Coconut crabs are terrestrial hermit crabs found widely throughout the tropical western Indo-Pacific Ocean, from Mauritius and the Aldabras Islands in the Indian Ocean to the Pitcairns, Tuamatus, and Easter Island in the Pacific Ocean, as well as on Madagascar and the Seychelles. There are also populations found in Tanzania. ("Biological studies on the coconut crab (Birgus latro) in the Mariana Islands", 1980; Fletcher, 1993; McLaughlin, 2013; Morris, et al., 1988)
Coconut crabs are found in coastal habitats on marine islands, or small islets near larger, continental islands, as much as 6 km from shore. They create burrows in the substrate, which provide protection and allow for food storage. These crabs may bury themselves completely in the soil while molting. Adult coconut crabs are primarily terrestrial; eggs are released into the sea, where larval development occurs. ("Biological studies on the coconut crab (Birgus latro) in the Mariana Islands", 1980; Eldredge, 1996; Lavery, et al., 1995)
- Aquatic Biomes
Coconut crabs are the largest terrestrial arthropods known, with a maximum carapace length of 200 mm (up to 1 m from leg tip to leg tip) and a maximum weight of 4 kg. Coconut crabs are hermit crabs, and juveniles use mollusk shells for protection until they grow too large for available shells. When an individual reaches this size, its abdomen partially tucks under its body and is protected by a series of hardened tergal plates. The remainder of the abdomen is covered with a leathery skin that has tufts of small bristles. Body color is variable, depending on the population and location; most coconut crabs are deep blue in color, though some may have red tinges or be predominantly red or purplish-red. Like many other species of crabs, coconut crabs have asymmetrical chelae, with the left larger than the right. Additionally, they have two pairs of long periopods (walking legs) with pointed dacytls, which allow them to grip tree bark and other surfaces. They also possess a smaller pair of appendages with small claws; females use these to tend to their eggs, while males use them in sperm transfer during mating. Coconut crabs exhibit sexual dimorphism; males are larger than females (average carapace lengths of 75 mm and 50 mm, respectively) and females have three large, feathery pleopods located ventrally on their abdomens, used to carry egg masses. ("Biological studies on the coconut crab (Birgus latro) in the Mariana Islands", 1980; Fletcher, 1993; Greenaway, 2003; Grubb, 1971; Lavery, et al., 1995; Wells, et al., 1984)
Coconut crabs have only vesitgal gills, which do not aid in oxygen intake; instead, they have lungs that they use for gas exchange. Their lungs are located in the thoracic region, and are comprised of the inner lining of the gill chamber, which is well vascularised, with a thin ephithelium and large surface area. (Fletcher, 1993; Morris, et al., 1988)
- Other Physical Features
- Sexual Dimorphism
- male larger
- sexes shaped differently
- Range mass
- 4 (high) kg
- 8.81 (high) lb
- Range length
- 50 to 200 mm
- 1.97 to 7.87 in
Eggs are carried on a female's pleopods until hatching. Egg maturation lasts 25-29 days, depending on tidal rhythms (developmental periods as long as 45 days have been recorded); these crabs time larval release to align with high tides. When embryos are mature, a gravid female moves from land to shallow intertidal water and releases her eggs by shaking them into the water. Upon making contact with the water, eggs hatch and larvae are released. Larvae undergo four or five zoeal stages, which last approximately 17-28 days total. Each stage requires a different amount of time: stage 1 lasts 5-6 days, stage 2 lasts 3-5 days, stage 3 lasts 3-18 days, and stage 4 lasts 6-12 days. Not much is known about the fifth larval stage. This larval period is followed by a glaucothoe (amphibious) stage, which lasts 21-28 days. Glaucothoes typically move into an empty gastropod shell before migrating onto land; survival is highly unlikely otherwise. Upon reaching land, glaucothoes burrow into substrate and undergo metamorphosis into juveniles after 3-4 weeks. During this time they develop highly vascularized lungs. Juveniles continue to use gastropod shells for protection until developing protective tergal plates. There have been reports of crabs as large as 11.3 mm (carapace length) still utilizing shells, but also crabs as small as 8.4 mm (carapace length) without. Young crabs undergo a series of molts during which they increase in size but do not experience changes in overall morphology. ("Biological studies on the coconut crab (Birgus latro) in the Mariana Islands", 1980; Fletcher, 1993; Greenaway, 2003; Morris, et al., 1988; Reese and Kinzie III, 1968)
- Development - Life Cycle
Mating occurs on land, with neither individual needing to have recently molted. No significant courtship behavior has been observed for this species, unlike most hermit crabs. During mating, a male crab holds a female’s chelae with his and pushes her onto her back, with her abdomen flush to the ground. He transfers his spermatophore to her gonopore, located near the base of her walking legs, and sperm enters her spermatotheca. Ova are fertilized internally and pass out of her body onto her pleopods in an egg mass or egg ”sponge", containing tens of thousands of fertilized eggs which are orange in color. A female carrying eggs is known as berried or gravid. ("Biological studies on the coconut crab (Birgus latro) in the Mariana Islands", 1980; Fletcher, 1993; Greenaway, 2003; Schiller, et al., 1991)
- Mating System
- polygynandrous (promiscuous)
Females produce 50,000-138,000 embryos per spawn. Release of the eggs and hatching takes place in the evening and is thought to be tied to lunar and tidal rhythms. Females have been observed releasing eggs when tides are highest, within a few days after the new moon or full moon, allowing for the greatest number of larvae to be pulled away from shore and into the open ocean, where they will have the most food resources and lowest predation risk. Gravid females have been observed most often during summer months, with individuals previously observed as gravid no longer carrying eggs by October. On Christmas Island, spawning peaks coincide with peaks in the rainy season. Size at sexual maturity seems to vary by population, with median sizes ranging from 27 to 42.5 mm (total length). ("Biological studies on the coconut crab (Birgus latro) in the Mariana Islands", 1980; Fletcher, 1993; Greenaway, 2003; Schiller, et al., 1991)
- Key Reproductive Features
- seasonal breeding
- gonochoric/gonochoristic/dioecious (sexes separate)
- Breeding interval
- Females likely only reproduce once per breeding season.
- Breeding season
- Breeding occurs during summer months.
- Range number of offspring
- 51,000 to 138,000
- Average number of offspring
- Range gestation period
- 25 to 45 days
- Average age at sexual or reproductive maturity (female)
- 5 years
- Average age at sexual or reproductive maturity (male)
- 5 years
Females carry developing embryos on their pleopods and care for them, keeping them clean and aerated, until hatching. Males exhibit no parental investment. ("Biological studies on the coconut crab (Birgus latro) in the Mariana Islands", 1980; Fletcher, 1993)
- Parental Investment
- female parental care
Coconut crabs are long-lived, only reaching their maximum size after 40-60 years. (Greenaway, 2003)
- Range lifespan
- 60 (high) years
- Range lifespan
Coconut crabs are mainly nocturnal, though they may be active during the day as well; on islands with high levels of human activity, they are exclusively nocturnal, to avoid predation. They are able to use their long legs to climb trees to find food, climbing to heights of two meters. These crabs do not engage in combat but do appear to have size-based dominance/submission relationships. Individuals are primarily solitary, venturing out of their burrows only to forage or mate. Crabs living on larger islands are nomadic, moving to new burrows frequently, while crabs on smaller islands tend to maintain one burrow. When it comes time to molt, coconut crabs dig burrows which may be up to 1 m long, staying in these burrows for 3-16 weeks; larger crabs take longer to complete molting. In preparation for this time, crabs will overfeed and produce greater volumes of haemolymph. When molting is completed, a crab will feed on its shed exoskeleton. ("Biological studies on the coconut crab (Birgus latro) in the Mariana Islands", 1980; Fletcher, 1993; Greenaway, 2003; Grubb, 1971)
- Range territory size
- 40 to 250 m^2
Depending on the size of the island, home range can vary from 40 to 250 m^2. (Greenaway, 2003)
Communication and Perception
Based on similarities in brain structure and neuropils, it is likely that coconut crabs have visual and mechanosensory abilities similar to those of other decapods. Coconut crabs have fully developed compound eyes on eyestalks. They perceive olfactory cues with their antennules and are able to differentiate between odors, allowing them to locate preferred food sources. The ways in which they process olfactory cues are very similar to those of insects. Bristles located on their claws function in tactile sensation. Communication between crabs is accomplished using visual cues; for instance, up-and-down movement of claws and legs is a signal for a smaller crab to stand down when confronted with a larger crab. (Fletcher, 1993; Grubb, 1971; Krieger, et al., 2010; Stensmyr, et al., 2005)
As planktonic larvae, coconut crabs feed on other planktonic organisms. There is no information currently available regarding diet during the glaucothoe stage; however, in an experiment studying the effects of enriched diets on the crab’s developmental stages, glaucothoes fed on shrimp and clam meat. Adults are omnivorous scavengers and have been observed feeding on carrion (including other crustaceans such as red crabs (Gecarcoidea natalis)), molted exoskeletons of other crustaceans, tropical fruits (such as Pandanus fruits, one of their primary food sources in many locations), and coconut meat. These crabs use a variety of methods to obtain meat from a coconut. A crab may carry the coconut up a tree and then drop it, cracking it open by the force of its impact on the ground. Individuals have also been observed using their claws to poke the coconut in a soft spot (through one of the “eyes”), splitting it open. Alternatively, a crab may beat the coconut open using its claws. Coconut crabs will bring large food items back to their burrows to consume and store them. In captivity, coconut crabs are known to eat various types of vegetation, such as lettuce and cabbage, as well as live giant African snails (Achatina fulica), though it is unknown if they would consume these animals in the wild. ("Biological studies on the coconut crab (Birgus latro) in the Mariana Islands", 1980; Fletcher, 1993)
- Plant Foods
- Foraging Behavior
- stores or caches food
The only documented predators of adult coconut crabs are humans. It has been suggested that juveniles and smaller invidiuals may be consumed by mangrove monitor lizards (Varanus indicus), cane toads (Rhinella marina), and feral pigs (Sus scrofa), but this has not been confirmed. ("Biological studies on the coconut crab (Birgus latro) in the Mariana Islands", 1980)
The scavenging habits of these crabs assist in the dispersal of coconut seeds, as they may abandon the fruit before returning with it to a burrow to feed. Interspecific competition may exist between coconut crabs and other terrestrial crabs with similar diets, such as Coenobita sp., although competition is typically indirect and, if confronted, a Coenobita crab is likely to withdraw. Although few are known and infections seem to be uncommon, it is possible for coconut crbas to serve as hosts to some parasites. ("Biological studies on the coconut crab (Birgus latro) in the Mariana Islands", 1980; Alexander, 1979; Anderson, 2000; Carson and Wheeler, 1973; McDermott, et al., 2010)
- Ecosystem Impact
- disperses seeds
Economic Importance for Humans: Positive
Coconut crabs are considered a delicacy in some cultures and are served at weddings or other ceremonies, as well as some restaurants. Many tourists are attracted to the unique experience of seeing and eating coconut crabs. The crabs are not difficult to catch and are an easy source of income for hunters. Their popularity as a food item has necessitated the import of larger crabs to some smaller islands. (Fletcher, 1993; Lavery, et al., 1995)
- Positive Impacts
Economic Importance for Humans: Negative
There are no known adverse effects of coconut crabs on humans.
Coconut crabs are currently listed as "data deficient" by the International Union for the Conservation of Nature and Natural Resources. It could be very easy to over harvest these animals, particularly due to their slow growth rate and the ease with which they are caught; however, they are not currently considered threatened or endangered by any agency. (Eldredge, 1996)
Meaghan Ly (author), The College of New Jersey, Yesenia Werner (author), The College of New Jersey, Keith Pecor (editor), The College of New Jersey, Jeremy Wright (editor), University of Michigan-Ann Arbor.
living in sub-Saharan Africa (south of 30 degrees north) and Madagascar.
an animal that mainly eats meat
flesh of dead animals.
uses smells or other chemicals to communicate
the nearshore aquatic habitats near a coast, or shoreline.
- dominance hierarchies
ranking system or pecking order among members of a long-term social group, where dominance status affects access to resources or mates
animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature
- female parental care
parental care is carried out by females
union of egg and spermatozoan
an animal that mainly eats leaves.
A substance that provides both nutrients and energy to a living thing.
an animal that mainly eats fruit
An animal that eats mainly plants or parts of plants.
- internal fertilization
fertilization takes place within the female's body
offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).
A large change in the shape or structure of an animal that happens as the animal grows. In insects, "incomplete metamorphosis" is when young animals are similar to adults and change gradually into the adult form, and "complete metamorphosis" is when there is a profound change between larval and adult forms. Butterflies have complete metamorphosis, grasshoppers have incomplete metamorphosis.
eats mollusks, members of Phylum Mollusca
having the capacity to move from one place to another.
- native range
the area in which the animal is naturally found, the region in which it is endemic.
active during the night
generally wanders from place to place, usually within a well-defined range.
- oceanic islands
islands that are not part of continental shelf areas, they are not, and have never been, connected to a continental land mass, most typically these are volcanic islands.
an animal that mainly eats all kinds of things, including plants and animals
found in the oriental region of the world. In other words, India and southeast Asia.
reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.
photosynthetic or plant constituent of plankton; mainly unicellular algae. (Compare to zooplankton.)
the kind of polygamy in which a female pairs with several males, each of which also pairs with several different females.
- saltwater or marine
mainly lives in oceans, seas, or other bodies of salt water.
an animal that mainly eats dead animals
- seasonal breeding
breeding is confined to a particular season
reproduction that includes combining the genetic contribution of two individuals, a male and a female
- stores or caches food
places a food item in a special place to be eaten later. Also called "hoarding"
uses touch to communicate
Living on the ground.
the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.
uses sight to communicate
animal constituent of plankton; mainly small crustaceans and fish larvae. (Compare to phytoplankton.)
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