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The Saga of Diego Garcia Continues with:

The PROPEOPDEMREP
Division of Ecological Perplexities Presents:
Nature: Red in Tooth and Claw!
MAN EATING CRABS!

Ken Alsip holds a mature Coconut Crab in 1987.  Based on it's size, this crab is probably between 30 - 40 years old.  That's Francis Salvador with him.
CRABS:  One of the most enduring nightmares of Diego Garcia are the tales of what happens if you were foolish or drunk enough to fall asleep in the jungle - You WOULD be eaten by crabs.  The photos that follow are of Coconut Crabs (also called Robber Crabs, but not by anyone I ever met on Diego),  hermit crabs, and the ubiquitous land crabs.

Hermit Crab
Can you tell the difference between a Hermit Crab and a baby Coconut Crab?
I couldn't!  Above is an adult Orange Hermit Crab (Coenobia perlatus)
For a really good site about identifying Hermit Crabs, see http://www.tonycoenobita.com/species_analysis_eng.htm

If you can tell the difference, or even if you can't, send me your pictures of Hermit Crabs on Diego, o.k.?

COCONUT CRABS
 (Birgus latro)


Let's face it.  Even though those red Land Crabs were everywhere and we saw them all the time, it was the legendary Coconut Crabs that fascinated everybody.  Coconut Crabs evolved in the Indian Ocean region, probably right there in the Chagos (during the last ice age, there were 5,000 square miles of land in the Chagos - compared to 5 today, so they had lots of room).   Based on mitochondrial DNA studies, it appears that they spread into the Pacific Ocean region during the last ice age.  They live in tropical regions, or areas washed by tropical currents (i.e., they live on Okinawa - where they are often captured and sold as pets in Tokyo). 

Almost everywhere they live, they are considered a delicacy by the natives, and reportedly taste very much like lobster.  They were eaten by the workers on Diego Garcia in the plantation days and by the Navy folks back in the 1970s and early 1980s.  In some places throughout their range they are disappearing, but in many places where they live, there are legal protections.  For example, on Guam there are minimum size limits for taking, and females with eggs cannot be killed. 

It is reliably reported that Coconut Crabs are absent from cultivated areas with livestock (cows, pigs, etc.) and as the various tropical islands are developed to feed their growing populations, it is likely that the crabs will disappear from some of the islands where they now exist.  Because they take so long to mature, it is easy for a hungry human population to eat every adolescent and adult crab on an island, resulting in extinction of that population.  This is believed to be what happened in Indonesia, the Seychelles, and various Pacific Islands.  However, it did not happen in the Chagos or on Diego Garcia.  Even so, the number of Coconut Crabs on the islands in the Chagos varies widely from overabundant to scarce.  In fact, it is believed that the east arm of the Diego Garcia atoll now has the most dense population of Birgus latro anywhere in the world, thanks to the the current protection measures in place.  Coconut Crabs will not disappear from the Chagos in the foreseeable future.

The first Coconut Crab I ever saw on Diego Garcia was a stuffed one on the Brit Rep's mantle.  It looked like a giant hermit crab with a lobster's tail.  It is my recollection that coconut crabs were completely absent from the airfield to downtown when I was first assigned to DG back in 1982.  By 1987 they were completely protected, though I never saw a live one in the main base area.  There were plenty of juveniles - juvenile Coconut Crabs look and act pretty much like 'hermit crabs' - and you could see those all over the beaches on both the ocean side and lagoon side.  Today (2008) adults are seen around the main base quite regularly, so it appears the conservation measures are working.

They are amongst the largest terrestrial arthropods, with reliable reports giving an adult weight of 10 pounds, and a leg span of about 3 feet.  Some unconfirmed reports have these crabs weighing over 35 pounds and having a leg span of about 10 feet.  I don't remember anyone making those kinds of claims on DG, although 10 pounders are now seen frequently as you can tell from the photos below.

Female Coconut Crab with Eggs
Female Coconut Crab showing developing eggs glued to abdomen.  Photo by Bob Ralph

The female Coconut Crab carries her eggs glued to her abdomen for a couple months, then lays her eggs at high tide, at night, in the ocean, and the eggs hatch immediately.  Some say they lay the eggs individually when they are ready, and other reports say ALL the female crabs on a given island will lay their eggs on the same night, but only once a year.

The larvae stay in the water, swimming near the surface, for about a month, and then sink to the bottom and spend another month or so in the water and on the beach, and then move up on the beach permanently.  During all this, they act just like their close cousins, the Hermit Crabs.  In fact, many of the 'hermit crabs' on the beaches of Diego are in fact juvenile Coconut Crabs.

After about 6 months, they grow large enough to graduate from 'juvenile' to 'adolescent', and leave the beaches and head inland, sometimes using empty coconuts as 'shells'.  However, this is not why they are called Coconut Crabs - instead it's because they are the only creatures in creation that open and eat Coconuts!  As they grow, they'll use whatever they can find to continue to protect their soft abdomen, and if there's nothing big enough, they'll dig holes in the ground and plug the hole with a dirt clod or rock. 

Eventually, adolescent Coconut Crabs become too big to use any readily available thing for a 'shell' and their outer shell hardens, and they wander around with their tails curled underneath (to protect to bottom side, which remains relatively soft).  When they aren't out looking for food or companionship, they hide in cracks in the rocks, burrows in the ground, or holes in trees.

Reports vary about how long they live; they have not yet been raised successfully in captivity, nor has a long-term tagging program been done, so there's no 'Sea World' type record to compare with crabs in the wild.  Some say they live about 30 years, some say 60.  They have to moult their shells as they grow, and there is data about this that shows they gain about 3% in weight with each moult, but that the moults only happen every 6 months or so.   The most repeated age in the literature for 'adulthood' (meaning they're sexually mature) is about 5 years.

Although they have rudimentary gills, these aren't very functional and adult Coconut Crabs will drown in water. They have a special breathing organ, sort of an intermediate step between gills and lungs, which must be kept moist.  So they are observed drinking often.  They lift the water to their mouths just like they do food, but also use their 10th, almost tiny, set of legs to clean these breathing organs and keep them moist.  They can drink salt or fresh water.

They locate their food by smell and can sniff out rotting meat or fruit from long distances.  They'll eat just about anything that's organic, and have been know to catch and eat newly hatched sea turtles, and inquisitive rats.  They will guard a food source as adults, will steal food from other Coconut Crabs, and are widely known to carry food back to their holes.  They get the name Robber Crab from these practices.

Adult Coconut Crabs typically come out of their burrows and holes at night, when their only known predator (humans) aren't out and about.  They sense human presence by vibrations through the ground, and if there is not too much disturbance they do come out during the day, especially if it's wet (like during the monsoon) so they can keep their breathing organs moist.

Since they eat anything that's dead or too slow to escape, there is a downside to having too many of these crabs around - they will seek out and eat the eggs and chicks of birds, especially those that nest on the ground.  So on Diego, where there are efforts to restore the sea-bird colonies, it might be wise to establish some 'population control plan' for Coconut Crabs in those areas where the birds are nesting.  Mmmmmm... I can almost taste the 'lobster' now...

Juvenile Coconut Crabs - Diego Garcia, 2002
Above:  Juvenile Coconut Crabs on the sea-ward beach, 2002.

FINDING COCONUT CRABS:

If you want to see adult Coconut Crabs on Diego Garcia, it's pretty easy.  Find a secluded spot with the potential for holes and burrows (somewhere in the jungle), and just sit quietly for about 15 minutes at night.  If there are any Coconut Crabs around, they'll start to move about after they're convinced you aren't there anymore.  Shine your flashlight at the sound, and you'll be rewarded.  By the way, that same technique works for the Land Crabs, although there are enough of them squished on DG1 that you don't really need to seek them out.
 

Typical Coconut Crab Habitat and Hiding
                      Place
Above:  Typical Coconut Crab habitat and a hiding place - in the rotted trunk of an old tree.  Photo by Bob Ralph.

Below - the crab that was in that hole:

Below:  The easiest way to find Coconut Crabs - they (foolishly) wander the highways!


 

One last note of caution:  Never get drunk and pass out in the jungle...

Unless You Want to Meet the
Adult Coconut Crabs of Diego Garcia:

Diego Garcia Mature
                      Coconut Crab





HERMIT CRABS!

Below:  Another adult Orange Hermit Crab (Coenobita perlatus), showing the fine hairs that they use to sense vibrations in the ground, and also act as hair triggers for closing their pincers when they are touched.

Below left is an Indonesian Hermit Crab (Coenobita brevimanus).
The one in the old gear is probably a Calico Hermit Crab (Coenobita variablis), which are normally found on the beaches of Australia.


Below:  Here're some photos by James Cox, taken Christmas Day 2007.
The top one is an Indonesian Hermit Crab.  Not sure about the other two, but they look like Calico Hermit Crabs.




 



LAND CRABS
 a.k.a. Warrior Crabs, a.k.a. Giant Land Crabs

(Cardisoma carnifex)
They are everywhere, so nobody bothers to take pictures of them!
If you've got any photos of these land crabs, PLEASE send them to me.

Land Crabs, Diego
                      Garcia

 Below:  Here's one that did not get crushed on DG1.  Well, at least the day this picture was taken.


 
Below:  Here're a couple photos from Elisha Carling



Below:  Here's one from Steve Swayne, taken in 1982:


MISCELLANEOUS CRABS

This is a picture of is what we used to call Ghost Crabs. 
They live on the ocean-side beaches.

Photo by James Cox, Christmas Day 2007:

Ghost Crab,
                      Diego Garcia 2007
Here's another kind sort of beach crab - Photo by Elisha Carling:

And another kind of crab - Photo by Elisha Carling


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