FEATURED OKINAWAN MARINE INVERTEBRATE

Birgus latro (Linnaeus, 1767) (size unrecorded)

Featured Invertebrate Data

Taxonomy:

Frequency on Okinawa: Collection Data:
[brackets indicate range for all Okinawa-collected specimens of the species]


Species Account:

        Birgus latro is considered to be rare on Okinawa as I have to confess to having never seen nor collected a specimen in the wild. The single featured animal that I've personally seen was received as a gift from one of my University of Maryland biology students (collected on 06 Nov '99 by Douglas Erickson who found and collected it from Ikei Island, Okinawa. Douglas brought the specimen to me at the Kadena Education Center during class and I placed it into my truck (an old Toyota Surf, the US export model to the US is called the 4Runner). Somehow the crab escaped into the heavily SCUBA-laden rear of the truck and I couldn't locate it for several days . . . in fear of my life (well, apprehensive) that while I was driving the crab would climb from the rear of the truck onto my neck (a bit of paranoia, but this is a formidably-sized animal). I finally found it w/o being attacked (grin) and I later (25 Nov '99) took a series of photographs in the area of my Sobe home on Okinawa.

I've added the following information from The University of Michigan's Museum of Zoology / Animal Diversity Web (ADW) site (I didn't add the numerous literature citations cited by the authors):

Birgus latro
By Meaghan Ly and Yesenia Werner

Geographic Range
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.

Habitat
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.

Physical Description
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.
Coconut crabs have only vestigial 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.

Development
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 vascularised 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.

Reproduction
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.

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).
Females carry developing embryos on their pleopods and care for them, keeping them clean and aerated, until hatching. Males exhibit no parental investment.

Lifespan/Longevity
Coconut crabs are long-lived, only reaching their maximum size after 40-60 years.

Behavior
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 over-feed and produce greater volumes of haemolymph. When molting is completed, a crab will feed on its shed exoskeleton.

Home Range
Depending on the size of the island, home range can vary from 40 to 250 m2
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.

Food Habits
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.

Predation
The only documented predators of adult coconut crabs are humans. It has been suggested that juveniles and smaller individuals may be consumed by mangrove monitor lizards (Varanus indicus), cane toads (Rhinella marina), and feral pigs (Sus scrofa), but this has not been confirmed.

        I've added a second page w/ a series of images of the species.

        In addition, there's a page w/ links to misc. phyla including this species phylum, as well as additional resources.

Etymology:

Literature Cited:


Page Date: 15 Aug '17
Page Modification Date: 15 Aug '17
Digitally manipulated photos
Copyright © 2017 Robert F. Bolland