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Coconut Crabs, or 'Robber Crabs' as they are called in the islands of the Indian Ocean, are the undisputed giants among land crustaceans.
Photo: Genevieve Johnson

July 17, 2002
The Diversity of Crabs
  Real Audio

Log Transcript

This is Genevieve Johnson speaking to you from the Odyssey in the Indian Ocean.

The Odyssey crew enjoys spending brief periods of time ashore in between research legs. When visiting the islands of the Chagos Archipelago, we always encounter substantial numbers and varieties of crabs - land crabs, shore crabs and hermit crabs. Several species are represented in the tropical areas of the world, some are small, reclusive and well camouflaged, others large, spectacular and brilliantly colored, while many exist as variations of these extremes.

All crabs are crustaceans and belong to the Phylum Arthropoda - meaning jointed legs. The Arthropods comprise the largest phylum of living creatures, roughly one million known species (plus many more as yet undescribed). Arthropods include all the various kinds of insects. The subphylum Crustacea - (the Latin word for shell) contains perhaps 45,000 species (some authorities believe that crustaceans should be placed in their own Phylum). Interestingly, crustacea are distinguished from other arthropods by a seemingly unimportant feature: they possess two pairs of antennae rather than one.

Hermit Crabs have a soft abdomen that is adapted to fit abandoned snail shells. We often witness battles as these crabs squabble over a shell that they both prize.
Photo: Chris Johnson

Decapods are perhaps the most familiar and largest order of crustaceans. They include crabs, prawns, lobsters and crayfish. They all have the characteristic hardened outer shells or exoskeletons, segmented bodies and jointed limbs-in many ways their outer shells are similar to the suits of armour worn by medieval knights.

In the true crabs, the abdomen is greatly reduced and is tucked snugly into a perfectly fitting depression beneath the cephalothorax - the single structure that combines head and thorax. Crabs, such as the hermit and robber crabs (also called coconut crabs), have a much longer abdomen. In hermit crabs, the soft abdomen is asymmetrical and adapted to fit into the helical chambers of abandoned snail shells. We often witness comical battles between these little crabs as they squabble over some shell that both prize. The coconut crab-the undisputed giant among land crustaceans has a hardened abdomen, which does not need the protection of a borrowed shell even though it is a species of hermit crab.

A crab's mouth is flanked by six pairs of feeding appendages that the crab uses to crush food and feed itself. Most crabs are scavengers but will also eat live prey such as turtle hatchlings-even other crabs. We often observe crabs in huge numbers, feeding on debris that has been left along the high tide mark, and which can include just about anything. Hermit crabs are especially inclined to feed in large groups. Species like the robber crab are more specialized in their eating habits. They wait for their preferred meal, coconuts, to fall from the tree, rather than settling for one that has washed in from the sea.

The legs of all crabs are segmented, like a series of long, stiff, hinged cylinders. Each hinge is capable of moving the portion of the leg beyond it in one plane only. But with each hinge having the ability to move in a different plane, the result is a limb capable of movement in almost any direction.

Hermit Crabs will feed on just about anything (like this coconut) left along the high tide mark.
Photo: Chris Johnson

Hermit crabs and robber crabs walk forward for the most part, but the true crabs are experts at walking or running sideways, often at lightening speed. Ghost crabs are among the most fleet of foot, capable of speeds of 1.8 meters per second. The leading legs pull and the trailing ones push. Land crabs such as the Christmas Island crab move in the same way, but at less than a tenth of that speed, only managing to amble along at a mere 1meter every 10 seconds.

It seems that crabs require so many legs to carry them, what happens if they lose one, or even a few? Curiously crabs have the ability to amputate a limb that has been injured or is being held by a predator. Yesterday, we watched in astonishment as two small moray eels launched themselves up onto the sand to snatch an unwary ghost crab. But they only came away with a couple of the crabs' legs before they squirmed back into the water. We watched as the crab, its gait now lopsided, raced for cover at a furious pace. Crabs usually get caught by their legs, but because the legs come off the crab they can often make an escape. At the base of each leg is a breakage plane where muscular action can achieve a self-amputation. Amazingly, a crab that has lost a limb in this way can subsequently regenerate it .

Apart from locomotion, limbs are important to different species for different purposes, some use them to dig burrows, others to climb trees. Ghost and hermit crabs use their legs to make characteristic sounds by rubbing a claw against their exoskeletons. We often watch coconut crabs -(they're always a thrill to see). When they are unexpectedly confronted by us, they tap their large claws or walking legs on the ground. We get the impression that they may be threatening us too-telling us to stay away. Other crabs are able to detect these vibrations through the ground via receptors in their legs, so this behavior probably evolved to work on other crabs, only when people came along, it turned out to work on them as well, even though we humans only hear the small part of the sound that comes through the air.

The Christmas Island Red Crab is found only on Christmas Island and the Cocos (Keeling) Islands in the Indian Ocean.
Photo: Genevieve Johnson

As we get to see first hand every day we are here, crabs survive in some rather difficult, but often spectacular ecosystems. They are extraordinarily adaptable creatures that have taken up residence both on land and in the sea. It's always a treat to observe them in their natural environment.

Stay tuned for the next Odyssey log when we talk about how crabs moult and grow a new suit of armor.


Log by Genevieve Johnson

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