Coconut Crab

Image BlueBec via Flickr

A little while ago we took a look at arthropods that have made their home underwater. I said then that crustaceans are so utterly aquatic, they require all sorts of adaptations to make a living on land. One such crustacean is today's monstrosity, the Coconut Crab. It's also known as the Robber Crab, but I have always known it as the former and I would like to get to know them a bit before I start slinging those kind of accusations around. It's only fair.

The first and most obvious thing about coconut crabs is that they are really big. They also come in a wide variety of beautiful colours with a stripy, dappled pattern, but probably the sheer size is more immediately striking. They can reach 40 centimetres in length with a leg span approaching a metre and weigh about 9 pounds. This makes them the biggest arthropod on land. Wow! That's not bad for a crustacean. It's weird that they just walked up a beach and beat insects and arachnids on their own turf. I guess beetles just didn't see it coming.

They can be found on all sorts of islands throughout the Indian Ocean and into the central Pacific. A lot of them do actually stay near beaches and shores, but they have also been found almost 4 miles inland, so they can get around pretty well.

Perhaps they're following their nose? I mean antennae. Coconut crabs have had to adapt to smelling in air rather than water and the solution turns out to be remarkably similar to that of insects. They've taken to it with aplomb, and can smell over large distances. They then have to get there, of course.

Coconut crabs have 8 legs and a pair of powerful pincers that can lift objects 64 pounds in weight. That's 7 times its own weight! Clearly, that is more than enough for climbing trees, which is a habit coconut crabs are famous for. The suggestion that they actually cut coconuts down seems to be entirely based on hearsay, though. It seems to be more about getting to fruit or getting away from overly exuberant heat. The claws are used for cracking open coconuts and maybe, just maybe, giving a painful nip to a foolish human. You have been warned! Not to say that you, dear reader, are foolish. It's just with all the litigation going on these days, one can't be too careful.

The first two pairs of legs are long, strong and also help with breaking coconuts. Incidentally, they also eat fruit, other nuts and seeds. Actually, they're not vegetarians and will also scavenge on carrion and eat little baby turtles. One was even seen catching and eating a rat! Seriously, don't mess with those claws.

Image: fearlessRich via Flickr

The third pair of legs are smaller and have little tweezer type things at the end. You see, coconut crabs are a kind of hermit crab. The female can carry fertilised eggs under her abdomen for months, releasing them into water during a high tide. The larvae float around as teeny tiny plankton for about a month, before sinking to the sea floor. At this point they become the hermit crabs we know, with soft abdomens they protect with snail shells or sometimes, coconut shells. The little tweezer legs are used to grip onto the shell. After a while, the youngsters climb onto land, loose the ability to breathe underwater and their abdomen hardens. They are now the coconut crab we all know and are a little bit scared of.

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The fourth and final pair of legs are absolutely puny and are kept protected beneath the carapace, the large piece of exoskeleton on the back. In this same place is the branchiostegal lung, which is kinda in between gills and lungs. It is this that allows coconut crabs to breathe air on land. The tiny little legs are used to keep it clean and moist.

Indeed, they may be terrestrial, but they still like a nice, cool, moist atmosphere. They dig burrows for themselves where they can hang out during the heat of the day and just in case you didn't quite get the message, they line it with bits of coconut husk. The coconut crab is a crab that really likes coconuts.

So, a brief recap, exploring the most salient points we have learnt today:

1. Coconut crabs are BIG and they are STRONG
2. Coconut crabs have pincers that are BIG and are STRONG
3. Coconut crabs eat coconuts
4. Coconut crabs may live in coconut husks when young
5. Coconut crabs live in coconut lined burrows when adult
6. Coconut crabs really like coconuts
7. Coconut crabs also have a taste for flesh, living or dead
8. Do NOT stand between a coconut crab and his or her coconut