Coconut crabs are a truly fascinating creature As the world’s largest land-dwelling arthropod, these enormous hermit crabs have captured people’s imaginations for centuries But just how big do coconut crabs get? In this article, we’ll take an in-depth look at coconut crab sizes and weights.
Coconut crabs, also known as robber crabs or palm thieves, are a type of hermit crab that lives on islands in the Indian and Pacific Oceans. Unlike other hermit crabs that live in the ocean, coconut crabs have adapted to life on land. Their name comes from their ability to climb coconut palms and crack open coconuts with their massive pincers.
Coconut crabs are astonishingly large, especially compared to other land invertebrates. An adult coconut crab can have a leg span of up to 1 meter (3 feet) and can weigh up to 45 kg (10 pounds). Their size makes them the largest land-living arthropod in the world.
Average Weights of Coconut Crabs
On average, a fully grown adult coconut crab weighs between 4-9 pounds. However, some individuals can reach even more impressive sizes. The largest coconut crabs can weigh up to 20 pounds!
Coconut crabs exhibit a high degree of size variation between individuals There are a few key factors that influence their growth
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Gender – Male coconut crabs are generally larger than females. Males may reach 10 pounds, while females max out around 6 pounds on average.
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Age – Coconut crabs grow slowly, taking decades to reach full size. Older crabs are typically larger. Lifespans in the wild are 40-60 years.
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Food availability – Crabs with access to abundant food sources tend to grow bigger over their long lives.
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Genetics – There seems to be natural variation between individuals, where some crabs are genetically predisposed to grow larger.
So while 4-9 pounds is average, coconut crabs can vary substantially in their final sizes depending on several biological factors.
Why Can Coconut Crabs Get So Big?
There are a few key reasons why coconut crabs are able to achieve such mammoth proportions compared to other terrestrial animals:
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Slow metabolism – Coconut crabs have an extremely slow metabolic rate. Their bodies convert food and oxygen at a sluggish pace compared to other animals. This slows their growth, but allows them to accumulate body mass over decades.
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Few predators – On remote islands with no large land predators, coconut crabs can live to old age and gain size without constraints from predation.
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Abundant food – Coconut crabs inhabit tropical islands filled with coconuts, nuts, fruits, and other bountiful food sources. This nutritional richness fuels their growth.
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Ideal habitat – Islands provide a hot, humid climate and sandy soils ideal for burrowing. This habitat supports the needs of giant coconut crabs.
Thanks to these key advantages, coconut crabs are able to achieve their massive size and unique distinction as the heaviest land invertebrate.
Weight Changes Through Life Stages
Coconut crabs undergo dramatic changes in size and form throughout their complex life cycle:
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Larvae – Coconut crabs begin life as tiny larvae about 3 mm long. Larvae float and drift in the ocean for 1 month, feeding on plankton.
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Juveniles – After transitioning to land, juveniles use snail shells and inhabit burrows near shore. At 8 mm, they abandon shells and begin looking more crab-like.
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Adults – Coconut crabs slowly grow across decades, molting and incrementally increasing in size each year. Growth rate varies substantially between individuals.
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Females – Mature around 2 inches long and 6 years old. Max out around 6 pounds.
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Males – Mature around 3 inches long and 10 years old. Can eventually reach 10 pounds or more.
So coconut crabs undergo an incredible journey from microscopic larvae to lumbering giants weighing as much as a housecat!
The Incredible Strength of Coconut Crabs
Given their large size, it’s no surprise that coconut crabs are incredibly strong. They use their massive pincers to crack open coconuts, climb trees, and defend themselves. Here are some astonishing facts about coconut crab strength:
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Coconut crabs can lift over 30 kg (66 pounds) – more than 30 times their own weight!
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Their pincers generate the strongest pinching force of any terrestrial animal.
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They can break open and eat entire coconuts, exerting up to an estimated 3,300 newtons or 740 pounds of force with their claws.
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Coconut crabs are able to climb trees up to 10 meters high (33 feet).
Clearly, the coconut crab’s large size translates into remarkable strength. These hermit crabs are not to be underestimated!
Conservation Status of Coconut Crabs
Due to habitat loss, climate change, overhunting, and predation by invasive species, coconut crab populations have declined substantially across their native range. They are listed as Vulnerable on the IUCN Red List.
Some key threats facing coconut crabs:
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Habitat destruction as tropical forests are cleared.
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Collection for human food or the pet trade.
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Predation on juveniles by introduced rats, pigs, and cats.
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Motor vehicle strikes as roads fragment their habitat.
More research and conservation action is urgently needed to protect these amazing animals into the future. Preserving their island habitats will give coconut crabs space to continue reaching their mammoth sizes.
Fascinating Giant Crustaceans
Coconut crabs are truly captivating creatures. Their great size and strength are a testament to the wonders of evolution in producing giant invertebrates adapted to land life. With proper habitat conservation, hopefully coconut crabs can continue thriving at their huge sizes for years to come.
So next time you crack open a coconut, spare a thought for these colossal crustaceans! Learning more about the lives of coconut crabs deepens our appreciation for the diversity of life on our planet.
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About This Fact Sheet
For detailed information, click the tabs at the top of this page.
© 2021 San Diego Zoo Wildlife Alliance
How to cite: Coconut Crab (Birgus latro) Fact Sheet. c2021. San Diego (CA): San Diego Zoo Wildlife Alliance; [accessed YYYY Mmm dd]. http://ielc. libguides. com/sdzg/factsheets/coconut-crab. (note: replace YYYY Mmm dd with date accessed, e. g. , 2019 Dec 31).
Disclaimer: The San Diego Zoo Wildlife Alliance does its best to give accurate information, but some of the facts it gives may become out-of-date or be replaced by new research. Questions and comments may be addressed to library@sdzwa. org.
Thank you to Prof. Katsuyuki Hamasaki for providing expert content review of this fact sheet.
Prof. Hamasaki studies the life cycles of marine crustaceans and invertebrates. He is especially interested in the biology of larvae, aquaculture techniques, and the biology of protecting marine life. Over his 30-year academic career, Prof. Hamasaki has authored nearly 200 research articles on crustacean and fisheries biology.
He has written important journal articles about how coconut crabs grow and change, how larvae move and settle, genetics, survival, growth, and aquaculture propagation.
Dr. It is his job to teach in the Department of Marine Bioscience at Japan’s Tokyo University of Marine Science and Technology. He currently serves on the editorial board of Invertebrate Reproduction & Development, a Taylor & Francis journal.
View Prof. Hamasaki’s publications list on Google Scholar.
Coconut Crab (Birgus latro) Fact Sheet
Coconut Crab (Birgus latro)
credit: © San Diego Zoo Wildlife Alliance. All rights reserved.
Taxonomy |
Physical Characteristics |
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Kingdom: Animalia Phylum: Arthropoda — arthropods Subphylum: Crustacea Class: Malacostraca Infraorder: Anomura Order: Decapoda — ten-legged arthropods Superfamily: Paguroidea — hermit crabs Family: Coenobitidae — terrestrial hermit crabs Genus: Birgus (Leach, 1816) Species: Birgus latro (Linnaeus, 1767) — coconut crab, robber crab Sources: Eldredge (1996); Drew et al. (2010); Cumberlidge (2020) |
Body sizeCan grow to more than 200 mm (8 in) in carapace width (Drew et al. 2010) and have a leg span of about 800 mm (30 in) across (Laidre 2018; Cumberlidge 2020). Striking size dimorphism in adult age class (e.g., Chauvet and Kadiri-Jan 1999; Laidre 2018). Males larger than females (Sato et al. 2013; Anagnostou and Schubart 2014; Drew and Hansson 2014; Oka et al. 2015; Appoo et al. 2021). Males: up to approximately 75 mm (3 in) thoracic length (Amesbury 1980; Buden 2012; Drew et al. 2013) Females: up to approximately 50 mm (2 in) thoracic length (Amesbury 1980; Drew et al. 2013) Extremely slow growing (Fletcher et al. 1990b; Drew et al. 2013; Sato et al. 2013; Oka et al. 2015). WeightUp to 4 kg (9 lb) (Brown and Fielder 1991) ColorationVariable light violet to blue, deep purple, brown to orange-red, with lighter variegated markings, claws, and leg tips (Nokelainen et al. 2018; Cumberlidge 2020). Red and blue color types identified. May provide camouflage. Color saturation greater on underside (Nokelainen et al. 2018). ClawsIncredibly strong claws; exert the greatest force — pinching or biting — of any terrestrial animal. Use for digging, defense, hunting, and opening coconuts (Oka et al. 2016). Other characteristicsExcellent sense of smell, similar in structure/ability to insects (Stensmyr et al. 2005). Can survive air temperatures up to approximately 37°C (100°F), on average (Kasuya et al. 2020). |
Distribution & Status |
Behavior & Ecology |
RangeWide distribution. Indian and western Pacific Oceans, from eastern coast of Africa to French Polynesia to Japan. Occurs almost exclusively on oceanic islands (coral atolls) or small islands adjacent to large continents (Eldredge 1996; Drew et al. 2010). HabitatLarvae and juveniles: Live in ocean water column or on ocean bottom (Reese and Kinzie 1968; Eldredge 1996). After transition to land, seek complex structures for protection, such as coral rubble, seaweed piles, and palm fronds (Kadiri-Jan and Chauvet 1998; Chauvet and Kadiri-Jan 1999; Hamasaki et al. 2015). Move farther inland as adults (e.g., Kadiri-Jan and Chauvet 1998). Adults: Live on land. Use tropical rain forest, coastal scrub woodlands, coconut palm groves, and seashore habitats (Helfman 1973; Brown and Fielder 1991; Greenaway 2003; Cumberlidge 2020). Reported to live up to 6 km (4 mi) from the ocean, if suitable humidity and shelter sites are available (Gibson-Hill 1947; Drew et al. 2010). During periods of inactivity, hide in burrows, rock crevices, hollow logs, and trees (e.g., Fletcher et al. 1990a; Eldredge 1996). IUCN StatusVulnerable (2018 assessment) (Cumberlidge 2020) CITES AppendixNot listed (UNEP 2021). Other DesignationsProtected to different degrees in different countries (Cumberlidge 2020; Caro et al. 2021). Listed as “vulnerable” in Ministry of the Environment of Japan’s Red Data Book (Hamasaki et al. 2011). Protected under India’s Wildlife Protection Act (Patankar and D’souza 2012). Limited protection in the Northern Mariana Islands (Amesbury 1980), Guam, the Philippines and Indonesia (Caro et al. 2021). Endangered status in Vanuatu (Cumberlidge 2020). Full protection on some islands (Poupin et al. 2013; Caro et al. 2021). Regulations such as hunting bans, catch size limits, minimum size limits, and a protected breeding season enacted in some countries. In many locations, may be legally protected, but laws not enforced or hunters unaware of regulations (Patankar and D’souza 2012). Populations in the WildPopulations have declined or gone extinct in many areas of this species’ range, mainly due to overhunting and habitat loss (Brown and Fielder 1991; Drew et al. 2010; Patankar and D’souza 2012; Laidre 2018; Cumberlidge 2020). Still common on some islands (Laidre 2018), particularly those uninhabited (or sparsely populated) by humans or where the coconut crab is actively protected (Cumberlidge 2020; Appoo et al. 2021; Caro et al. 2021). See Cumberlidge (2020), Drew et al. (2010), and Caro et al. (2021) for detailed information. Indian and Pacific Ocean populations differ genetically. Additional structuring among Pacific populations; influenced by inter-island distance (Lavery and Fielder 1991; Lavery et al. 1995; Lavery et al. 1996; Hamasaki et al. 2015; Veldsman et al. 2020; Yorisue et al. 2020). Threats to SurvivalOverhunting by humans; habitat loss; juvenile predation by introduced species; strikes by motor vehicles; natural disasters, such as tsunamis (Amesbury 1980; Patanakar 2007; Laidre 2018; Cumberlidge 2020). Conservation propagation programs being developed (Hamasaki et al. 2013; Hamasaki et al. 2014; Hamasaki et al. 2016). |
Activity CycleVariable. Nocturnal in many locations to avoid predators, stay cool, and reduce water loss (Held 1963; Amesbury 1980; Eldredge 1996). Appear to be less active during the day, where humans live (Reyne 1939; Brown and Fielder 1991). Active day and night in some locations, or during humid conditions (Amesbury 1980; Fletcher et al. 1990a; Greenaway 2003; Buden 2012). MovementsSome individuals show site fidelity to burrow and foraging sites, while others move substantial distances over short periods (Fletcher et al. 1990a). Migrate between inland and coastal areas during breeding, females to lay eggs and males for mating opportunities (Sato and Yoseda 2013). Do not migrate between islands but rather disperse as larvae (Helfman 1973). Social BehaviorLive singly in underground burrows and rock crevices (Eldredge 1996). Larger individuals dominant (Helfman 1973). Males more commonly fight to compete for mates, and to defend food and other resources (Drew et al. 2010; Drew et al. 2013). CommunicationProduce different kinds of tapping and pulsing sounds by moving structures in airway system. Likely used in agonistic and mating interactions (Oka et al. 2019). BurrowsAdults dig large burrows in sand or loose soil. Often located beneath roots of coconut trees. Burrow chambers lined with coconut husks. Vigorously defend burrows (Amesbury 1980; Eldredge 1996). Remain underground when molting (a period of 3 to 16 weeks, depending on body size). Block burrow entrance with sand or soil (Held 1963; Amesbury 1980; Fletcher et al. 1990b; Fletcher et al. 1991). Diet and FeedingLarvae: No data available for wild larvae. Fed zooplankton in managed care, so presumably carnivorous (Drew et al. 2010; Katsuyuki Hamasaki, personal communication, 2021). Adults: Omnivorous. Eat a wide range of plant and animal foods (fruits, nuts, vegetation, insects, carrion, etc.). Commonly eat fruits of coconut, pandanus, and other palm species (Reyne 1939; Gibson-Hill 1947; Amesbury 1980; Sato and Yoseda 2013; Laidre 2018). Named for ability to open coconuts, a process that sometimes takes several days (Drew et al. 2010). Herbivory and scavenging most important feeding modes. Observed to prey and scavenge on invertebrates, small mammals, and even birds (Helfman 1973; Greenaway 2003; Kessler 2005; Hennicke 2012; Laidre 2017; Laidre 2018). Bring large pieces of food to burrow, where they remain feeding for several days (Amesbury 1980). Large males often exclude smaller individuals from desirable feeding locations (Helfman 1973; Drew et al. 2012). PredatorsAdults: humans (e.g., Amesbury 1980) Juveniles: Not documented. Possibly rats, feral pigs, and birds (Helfman 1973; Whitten et al. 2002, as cited by Buden 2012). LocomotionWalking/crawling and climbing. Climb trees using strong, heavily calcified legs with pointed tips for gripping trunk (Greenaway 2003; Drew et al. 2010). Relationship with HumansAlso called “robber crabs” (Brown and Fielder 1991). Known to take objects from porches, camp sites, etc. Appear to be attracted to novel objects and odors — opportunistic tendencies likely an adaptation for living in food-limited, remote locations (Laidre 2018). Hunted by some island communities; desirable as food (Brown and Fielder 1991; Caro et al. 2021). Hunting regulations passed by some governments but generally not well enforced (Buden 2012). Social taboos against hunting/eating coconut crab in some locations (Patankar and D’souza 2012; Forth 2019). In the past, flesh considered an aphrodisiac. Crabs also sold in dried form or as a novelty/luxury food dish to tourists (Brown and Fielder 1991). Human-wildlife conflict on Vanuatu coconut palm plantations, where perceived to feed on coconut tree seedlings (Cumberlidge 2020). |
Reproduction & Development |
Additional Species Highlights |
Sexual MaturityApproximately 22 to 28 mm (0.87 to 1.1 in) thoracic length (Helfman 1973; Amesbury 1980; Fletcher et al. 1990b; Fletcher and Amos 1994; Kadiri-Jan and Chauvet 1998; Sato et al. 2008; Sato and Yoseda 2008), or approximately 5 to 10 years of age (Sato et al. 2008; Sato and Yoseda 2008). BreedingReproductive activity thought to occur year-round; varies geographically (see e.g., Schiller et al. 1991; Drew et al. 2010). Mating commonly occurs during summer (e.g., Helfman 1973; Horstmann 1976, as cited by Brown and Fielder 1991) or during a region’s wet season (e.g., Appoo et al. 2021). Facing each other, male holds female’s front limbs (chelipeds) and mounts after turning female onto her back. Then passes a packet of sperm to female. Pair separates (Helfman 1977; Schiller et al. 1991; Tudge and Jamieson 1991). Female migrates seaward after mating. Extrudes eggs and fertilizes them with male’s sperm. Female then broods sponge-like egg mass on her abdomen for 25 to 45 days (Schiller et al. 1991; Eldredge 1996; Hamasaki et al. 2016). Egg color changes from bright orange to gray, as embryos develop (Reese and Kinzie 1968; Fletcher and Amos 1994; Sato and Yoseda 2009a). Female releases eggs near cliffs or in intertidal zone, timed so that waves or high tides wash eggs into the ocean (Amesbury 1980; Sato and Yoseda 2009b; Sato and Yoseda 2009a; Schiller 1993, as cited by Drew et al. 2010; Drew and Hansson 2014). Then returns inland, sometimes migrating with other coconut crabs (Schiller et al. 1991). Females thought to lay eggs once per reproductive season (Helfman 1973; Sato and Yoseda 2013). DevelopmentBegin life as ocean-drifting larvae (microscopic plankton, less than 3 mm in length), moving between inshore and offshore waters. At sea approximately a month before becoming benthic (bottom dwelling) and migrating back to inshore zone (Reese and Kinzie 1968; Schiller et al. 1991; Wang et al. 2007; Hamasaki et al. 2013). Move into small empty snail shells and permanently transition onto land. Cease using shells after reaching approximately 8 mm (0.3 in) thoracic length and exoskeleton re-calcifies (Reese 1968; Amesbury 1980; Kadiri-Jan and Chauvet 1998; Hamasaki et al. 2011; Hamasaki et al. 2013; Hamasaki et al. 2015). Clutch SizeRange: 45,000 to 250,000 eggs per clutch. Increases with female body size. Egg mass may weigh as much as one-third of female’s body weight (Helfman 1973; Sato and Yoseda 2008). Life StagesHatchlings: Larvae hatch from egg, then undergo 2 developmental stages. Adapted for swimming in open water. Spines for protection. After molting 4 to 5 more times, become bottom-dwelling (benthic), with shrimp-like body form. Find tiny shell. Migrate to shore after 3 to 4 weeks. Live near high intertidal zone for about 4 weeks, then become juveniles (Reese and Kinzie 1968; Eldredge 1996; Hamasaki et al. 2011). Juveniles: Little known. Live cryptically in burrows, foraging infrequently and only for short periods (Held 1963; Fletcher and Amos 1994). Use snail shell for 1 to 2 years (Reese 1968), then cease using shell and morph into adult body form (Greenaway 2003). Move from coastal to more inland areas (Kadiri-Jan and Chauvet 1998). Juvenile mortality is high (Hamasaki et al. 2015). Adults: Molt annually across life span, often taking at least several decades to reach full size. Some adults grow to large size, others do not grow larger (remain juvenile sized) (Fletcher et al. 1990b; Oka et al. 2015). Typical Life ExpectancyWild populations: typically, 40 to 60 years (Fletcher et al. 1990b; Sato et al. 2013; Oka et al. 2015) Managed care: no AZA estimates |
Feature Facts
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Coconut Crab: Your Worst Nightmare
FAQ
How big is the biggest coconut crab?
How strong is a coconut crab grab?
How strong is coconut crab crushing?
How many coconut crabs are left?
How big is a coconut crab?
The coconut crab ( Birgus latro) is a terrestrial species of giant hermit crab, and is also known as the robber crab or palm thief. It is the largest terrestrial arthropod known, with a weight of up to 4.1 kg (9 lb). The distance from the tip of one leg to the tip of another can be as wide as 1 m (3 ft 3 in).
What is a coconut crab?
Then perhaps you’ve never had the privilege of meeting a coconut crab. Coconut crabs are the world’s largest land-based crustaceans. These mostly gentle giants can weigh as much as a small terrier and have a leg span of up to three feet (one meter). But their most impressive assets are their front claws, which grip with tremendous force.
How long does a coconut crab take to grow?
The coconut crab reaches sexual maturity around 5 years after hatching. They reach their maximum size only after 40–60 years. They grow remarkably slowly, and may take up to 120 years to reach full size, as posited by ecologist Michelle Drew of the Max Planck Institute.
Are coconut crabs extirpated?
While its range broadly shadows the distribution of the coconut palm, the coconut crab has been extirpated from most areas with a significant human population such as mainland Australia and Madagascar. B. latro is both the largest living terrestrial arthropod and the largest living terrestrial invertebrate.