Moon Crab

Latin Name Gecarcinus quadratus Order: Decapoda Family: Gecarcinidae

The common name “moon crab” is sometimes used for other Gecarcinus species or crabs in the genus Cardisoma. G. quadratus is also commonly known as the Halloween crab.

Overview

As one of the most surprising, yet familiar arthropod species, land crabs can make very good ambassadors – but they can’t be used just like a hissing cockroach or tarantula might be. Successful moon crab keeping revolves around three major factors: habitat construction, proper diet, and managing their stress. Captive moon crabs require some unique considerations, but can be an eye-catching addition to an ambassador collection.

Unfortunately, this crab’s planktonic larval stage means that all currently available Gecarcinus are wild-caught.

Natural History Information

Range and Habitat

G. quadratus is found throughout the Pacific coast of Central America, north into Mexico and south to Panama. They live in forests, beaches, and mangrove swamps within about a half mile of the coast.

Longevity

Because this species is not available captive-bred, and relatively few studies of its wild life history have been made, many details about its longevity are unknown. Anecdotal reports from keepers indicate adult lifespans of at least 10 years.

Ecosystem Role

As a large, omnivorous invertebrate, the moon crab occupies an intermediate role in the food web. It is an important food source for ground-living mammals, such as raccoons, and it in turn preys on a variety of other invertebrates. As burrowers, moon crabs are responsible for some tilling and aeration of soil. Their larvae are planktonic and feed on other tiny marine organisms.

Husbandry Information

Housing Requirements

Life Cycle Natural History Relevant Information

Breeding moon crabs is not practical for most institutions; their larvae require separate marine setups which replicate natural light cycles, tides, and saltwater composition.
Crepuscular and nocturnal. Moon crabs choose to leave their burrows most often in the evening, though sometimes they will be seen exploring by day.
They’re burrowers by nature, and will construct tunnels and depressions in soil or sand. You won’t see them in their enclosure much unless you have their only or most inviting burrow set up against the front glass.
Male/female pairs can be maintained together if they have multiple hides available (at least 2/crab) but same-sex pairs will probably fight. Fights leading to injuries and even death are possible in male/female pairs as well, especially if the crabs are recent imports and stressed.

Habitat

This crab’s wild habitat is humid, warm, and densely vegetated.
Halloween moon crabs are truly terrestrial, not amphibious. Any pool or aquatic section of their habitat must be easy for the crab to enter and exit.
They are very capable of climbing rocks, branches, and other irregular surfaces. Be sure your crab can’t reach the lid of its enclosure to avoid it escaping.
A 10-gallon aquarium is suitable for a small crab (carapace width of around 1 inch). Larger adults should be kept in a 20-gallon or larger enclosure.
Water
- Iowa State Insect Zoo: They can survive just fine with only a fresh water bowl, but sea salt is supposed to help with molting. We have seen our crabs using both their pools.

Temperature, Humidity, Light Cycles

Temperature: Moon crabs are most comfortable at temperatures of between 75 and 85 degrees Fahrenheit. They can tolerate lows of 60 degrees, but prolonged low temperatures are unhealthy.
Humidity: Although this crab can survive for several days without access to water, they are healthier and more active in humid environments.
- Iowa State Insect Zoo: We maintain our crabs’ humidity by daily misting, ensuring that the water bowls are full, and occasionally pouring water onto the substrate.
Light: As a nocturnal animal, Halloween moon crabs don’t require UV light. Adults will come out more frequently in the dark, making them a consideration for a dimly-lit nocturnal exhibit.

Substrate

Deep substrate which allows the crab to burrow is a necessity. Substrate between three and six inches deep is sufficient. It should never be soaking wet, but regularly dampened.
- Iowa State Insect Zoo: We have used pure play sand, pure soil, coco fiber, and a mixture of sand and soil. We prefer the sand/soil mix, as it allows for growing plants and holds burrows well.

Other General Housing Requirements or Management information

They will take shelter in pre-made hides; bury a burrow such as a half-flowerpot at an angle against the side of the tank and cover it with substrate. This way, you can still observe the crab while it’s “hiding.” This can also be useful to helping them warm up to your presence. Using the observable burrow can be encouraged by burying rocks near the surface of the substrate in the back of the enclosure. A determined crab will burrow wherever it is able to, and is more than capable of pushing aside rocks its size or smaller as it digs.
- Iowa State Insect Zoo: Moon crabs will occupy multiple hides if they have them available. They will also dig new burrows even if they have one or more safe shelters that they already use.
The crab’s pool(s) should be deep enough that it can entirely submerge itself.
Moon crabs can molt on land or underground, but underground molts tend to be more successful. If the crab burrows and does not resurface for two weeks, don’t dig it up and consider it off-use until evidence of its resurfacing is seen (eaten food, fresh footprints, etc).

Diet Requirements

Diet in the Wild

Wild moon crabs eat living and fallen leaves, fruit, carrion, and invertebrates. They gather food at night and hoard it in their burrows.

Diet under human care

Iowa State Insect Zoo: we’ve had the most success with feeding them a base diet of plants such as short grasses, dandelion, clover, vegetables, and other nontoxic plants that are planted in his enclosure. He cuts them to eat at his own pace, and we replace them when they are destroyed. We add small amounts of fish flakes, crustacean pellets, apple, grape, banana, mango, lettuce, broccoli, spinach, and incapacitated or dead crickets or cockroaches three times a week. Bones are great sources of calcium, we leave small chicken and rodent bones in the enclosure and offer eggshell periodically as well.
Iowa State Insect Zoo: High-protein treats like peanut butter and fish are accepted readily by most crabs. Some can even be lured out of their burrows to feed on peanut butter presented on a stick.

Veterinary Concerns

Signs of mild stress include a threat display (leaned back with raised claws), escape behavior, and clicking sounds. Signs of advanced stress include bubbling from the mouth, spitting up liquid, and autotomizing (detaching) claws.
Limb injuries and losses are usually recoverable. Crabs can regenerate all limbs if cared for until their next molt. Regenerating limbs appear as buds encased in membrane – upon the next molt they fully inflate and become usable.

Enrichment & Training

Enrichment

Behavioral Relevant Information

Though their nocturnal habits may make moon crabs seem very sedentary, they use the entirety of their space at night.
Iowa State Insect Zoo: Materials such as grasses, leaves, and snake sheds may be taken into the burrow to eat or tear apart. We have also observed our crabs burying food and enrichment items in other locations.

Environmental Enrichment

Iowa State Insect Zoo: Novel materials to destroy or manipulate, such as cardboard tubes, plastic balls, snake sheds, new grass, bark, and sticks are popular; particularly if smeared with peanut butter or jelly, or containing other foods.

Behavioral Enrichment

Iowa State Insect Zoo: Live insects can be added to elicit hunting behaviors and serve as food enrichment. Because moon crabs hunt infrequently, use insects that cannot escape if the crab doesn’t go after them right away.

Schedule

Other Enrichment Resources

Training

Behaviors Trained

Iowa State Insect Zoo: Moon crabs appear intelligent for invertebrates, but are difficult to motivate. Foods such as peanut butter, fish, or preferred pellet/flake foods can be used to help habituate a crab to human contact. Habituation to being handled and viewed is the only training we have done.

Reinforcers used & schedule of reinforcement

Other

Social Housing/Colony Management

Unless captive breeding is the intent, the only reason to house multiple moon crabs would be to have a backup if one individual is molting or very reclusive.
Iowa State Insect Zoo: Housing multiple moon crabs in the same enclosure always brings the risk of injuries or death from fighting. This risk can be minimized by housing opposite-sex pairs of similarly-sized crabs in large enclosures (at least 10+ gallons/crab) with multiple hides and pools.

Colony or Breeding Management

Individual Identification

Color can be a useful identifier, but is unreliable as it can change between molts. Diet is believed to influence coloration, with diets higher in fruits producing brighter crabs. Males usually have brighter, higher-contrast coloration.
The abdomen of male moon crabs is very thin, whereas a female’s stretches across most of the underside of her body. Males usually possess larger claws, one of which is sometimes larger than the other.

Programmatic Information

Messaging Themes

Adaptations to land/amphibious lifestyles – moon crabs’ lung-like gills and lifestyles which balance interaction between aquatic and terrestrial ecosystems fit in well to habitat-based programs.
Arthropod characteristics – as a large, charismatic arthropod, they are an inviting way to demonstrate exoskeletons, multiple jointed legs, and compound eyes.
Responsible pet trade messaging – as animals which can’t be captive-bred by most owners, they are in a unique position here.
Culturally-relevant animals – moon crabs are a food source for native peoples of Central America. Crabs as a whole have been a staple food of coastal civilizations, and feature prominently in art and stories.

Threats and Conservation Status

Though most of its populations are currently stable, collecting of individuals for the pet trade or for food are potential threats.
Large-scale pollution, destruction, and warming of both forest and ocean habitats may endanger this species in the future.

Interesting Natural History Information

Wild moon crabs spawn in a great migration, when all the egg-bearing females travel to the ocean and release their eggs.
Moon crabs’ gills are inflated and act similarly to lungs to pull oxygen from moist air. Their exoskeleton and abdomen seal very tightly to hold in moisture.
Their eyestalks give them 360-degree vision, which is their primary sense for avoiding predators and finding food.
Moon crabs have four tiny antennae, positioned between their eyes and mouth. Land crab antennae tend to sense smells better in humid air.

Did you know…

The origin of the name “moon crab” may be speculated, but its exact etymology is unclear.

Handling & Presentation Tips

Iowa State Insect Zoo: Most crabs are shy by default, but Halloween crabs are extra shy. Some individuals will warm up to you with repeated hand-feeding of things like peanut butter, fish, or banana, but in general they won’t approach you voluntarily. A habituated crab will walk and climb on you calmly once you get it out, though, and pinching decreases to zero with enough gentle handling. They can climb effectively on most clothes, but two hands should always be at the ready. A fall from sufficient height onto a hard surface could be fatal.
Positive behaviors while on programs include sitting calmly in hands, exploring the area, and feeding if food is offered.
Signs of mild stress include a threat display (leaned back with raised claws), escape behavior, and clicking sounds. Signs of advanced stress include bubbling from the mouth, spitting up liquid, and autotomizing (detaching) claws.

Use Guidelines

Iowa State Insect Zoo: We wouldn’t consider it safe to get them out for handling programs more than twice a week – they need time to recharge and with their good crustacean memories, I’ve found that that doesn’t make habituation too much of a problem.
- When possible, they can be brought for outreach in a smaller enclosure that they go into just for the show, which is also definitely the safest option for people who’re not used to the very awkward ordeal that handling a medium-sized crab is!

Public Contact and Interaction Guidelines

Public contact is not recommended, as these crabs typically do not tolerate petting (even on their backs). They are wary of strangers and probably will not accept food from others.

Transportation Tips

Crating Techniques

Temperature Guidelines

Acquisition Information

Unfortunately, this crab’s planktonic larval stage means that all currently available Gecarcinus are wild-caught. Keep this in mind when seeking them from animal dealers or pet shops.

Look for specialty/exotic rescues such as:

Resources

Contributors and Citations

Iowa State Insect Zoo

Comments from the Rating System

Top Photo Credit: Thomas Splettstoesser [CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)%5D