661.1000104.
- Hemolymph Oxygen Transport, Acid-Base Status, and Hydromineral Regulation During Dehydration in Three Terrestrial Crabs, Cardisoma, Birgus, and Coenobita. Biology. Published 1981. DOI:1002/jez.1402180107.
- Goussanou et al., J. Appl. Biosci. 2017 Morphological, structural characteristics and growth relationship of crabs Callinectes amnicola and Cardisoma armatum in the Nokoué lake Porto-Novo lagoon in South Benin.
- Sizes at first sexual maturity and capture and demographic parameters of crabs Callinectes amnicola and Cardisoma armatum in the complex Nokoué Lake Porto-Novo lagoon in South Benin. International Journal of Fisheries and Aquatic Studies 2018; 6(1): 195-201.
- Some ecological aspects of the gecarcinid land crab, Cardisoma armatum Herklots, 1851 (Crustacea, Brachyura, Gecarcinidae) from the estuarine region of the Comoe River, Côte d’Ivoi. International Journal of Biological and Chemical Sciences. September 2016. DOI: 10.4314/ijbcs.v10i2.1.
- A synopsis of the burrowing land crabs of the world and list of their arthropod symbionts and burrow associates. Contributions in science. 1972. Volume 220. Pages 1—58.