Moon Crab: Complete Guide to Care, Breeding, Tank Size & Disease

Moon Crab, also known as ‘Halloween Crab’, ‘Halloween Hermit Crab’ or ‘Purple Moon Crab’, is a gorgeous, purple-colored terrestrial crab. It is quite a unique specimen for any hobbyist. It is native to the Pacific Coast and found in Costa Rica, Panama, and Mexico.

A Moon Crab makes for a lovely pet for hobbyists of any level as they are vibrant to look at and easy to care for. They need some unique aspects of care, but they make up for it through their eye-catching beauty.

They are also known as ‘Halloween Crab’ because of their black carapace with bright orange-colored legs. They look exactly like jack-o-lanterns. Their purple claws show off their perfect Halloween outfit!

A striking feature about them is that they do quite well in captivity. Even though they are quite territorial, they love playing and sometimes fighting. Sometimes, they even fight until death with other crabs of the same species. If you plan on getting a pair of them, remember to keep them in separate enclosures. Only let them play together under strict supervision.

Information Chart 
Scientific Name:Gecarcinus quadratus
Care Level:Easy – Medium
Colour:Purple, black, and orange
Lifespan:Up to 8 years
Size:Up to 2.5 inches
Diet:Herbivore and Omnivore
Minimum Tank Size:15 gallons
Temperature:79°F to 88°F
WaterConditions:Freshwater and saltwater
Tank Mate CompatibilityTerritorial; not advisable

Moon Crab: A Quick Summary

Moon crabs, also known as Gecarcinus quadratus, are small, semi-terrestrial crabs native to Central and South America. They are popular in aquariums due to their unique appearance and behavior, and are known for digging and burrowing in the substrate.

Moon Crab Appearance

Moon Crab Overview

As mentioned earlier, they get their name as the Halloween Moon Crab because they have all the Halloween colors. Moon Crabs look quite breath-taking with their black carapace, orangish-red legs. Their purple claws appear whiter near the tips.

Moon Crabs also have two minuscule white dots on the back of their black carapace. They have orange dots near the eyes which appear as thin stalks.

As they get older, their bright colors appear fainter and duller. This also happens when they are about to molt, as the carapace loses its brightness. This should not be a cause of worry, as after the molting process is complete, their colors will be bright again and even more vivid. 

Moon Crabs also have a gill chamber, which is placed under their carapace near the legs. If they are to stay on land, outside water for a long time, these gills must stay moist.

The lifespan of Moon Crab

The typical lifespan of a Moon Crab is up to 8 years in captivity. The average lifespan can be anywhere between 4 to 8 years, depending on multiple factors such as care, diseases, etc. 

Moon Crab Size

Moon Crabs are usually not that big. Adult Moon Crabs in captivity can grow up to 2 to 2.5 inches, and their legs can grow up to 4 inches. In the wild, however, their carapace might grow up to 4 inches!

Natural Habitat and Origin

Moon Crabs, scientifically known as Gecarcinus quadratus, are terrestrial creatures and were originally described by Ferdinand de Saussure in 1853. 

The natural habitat for Moon Crabs is the Pacific Coast, along the coast of Mexico, Panama, and Costa Rica (Bahía de Culebra, Boca de Barranca, Puntarenas, Cabo Blanco, Pigres, etc.) and some of the Atlantic Coast as well, particularly from Florida to Guyana, in South America. 

Moon Crabs live in forests, beaches, and even swamps of mangroves.

Moon Crabs live high above the tide line. They can be found around 700 yards away from these water bodies.

The natural habitat for Moon Crabs is humid, densely vegetated, and quite warm. Since the Moon Crabs are not amphibians, but terrestrial creatures, they need easy entry as well as exit from water areas in their tanks.

Moon Crab Care and Tank Set-up

Tank Size and Specifications

Optimum Tank Size for Moon Crab

Optimum Tank Size for Moon Crab

The recommended tank size for Moon Crab is 15 gallons. Another requirement is that the tank should have a screen top, and a tight-fitting lid as Moon Crabs will escape if the lid is loose or if they are presented with an opportunity — even the smallest gaps. In case they do escape and are outside the tank, the chances are that they will die of dehydration within one day.

Tank Shape for Moon Crab

Bigger tanks are suitable for Moon Crabs. They need enough space to move around, and so any tank smaller than 15 gallons will not do. 

If you are housing two Moon Crabs, then the tank size needs to be increased twofold as well. The ideal group to put together would either be all females, or one male and one female. It would be impossible to keep males together as they will fight constantly. Since females of this species are less aggressive than males, their coexistence might be possible — but not guaranteed.


A Moon Crab tank needs to replicate its native habitat, and so the proper substrate should be a mixture of coconut fiber, peat, sand, and soil, often topped with a thick layer of leaf litter and moss. 

The proper substrate should be around 4 to 6 inches of sand and 3 to 6 inches of peat and moist moss. 

The substrate needs to be moistened to the consistency of building a sandcastle at all times. To test the consistency, you can try sticking a pencil in all the way, and pull it back. If it doesn’t fall apart, then you’re good to go. 

Crabs love digging, and they will need enough space to dig and burrow into the moist substrate, especially to molt. Hence, providing a free substrate, which is easy to dig in, is essential.

The substrate needs to be changed every 2 to 4 months.

How Many Moon Crabs in a 15 Gallon Tank?

The thumb rule for deciding this is to house one Moon Crab in a 15-gallon tank and add 10 gallons as the number increases. 

However, as advised earlier, Moon Crabs are extremely territorial and will not do well with others, especially if they are males. It is never a good idea to house more than one Moon Crab in a tank without strict supervision.

Water Parameters

Water Parameters for Moon Crabs

Since Moon Crabs are native to temperatures ranging from 78 to 85 degrees F, which means highly humid coastal regions, they need a specific set of water requirements.

As Moon Crabs prefer warm and humid environments, one can also use a heat mat below the tank to keep the tank warm.

Moon Crabs need two large water dishes or water bowls in their tank — a bowl of fresh water and a bowl of saltwater. The freshwater bowl must be a calcium and water mixture to provide sufficient minerals for the Moon Crab. The saltwater bowl should be a mixture of marine salt and water, to mimic the coastal ocean. You can also add some water dechlorination drops to ensure purified water, as any harsh water conditions will harm Moon Crabs.

These bowls should be easily accessible and large enough for the Moon Crab to climb into it. Since they cannot breathe underwater it is crucial to provide them with some kind of a ladder to climb in and out of these bowls. The depth of the water bowls need not be too deep, but only deep enough for them to be able to partially submerge. A depth of 1 inch should be enough for this. 

Tank Landscape

Best Plants for Moon Crab Tanks

The best item for tank decoration would be cork bark, as Moon Crabs love a humidified environment. Cork bark comes in a variety of shapes and sizes and is readily available. Apart from that, the best solution would be to use fake plants instead of any real ones as real plants might have a thick root system, which can be a problem to ensure their growth or maintain them with the crabs’ burrowing and digging habits. Some options for real plants can be spider plants, moss, pothos, etc. If you want to use real plants, they should be able to withstand the tropical climate — that is, humidity and heat, as the tank will have that temperature range.

Worst Plants for Moon Crab Tanks

Any plants that grow vertically or have thick roots should be avoided. Moreover, all live plants will get trampled on, chewed, and torn apart by the Moon Crabs. Hence, fake plants will save a lot of trouble for you and be suitable for a tank environment for the Moon Crab.

Decorations for Moon Crab Tanks

The natural habitat of Moon Crabs includes sand dunes, mangroves, and rainforests, and this should be kept in mind while trying to recreate its habitat.

Moon Crabs need plenty of places to hide and burrow into, and you can provide these structures for them using objects such as ceramic flower pots, plastic plants, rocks, fish netting, stones, rough wood, branches, foliage, bark, etc. 

Moon Crabs love climbing as well, so this should be encouraged by installing various types of decor that allows them to climb around the tank, and even to aboveground hiding spots, which can be created using coconut shells. 

Lighting for Moon Crab Tanks

Since Moon Crabs are nocturnal in nature, proper lighting and the lack of it is very important for the survival of Moon Crabs. As they are found in tropical climates, the lighting needs to be perfected to recreate the warm nighttime. To do this, a heating pad can be placed below the tank as mentioned earlier, or you can use a low wattage light bulb that hangs over the screen of the tank, as it helps maintain the temperature without making the tank sunny. Less light will make the Moon Crab more comfortable and safe. A red heat bulb can also be used to view the activities of the crab. 

Feeding Moon Crab

Best Diet for Moon Crab

Moon Crabs are herbivores to a large extent, but they occasionally scavenge for food in the wild. For this reason, they can also be called omnivorous scavengers. They can eat nearly anything — living and fallen leaves, pelleted food, seedling, invertebrates, fruits like mangoes, papayas or coconuts, and even carrion. They scavenge food at night and hoard it. Most of them suffer from calcium deficiencies, and the best way to avoid that is to give them a cuttlebone to make up for it.

How Often Should You Feed Moon Crab?

Moon Crabs don’t have a big stomach, and over-eating, as well as under-eating, can prove to be fatal to the crab. The best option is to feed them once, and provide more food than necessary and collect the leftovers the next day. 

Moon Crab Behaviour and Temperament

Moon Crabs are skittish and shy and do not like to be touched or handled. It may take weeks before you can handle the Moon Crab and it’s important to remember that over-handling can often lead to stress for them. 

Moon Crabs are nocturnal and they enjoy hiding and spending their time scavenging for food in their tank.

While they are easy to take care of, they can be a little aggressive, especially if there are other tank mates present. They may pinch and even claw you if they feel threatened.

For Moon Crabs, the molting process is very important and they need their privacy the most during this time. This process is the crabs shedding their exoskeleton (and sometimes eating that to fulfill their calcium requirements), and growing into a new one. There are four periods of molting, and the entire process can take up to weeks, during which time, the Moon Crab goes into hiding and only needs darkness, moisture, and heat to successfully molt. 

Is a Moon Crab societal or lone in nature?

Moon Crabs are solitary creatures. They do not like mingling and prefer to hide. It becomes quite important to give them their space and leave them be, especially when they’re molting or even exploring their tanks.

Moon Crab Tank Mates

Ideal Moon Crab Tank Mates

Before letting any tank mate inside a Moon Crab tank, some things need to be considered such as the fact that crabs are not social creatures. Having a tankmate poses a threat to both the tank mate and the Moon Crab as the latter is quite territorial and will fight and eventually kill one another. However, if you must, then only other Moon Crabs would be an ideal tank mate — and that too, females, as they are not as aggressive as males. This is, however, not a guarantee that they won’t fight or hurt each other.

Bad Tank Mates for Moon Crab

As mentioned earlier, all tank mates are bad for the Moon Crab and they should be in a solitary environment where they can live in peace.

Breeding Moon Crab

Moon Crabs are easy companions, but breeding them is a big task. As of now, the industry depends majorly on wild-caught species. The problem with breeding them domestically or in captivity is that it takes time for the larvae to develop into adults, and this process can only be done satisfactorily in saltwater, but never in freshwater.

Moon Crabs must go to the ocean to release their eggs, which latch themselves to the plankton. Although the females release more than thousands of eggs, often in the night and during the ebbing tide, only a handful of them go on to survive the larva stage to hatch, and fewer than them crawl back to land. 

Moon Crabs are particular about their breeding time, and they plan it to coincide with the monsoon, probably because it helps the adults not dehydrate on their way to the ocean.

In captivity, they can rarely breed, but in theory, this can happen under very stringent conditions. Female Moon Crabs are transferred to a different tank during the breeding time, where the water temperature is maintained at around 80 to 82 degrees F. The female follows the same method and releases eggs that latch onto the plants in the tank. It takes about 21 days for the larvae to metamorphose and hatch. Once they do, they should be fed on brine shrimp and other protein-rich food until they can eat pelleted food or other small foods. Since the larvae are attracted to light, the lights can be increased during water changes so the process can be completed smoothly. 

Even after all this time, the larvae will still take weeks before they resemble the adults in any way — making this process quite tricky and exhausting.

Moon Crab Breeding Level – Very Difficult. 

Moon Crab Sexual Dimorphism?

Moon Crabs are sexually dimorphic. Some indicators that help us differentiate between them are their abdomens, claws, coloration, and their size. In males, the underside of their abdomen is narrow and slim, whereas, for females, there is a broad plate on their abdomen. Male Moon Crabs also have bigger and more vibrant claws, whereas the size of the female claws is smaller. Males have a more vivid coloration, and sometimes their legs are completely red or orange. In females, the orange color appears dull or washed out.

Moon Crab Common Diseases and their Treatment

Most diseases that occur commonly in Moon Crabs can be eradicated by keeping up tank hygiene and making sure that the diet of the crab is well-maintained and balanced.

Calcium deficiencies

The biggest concern amongst Moon Crabs is having a calcium deficiency which can be avoided by giving them a calcium supplement such as a cuttlebone, and also sprinkling a little calcium powder in their food. 

Another thing that can be taken care of to prevent the Moon Crab from falling sick is to maintain the right temperature and moisture in the tank, to prevent infection and bacteria.

Facts About Moon Crab

  • Moon Crabs are found mostly on the Pacific Coast, along the coast of Mexico, Panama, and Costa Rica.
  • They have many names such as  ‘Halloween Crab’, ‘Halloween Hermit Crab’ or ‘Purple Moon Crab’, etc.
  • Moon Crabs are very territorial and solitary creatures.
  • Moon Crabs are almost impossible to breed in captivity.
  • When Moon Crabs molt, they disappear for weeks in the end and hide underground in the darkness and moisture. This is the most stressful time of their life!
  • Moon Crabs will often fight other crabs or tank mates to death to establish dominance and hence, they are a big no-no as tankmates for any species of crabs.

Is Moon Crab Right for You?

Author’s note: Moon Crabs make for the perfect pet to have because they are very unique looking, extremely easy to care for, and they prefer solitude so the chances are they will coexist peacefully with you, with some effort from you! Moon Crabs make for quite an engaging and rewarding observation as they burrow, dig, climb and explore their tanks, and you will get to notice their many moods and quirky personalities firsthand.


Can Moon Crabs live with other crabs?

It can be done, but it is highly unadvised to prevent aggression. One must only keep them by themselves to avoid fights within them and other crabs.

How many Moon Crabs can I put in a 15-gallon tank?

As mentioned earlier, Moon Crabs are highly territorial and must not be kept together. However, if the tank size increases twofold or more, you may keep one male and one female or two females together.

Do Moon Crabs breed easily?

No, Moon Crabs are very difficult to breed in captivity as they require very specific conditions to do so. Please refer to the section above to understand it in more detail.


Moon Crabs are quite exotic, being primarily tropical species, and are great pets, once we understand how to properly care for them. Their long lifespan allows them to be a longer-living species of pets. They make for a great viewing experience and should be considered if you are a little more experienced and don’t mind taking care of a unique, beautiful creature!

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