Now that’s a crab

January 28, 2013 • 6:18 am

Update: Reader Dennis Hansen, a biologist who works on the Indian Ocean island of Aldabra in the Seychelles, which (like the Galapagos) has giant tortoises, sends three coconut crab photos and a note:

Here’s a few photos of coconut crabs from Aldabra, for your perusal. They leave the giant tortoises alone, it seems. At least until a tortoise dies, by which time the crabs tear it apart from the inside. When staying in one of the remote field camps on the atoll, they do what they can to rob us of our meagre field rations, though. Don’t leave food on the table, or it will disappear within a few minutes.

crab & nut

field camp crab

crab & tortoise


This photo and caption are from Professor Brian Cox’s Facebook page, and apparently appear in his new book, Wonders of Life with the caption:

“These animals are known locally as robber crabs on their native Christmas Island because they have a reputation for curiosity and for stealing things. They wander into unlocked houses and steal knives, forks and even shoes.”

Picture 2

Actually, I know these beasts by the name “coconut crab,” but their scientific name is Bigus latro (“Bigus” is right!). According to Wikipedia, it’s the largest land-living arthropod in the world, and can weigh up to 4.1 kg (9 pounds).  (Question for readers: what is the largest living arthropod among all animals?) And they can live for up to 60 years!

While they’re reputed to climb trees and pick coconuts, they don’t appear to do that often, though they can open coconuts “collectively” or, individually, with great effort. Instead, their usual diet consists of fruit, nuts, seeds, and tree pith.

They’re terrestrial hermit crabs, but only the juveniles use shells, and their geographic distribution is shown on the map below. Wikipedia notes:

Coconut crabs live in the Indian Ocean and the central Pacific Ocean, with a distribution that closely matches that of the coconut palm.

Christmas Island in the Indian Ocean has the largest and densest population of coconut crabs in the world, although it is outnumbered there by more than 50 times by the Christmas Island red crab, Gecarcoidea natalis.  Other Indian Ocean populations exist on the Seychelles, including Aldabra and Cosmoledo, but the coconut crab is extinct on the central islands.Coconut crabs occur on several of the Andaman and Nicobar Islands in the Bay of Bengal. They occur on most of the islands, and the northern atolls, of the Chagos Archipelago.

Since they drown easily, one wonders how they got to all those islands. Perhaps a reader can enlighten us.


And the crabs are both smart and tenacious when it comes to getting coconuts:

It is a common perception that the coconut crab cuts the coconuts from the tree to eat them on the ground. The coconut crab can take a coconut from the ground and cut it to a husk nut, take it with its claw, climb up a tree 10 m (33 ft) high and drop the husk nut, to access the content inside.They often descend from the trees by falling, and can survive a fall of at least 4.5 metres (15 ft) unhurt. Coconut crabs cut holes into coconuts with their strong claws and eat the contents, although it can take several days before the coconut is opened.

Thomas Hale Streets discussed the behaviour in 1877, doubting that the animal would climb trees to get at the nuts. In the 1980s, Holger Rumpff was able to confirm Streets’s report, observing and studying how they open coconuts in the wild. The animal has developed a special technique to do so: if the coconut is still covered with husk, it will use its claws to rip off strips, always starting from the side with the three germination pores, the group of three small circles found on the outside of the coconut. Once the pores are visible, the coconut crab will bang its pincers on one of them until they break. Afterwards, it will turn around and use the smaller pincers on its other legs to pull out the white flesh of the coconut. Using their strong claws, larger individuals can even break the hard coconut into smaller pieces for easier consumption.

Here’s a video of one carrying his coconut prize:

This largest of all crabs is called a Coconut or Robber Crab. The Coconut Crab, Birgus latro, was found after dark on an unpaved country road on Christmas Island, Australia. It is seen here carrying a coconut. However, it also eats a large variety of other items, including dead crabs. . The headlights of a car serve as illumination here.

A photo of one attacking a coconut. What a beautiful animal! I must confess, though, that when I look at those claws I think of drawn butter (they’re apparently edible, but I wouldn’t eat one as they’re endangered when in contact with humans).


Here’s a monster, with a LOLzy Youtube comment after the video:

Picture 1

A few more Coconut Crab Facts:

Coconut crabs are considered one of the most terrestrial decapods, with most aspects of its life linked to a terrestrial existence; they will drown in sea water in less than a day.Coconut crabs live alone in underground burrows and rock crevices, depending on the local terrain. They dig their own burrows in sand or loose soil. During the day, the animal stays hidden to reduce water loss from heat. The coconut crabs’ burrows contain very fine yet strong fibres of the coconut husk which the animal uses as bedding. While resting in its burrow, the coconut crab closes the entrances with one of its claws to create the moist microclimate within the burrow necessary for its breathing organs. In areas with a large coconut crab population, some may come out during the day, perhaps to gain an advantage in the search for food. Other times they will emerge if it is moist or raining, since these conditions allow them to breathe more easily. They live almost exclusively on land, returning to the sea only to release their eggs; on Christmas Island, for instance, B. latro is abundant 6 kilometres (3.7 mi) from the sea.

Should a coconut crab pinch a person, it will cause pain and be unlikely to release its grip. Thomas Hale Streets reports a trick used by Micronesians of the Line Islands to get a coconut crab to loosen its grip: “It may be interesting to know that in such a dilemma a gentle titillation of the under soft parts of the body with any light material will cause the crab to loosen its hold.”


106 thoughts on “Now that’s a crab

  1. I find this part (presumably from Wikipedia) amazing: “The coconut crab can take a coconut from the ground and cut it to a husk nut, take it with its claw, climb up a tree 10 m (33 ft) high and drop the husk nut, to access the content inside.” Can anyone confirm that these arthropods really deliberately drop coconuts from trees to crack them open?

    1. We had quite a few “coconut crabs” on the island job in Tanzania the year before last. I never saw anything suggestive of the crabs eating coconuts. We did find “opened” coconuts regularly, but since the openings were quite small (around a centimetre) I ascribed them to the rats which had appeared between our 2004 job and 2011.

  2. The first Wonders of Life episode (BBC2 yesterday) was excellent! It is so rare for prime-time TV to have actual science, presented with the attitude that people might actually be interested in science, rather than the usual dumbing everything down and avoiding any concept that might stretch any viewer.

    And then there was the dissing of dualism and vitalism and the direct and explicit statements that living things are the laws of physics playing out, with nothing “mystical” or un-physical about them.

    1. Wonder how the biologists feel about a physicist mooching around in their territory! Great program though. Looking forward to the rest of the series.

      1. Well, Matthew Cobb, Brian’s colleague and an advisor on the series, iirc, should probably weigh in on that point.

        But as Coel’s comment suggests, Brian is really giving a physicist’s/physics perspective on life.

        /@ (ex-physicist!)

        1. Well I was very happy with the series, and with this opening episode in particular. Brian is brilliant on TV, and I think he brings something very different to the study of life, because he’s a physicist. And that’s fine – he’s not Attenborough. I haven’t heard too much grumping from biologists (maybe less than from astronomers regarding his previous series – he’s a particle physicist, don’t forget). Everyone here in Manchester is very happy – he’s a great draw and I think this will inspire a new generation of youngsters to study science. The #wonders twitterstream was overwhelmingly positive (though they did label DNA as ‘dioxyribose’ not ‘deoxyribose’).

          1. I hope you’ve had the announcer who talked over the specially commissioned final-credits Eric-Idle song hung, drawn and quartered pour encourager les autres.

          2. I watched it, too, and agree, it was actually really good. You Brits should be proud of your BBC, it so often produces the only popular science shows on tv worth watching. Great show, thank you!

          3. Perhaps, but one Attenborough series is worth ten years of programming on Discovery or NatGeo in the US. It is nice to have occasional programming that treats its audience like adults. Doesn’t happen too often elsewhere on tv.

  3. As to how they got to all the islands? I’m guessing they release their eggs into the ocean like other hermit crabs. There the eggs will drift, hatch and the young will spend some time as plankton before going to land. But it it does make you wonder why they aren’t more widely distributed.

    1. You’re right about the plankton stage. Wikipedia also says their distribution closely matches that of the coconut palm.

    2. What constrains wider distribution, I would guess was competition or predation – they can grow big because there are no mammalian or reptilian rivals.

  4. Very Interesting!Not exactly cuddly…maybe the babies are. Maybe they travel on coconut rafts. Here’s an interesting related story: Could Coconut Crabs have killed Amelia Earhart? ( they be related to Tree Lobsters?

  5. I saw a movie with one of those suckers in it and it didn’t end well (except for Ripley and Jones the cat).

    1. I don’t see the problem with unloicked doors. We would occasionally find them in our tents – use a bucket or a broom and sweep them out. Otherwise – the Somali pirates were a much bigger concern. And the typhoid poisoning.

        1. I still don’t see your point. What is so horrifying about the idea of an unlocked door? Or, for that matter, a mucking great arthropod wandering around in your accommodation?
          But yes, of course tents have doors. Made of canvas, held shut by zips and/ or ties. It’s a bit difficult to get your body in, or to keep the weather out otherwise, which rather defeats the point of carrying them then.

          1. No, you’re obviously not seeing my point- and that’s horrifying. You must be the one showing mucking great arthropods how to open unlocked doors – knock that shit off right this minute!
            Sounds like you need a bigger tent…or maybe the help of clearer instructions.

        1. Ahh … seeing “unlocked door” and thinking “unlocked door that will naturally stay shut without being wedged shut by a chair or bloody great lump of coral”.
          Where these things roam, with cheap industrial office cabins etc, doors that aren’t locked, or at least latched, NEVER fit properly in the doorways. Well, not for more than a few days.

          1. So you thought that everyone would see “unlocked door” the same way you would – interesting.

          2. Not the way I read it. To me he’s saying, ” my mind immediately thought of how doors usually behave in the environment these crabs are found in.” Nothing about everyone’s perceptions.

            You see to be bit touchy. This is not an issue for the ages. 😉

          3. All you did was just restate my previous point.
            I seem touchy? The one who’s trying to keep this light? Read my stuff again – I’ll be performing here on this scrap of beach where doors have a ‘usual’ behavior…
            If anyone’s touchy, it’s the ‘mansplainer’ who can’t seem to imagine how someone else might see things.

  6. Looks a lot like a lobster in the face. I definitely would not mess with one of these should I come near one!!!! Huge claws = pain!!!

    1. In (approximately) 90 people X 130 days on location =~ 12000 man-days, we had no reports of crab injuries. In the same time we (sucessfully) revived one local drowned fisherman, dealt with numerous minor injuries and lots and lots of “wife beaten up by husband” types of injuries.
      Not very nice people some of these islanders (though generally I liked Tanzania and Tanzanians).
      Crab injuries are not a major concern.

  7. As far as I know, the largest living arthropod is the Japanese Spider Crab.

    I’d be *really* impressed if something bigger than those was marauding about.

  8. And of course, there is the excellent paper by Marcus Stensmyr who did electrophysiology on these crabs *in the field* and showed an interesting sensory adaptation to its terrestrial life-style. Here’s the abstract:

    Insect-Like Olfactory Adaptations in the Terrestrial Giant Robber Crab

    The robber crab (Birgus latro), also known as the coconut crab, is the world’s largest land-living arthropod, with a weight reaching 4 kg and a length of over half a meter [1]. Apart from the marine larval stage 2 and 3, this crab is fully terrestrial, and will actually drown if submerged in water [4]. A transition from sea to land raises dramatically new demands on the sensory equipment of an animal. In olfaction, the stimulus changes from hydrophilic molecules in aqueous solution to mainly hydrophobic in the gaseous phase [5]. The olfactory system of land crabs thus represents an excellent opportunity for investigating the effects of the transition from sea to land. Have land crabs come to the same solutions as other terrestrial animals, or is their olfactory sense characterized by unique innovations? Here, we show that the robber crab has evolved an olfactory sense with a high degree of resemblance to the insect system. The similarities extend to physiological, behavioral, and morphological characters. The insect nose of the robber crab is a striking example of convergent evolution and nicely illustrates how similar selection pressures result in similar adaptation.

    1. Thanks for posting this, answering some questions I had on reading the OP.

      I wonder if drowning in (presumably) salt water is due to perhaps having lost the ability to clean algae & other stuff off their gills vs. outright incompatibility with water?

      Regardless, how they compare to blue crabs (Callinectes sapidus) in an environment of drawn butter is high on my list of unanswered questions.

      1. Oh, and related to the last part, if you search lobster dehydrogenase, you’ll find that a lot of work in the earlier days of enzymology / protein chemistry was done on lactate and glyceraldehyde-3-phosphate dehydrogenase from lobster. There was a reason for that.

        1. You mean like the trouble some graduate students got into during the early 1970s in a summer course at the Marine Biological Laboratory? The used the autoclaves for cookin g corn and lobster, thus contaminating the tissue culture plates that were sterilized in them subsequently.

      2. I would guess that the oxygen content in any water would not be enough for them if they evolved to live in air as adults.

        1. Absent factors like algal blooms in ponds, O2 content in water should be the same as ambient air. I was assuming that they have gills like aquatic crabs, since noted in the post they need moisture for respiration.

          But their Wikipedia page explains a lot. Excerpted from the Respiration section:

          They use a special organ called a branchiostegal lung to breathe. This organ can be interpreted as a developmental stage between gills and lungs, and is one of the most significant adaptations of the coconut crab to its habitat.[14] The branchiostegal lung contains a tissue similar to that found in gills, but suited to the absorption of oxygen from air, rather than water. …. Coconut crabs use their hindmost, smallest pair of legs to clean these breathing organs and to moisten them with water. The organs require water to properly function, ….

          1. Air is 20% O2. Water is almost 100% water.
            Terrestrial crustaceans such as wood lice or coconut crabs evolved in water so they still breathe with something like gills. The gills still must be kept moist because the gills still absorb their oxygen from the water as they always did. The water is oxygenated from the oxygen rich air so their gills don’t have to be as efficient as those in their marine cousins.
            They don’t so much drown in water as suffocate.

          2. But the O2 tension ought to be the same at the interface, shouldn’t it? Either way, they’re absorbing O2 dissolved in H2O. But if there’s something additional in terrestrial vs. aquatic crabs, it would be interesting to learn the difference in O2 binding of their respective hemocyanins.

  9. I can certainly attest to the fact that coconut crabs have an excellent sense of smell. On Aldabra, where they are a dominant component of the terrestrial ecosystem, they arrive on the floor of our huts’ porch, 3-4 feet above ground, within 5-10 minutes of us sitting down to eat dinner. Putting down a finished plate with a bit of sauce left and watching the crabs scuttle around, pushing each other on & off the plate, is our version of TV/evening entertainment.

      1. Whoops, I just saw I got three from you, which I’ll add to my post. But PLEASE take a photo of those crabs cleaning the dinner plates!

    1. My first eco-protest was over Aldabra in the 1960s – I wrote to my MP when aged about 7 particularly about the tortoises, & in fact the island was saved from development because of the outcry.

        1. Good question – I donate to the usual suspects like WWF etc, sign petitions online, but is that really enough? Is that just assuaging my middle class middle age white western male guilt? 🙁

      1. Yes, that was quite a drama back then, from what I have read! Thanks for the letter back then. A good recap of it all is given in Tony Beamish’s “Aldabra alone”, if you haven’t read it yet. His documentary & reporting from Aldabra certainly came at the right time!

    1. Jesus H Christ, that’s incredible! Those crabs in the first few shots reminded me of the orcs (?) in the battle scenes in Lord of the Rings, just hordes of ugly armoured creatures marching across the seabed. And then the stingray nukes them ; )

    2. Goddamit I’m on holiday and have crappy internet, so I can’t watch videos. I will have to save it for when I get home.

  10. Pretty sure I saw several of them killed and eaten on the survival scenario show, “Man, Woman, Wild”.

    A crab that eats coconuts, with claws the size of my hand, strikes me as temptingly good eating–endandgered or not.

    Are they really not a part of traditional island fare?

  11. Pacific land crabs are also interesting from a conservation point of view. Atkinson pointed out years ago that the effect of introduced rats on an island avifauna could be predicted by whether rats or crabs occurred naturally. On islands with crabs, birds were much less affected by introduced rats, presumably because they had already adapted to the predatory crabs (or been driven extinct by them and were not present).

    Atkinson, I.A.E., 1985. The spread of commensal species of Rattus to Oceanic Island and their effects on island avifaunas. International Council for Bird Preservation Technical Publication 3:35–81.

      1. Sorry! I couldn’t find my copy, and so relied on Jared Diamond’s description of the work for my capsule summary. The monograph it’s in was entitled “Conservation of Island Birds: Case Studies for the Management of Threatened Island Species” and was edited by P.J. Moors, which info might help you find a copy. I did search for an online copy, and couldn’t find one.

  12. I encountered them on the island of Bellona (Mungiki) in the southern Solomons in 1975. Yes, delicious. The Polynesian Bellonese appreciated them and did not squander them.

    1. These were hermetic; I didn’t know they later gave up shells.

      So far as not drowning goes, might they not get from one island to another by clinging to floating coconuts, thus sharing distribution with the coconuts?

          1. Didn’t realize that your not knowing they gave up their shells only applied to one island…

          2. Well maybe they kept their shells only on Mungiki because with no roads, and hence no Beetles, people didn’t put two and two together….

          3. You hit on something there – the only ways humans are safe around these critters are 1) not realize what these crabs are up to – or 2) pretend not to know and back away very slowly.

  13. What a wonderfully enjoyable post! Dennis Hansen’s shot of the crab climbing onto the table was a real laugh-out-loud moment!

    1. Diane, you should have been there when I first arrived on Aldabra, and Rich, our resident team member showed me how to pick up a coconut crab… …which then promptly grabbed on to his tshirt, causing Rich to do all sorts of acrobatics to keep fingers, face and nipples out of the crab’s way! 😀

  14. My guess for biggest arthropod would have been the (Tasmanian) giant lobster. However, the all-knowing Wikipedia claims it is the Japanese spider crab.

  15. I remember seeing coconut crabs, or something very similar to them, when I lived on Guam. (Hey, I was 2-3 years old, my biology was a bit…elementary…then.)

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