Coconut Crab

Birgus latro

Gabbi Dubansky

Taxonomic Classification

Kingdom: Animalia

Phylum: Arthropoda

Class: Malacostraca

Order: Decapoda

Family: Coenobitidae

Genus: Birgus

Species: B. latro

“Coconut Crab.” Wikipedia, 25 Nov. 2021. Wikipedia, https://en.wikipedia.org/w/index.php?title=Coconut_crab&oldid=1057076960.

“Call the Coconut Crab a Crustacean on Steroids.” HowStuffWorks, 21 May 2019, https://animals.howstuffworks.com/marine-life/coconut-crabs.htm.

Holy Crab! Fearless Australian Poses With Massive Coconut Crab | HuffPost Null. https://www.huffpost.com/entry/coconut-crab-christmas-island_n_56b75b07e4b08069c7a79b81. Accessed 5 Dec. 2021.

cyan-biologist. “Cyan-Biologist.” Cyan-Biologist, https://cyan-biologist.tumblr.com/post/166187048639/birgus-latro-l-1767-coconut-crab-done-in-ink. Accessed 5 Dec. 2021.

Description

As the largest known land-based arthropod, coconut crabs carapaces can reach 1 meter from the tip of one leg to the other, weighing in at up to 4 kilograms. Because coconut crabs are a kind of hermit crab, the young stages will use a gastropod shell as protection. Once they are in adulthood, they will have outgrown these shells, and will have developed an exoskeleton, which is then overgrown with tergal plates and bristles. The coconut crabs are also sexually dimorphic, meaning that there are visible differences between males and females - the males being larger than the females (Ly, Meaghan, and Yesenia Werner.)

Body color is varied, and most often is dependent on their location and the rest of the coconut crab population. Most commonly, though, they are dark blue with red accents, or they may be red with purple tints (Ly, Meaghan, and Yesenia Werner.)

Common among crab species, the coconut crab has a set of asymmetrical celae, or hinged claws that resemble pinchers, where the right side is smaller than the left side. The coconut crab also has two sets of walking legs that are tipped with dacytls, or digits, that help them grip and climb various objects and surfaces, like trees (Ly, Meaghan, and Yesenia Werner.)

The pinching forces of the coconut crab's pinchers, as with many crustaceans, is closely related to the size of their claws. In a study conducted by Oka, Shin-ichiro, et al., they discovered that contrary to their expectations, there was no sexual dimorphism found (in the Okinawan population studied) in regard to the pincher morphology, or in the force fo the pinch. (Oka, Shin-ichiro, et al.)

Behavior

Coconut crabs are both nocturnal and terrestrial, although some of their reproduction takes place in or near water. The larvae are released into the water, and once the crab can find a sufficient shell, it will return to land. Once the coconut crab has grown their own exoskeleton, they will abandon the gastropod shell they previously used, and will continue without it until they are in a position that another one is needed (Shell Use: An Adaptation for Emigration from the Sea by the Coconut Crab.). Coconut crabs have been observed to display combative, or agonistic, behavior, which is similar to many other hermit crab species (Helfman).

In a study by Helfman, several different forms of behavioral responses were observed, including various stances of the crab that ranged from defensive to offensive, depending on the sizes of the crabs involved. One example is the High Body Position, where the body is somewhat raised, while withdrawing their antenna. This position was observed when two coconut crabs of similar size approached each other.

Helfman, Gene S. “Agonistic Behaviour of the Coconut Crab, Birgus Latro (L.)1.” Zeitschrift Für Tierpsychologie, vol. 43, no. 4, 1977, pp. 425–38. Wiley Online Library, https://doi.org/10.1111/j.1439-0310.1977.tb00490.x.

Ocean Watch. “Coconut Crabs Burrow in Holes to Molt. Check out Their Matchstick Eyes. #coconutcrabs Http://T.Co/QCJuBt0SsS.” @ocean_watch, 30 Dec. 2014, https://twitter.com/ocean_watch/status/550004442469113856.

“Figure 1. Distribution of Coconut Crab (Re-Drawn from Fletcher 1993).” ResearchGate, https://www.researchgate.net/figure/Distribution-of-coconut-crab-re-drawn-from-Fletcher-1993_fig1_289380442. Accessed 5 Dec. 2021.

Habitat

Coconut crabs live primarily on the coasts of tropical islands near the western Indo-Pacific Ocean, like Madagascar, Easter Island, and Seychelles. ("Sea Wonder: Coconut Crab.")

As juveniles, the coconut crab lives in the water until they are further developed, and in adulthood, the coconut crabs keep to the land. Despite their protective shell coverings developed during their adult life, coconut crabs make burrows to live, keep food, and molt in ("Sea Wonder: Coconut Crab.")

Feeding & Diet

Coconut crabs in the adult stage will essentially eat anything they can acquire, meaning they are omnivorous scavengers. They have been observed eating anything from deceased animals, fruit, and of course, coconuts - they have even been seen taking a coconut up a tree in order to break it open and get to the meat by dropping it from a tall height (“Sea Wonder: Coconut Crab.”)

There are also reports of coconut crabs consuming live animals by attacking their prey, like rats, smaller crustaceans, and even smaller seabirds, and then dragging them into their burrows. This feeding behavior is often a response to food availability - if their regular diet of animal carcasses and coconuts are scarce, they may turn to opportunistic catches for meals (Coconut Crabs: The Bird-Eating Behemoths Thriving on Isolated Tropical Islands.)

In 2020, a family was hosting a barbecue, and their setup got ambushed buy a hoard of coconut crabs (called robber crabs in the DailyMail video attached). Coconut crabs have an incredibly strong sense of smell, and as opportunistic feeders, they sought out the family's food. The son counted at least 52 crabs. They are essentially harmless to humans if left unbothered, so the family only had to hide their food from the crabs. (Poposki, Claudia.)

Poposki, Claudia. “Family Trying to Enjoy a Spring Barbecue Surrounded by Robber Crabs.” Mail Online, 22 Sept. 2020, https://www.dailymail.co.uk/news/article-8758171/Christmas-Island-Family-trying-enjoy-spring-barbecue-surrounded-robber-crabs.html.

Reproduction

Fertilization occurs when the male coconut crab provides the spermatophore, a packet of sperm, to the female coconut crab. This fertilization is believed to most often take place in the water, but there have been cases of coconut crabs copulating away from the water (Amesbury). Summer months are the most often observed times for the females to be seen carrying the embryos that are in development (Helfman). The height of the larval releasing action is documented to occur most closely to highest tide, and it is believed that this is because the released larvae are most successful when they are able to be carried to open waters (Amesbury).

Predators

Adult coconut crabs do not have any known or observed predators in their natural habitat (Helfman). However, due to human influences, the International Union for Conservation of Nature has listed the coconut crab as a vulnerable species (Neil Cumberlidge (Northern Michigan University, Usa)).

While the adult coconut crabs do not have as many predators due to their exoskeleton and size, the larvae, which temporarily rely on an acquired gastropod shell, are relatively more vulnerable to predation by larger animals.

Conservation

Due to overharvesting by humans that consume the coconut crab, and due to intense habitat loss, there is a decline in their numbers. The coconut crab is considered a cash crop, and the growing and harvesting of this species has significantly contributed to its population growth (Fletcher, W. J., et al.) For many, the coconut crab is one of rather few food sources, so conservation fo the species has faced some obstacles.

In some places, the crabs are protected, either by limiting the amount that they can be hunted, or by protecting females carrying eggs. In terms of habitat loss, much more action would need to be taken to limit the negative impacts of climate change that threaten this species (Coconut Crabs: The Bird-Eating Behemoths Thriving on Isolated Tropical Islands.)

A study by Sato, Taku, and Kenzo Yoseda revealed that, based on the reproductive period of a population on Hatoma Island, closing parts of the island to visitors would be the most beneficial to the reproducing coconut crabs, allowing their population to increase (Sato, Taku, and Kenzo Yoseda.)

Citations & Photos (MLA9)

Amesbury, S. S. “BIOLOGICAL STUDIES ON THE COCONUT CRAB (BIRGUS LATRO) IN THE MARIANA ISLANDS.” Undefined, 2000. www.semanticscholar.org, https://www.semanticscholar.org/paper/BIOLOGICAL-STUDIES-ON-THE-COCONUT-CRAB-(BIRGUS-IN-Amesbury/de18222971b782c3bbf2014875fb2fb0b59c7226.

“Call the Coconut Crab a Crustacean on Steroids.” HowStuffWorks, 21 May 2019, https://animals.howstuffworks.com/marine-life/coconut-crabs.htm.

“Coconut Crab.” Wikipedia, 25 Nov. 2021. Wikipedia, https://en.wikipedia.org/w/index.php?title=Coconut_crab&oldid=1057076960.

Coconut Crabs: The Bird-Eating Behemoths Thriving on Isolated Tropical Islands. https://www.nhm.ac.uk/discover/coconut-crabs-bird-eating-giants-on-tropical-islands.html. Accessed 5 Dec. 2021.

cyan-biologist. “Cyan-Biologist.” Cyan-Biologist, https://cyan-biologist.tumblr.com/post/166187048639/birgus-latro-l-1767-coconut-crab-done-in-ink. Accessed 5 Dec. 2021.

Fletcher, W. J., et al. “Growth of the Coconut Crab Birgus Latro in Vanuatu.” Journal of Experimental Marine Biology and Ecology, vol. 141, no. 1, Aug. 1990, pp. 63–78. ScienceDirect, https://doi.org/10.1016/0022-0981(90)90158-9.

Helfman, Gene S. “Agonistic Behaviour of the Coconut Crab, Birgus Latro (L.)1.” Zeitschrift Für Tierpsychologie, vol. 43, no. 4, 1977, pp. 425–38. Wiley Online Library, https://doi.org/10.1111/j.1439-0310.1977.tb00490.x.

Holy Crab! Fearless Australian Poses With Massive Coconut Crab | HuffPost Null. https://www.huffpost.com/entry/coconut-crab-christmas-island_n_56b75b07e4b08069c7a79b81. Accessed 5 Dec. 2021.

Ly, Meaghan, and Yesenia Werner. “ADW: : INFORMATION.” Animal Diversity Web, https://animaldiversity.org/accounts/Birgus_latro/. Accessed 5 Dec. 2021.

Neil Cumberlidge (Northern Michigan University, Usa). “IUCN Red List of Threatened Species: Birgus Latro.” IUCN Red List of Threatened Species, Aug. 2018. www.iucnredlist.org, https://www.iucnredlist.org/en.

Oka, Shin-ichiro, et al. “A Mighty Claw: Pinching Force of the Coconut Crab, the Largest Terrestrial Crustacean.” PLOS ONE, vol. 11, no. 11, Public Library of Science, Nov. 2016, p. e0166108. PLoS Journals, https://doi.org/10.1371/journal.pone.0166108.

Sato, Taku, and Kenzo Yoseda. “Reproductive Season and Female Maturity Size of Coconut Crab Birgus Latro on Hatoma Island, Southern Japan.” Fisheries Science, vol. 74, no. 6, Nov. 2008, pp. 1277–82. Springer Link, https://doi.org/10.1111/j.1444-2906.2008.01652.x.

“Sea Wonder: Coconut Crab.” National Marine Sanctuary Foundation, https://marinesanctuary.org/blog/sea-wonder-coconut-crab/. Accessed 5 Dec. 2021.

Shell Use: An Adaptation for Emigration from the Sea by the Coconut Crab. https://www.science.org/doi/abs/10.1126/science.161.3839.385. Accessed 5 Dec. 2021.

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