Coconut Crab (Birgus latro) Fact Sheet: Summary

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 size

Can 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).

Weight

Up to 4 kg (9 lb) (Brown and Fielder 1991)

Coloration

Variable 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).

Claws

Incredibly 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 characteristics

Excellent 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).

Range

Wide 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).

Habitat

Larvae 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 Status

Vulnerable (2018 assessment) (Cumberlidge 2020)

CITES Appendix

Not listed (UNEP 2021).

Other Designations

Protected 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 Wild

Populations 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 Survival

Overhunting 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 Cycle

Variable. 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).

Movements

Some 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 Behavior

Live 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).

Communication

Produce different kinds of tapping and pulsing sounds by moving structures in airway system. Likely used in agonistic and mating interactions (Oka et al. 2019).

Burrows

Adults 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 Feeding

Larvae: 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).

Predators

Adults: 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).

Locomotion

Walking/crawling and climbing. Climb trees using strong, heavily calcified legs with pointed tips for gripping trunk (Greenaway 2003; Drew et al. 2010).

Relationship with Humans

Also 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).

Sexual Maturity

Approximately 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).

Breeding

Reproductive 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).

Development

Begin 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 Size

Range: 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 Stages

Hatchlings: 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 Expectancy

Wild 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

• World’s largest terrestrial invertebrate (e.g., Brown and Fielder 1991)
• Closely related to terrestrial hermit crabs (e.g., Brown and Fielder 1991)
• Very strong; can lift up to approximately 30 kg (66 lb) (Oka et al. 2016)
• Complex brain with specializations for smell (Stensmyr et al. 2005; Krieger et al. 2010)
• Range once included northeastern Australia, eastern Africa, and Madagascar (Reyne 1939; Cumberlidge 2020)
• Charles Darwin, who observed coconut crabs during his H.M.S. Beagle voyage, marveled at this species’ extreme size variation (Laidre 2018)
• Kenny, animal ambassador at the San Diego Zoo, is one of the few coconut crabs housed at a zoo in the United States (ZIMS 2021)