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LatestPhoto
A Ghost Crab.
Photo: Genevieve Johnson

July 21, 2002
Crabs - Putting on a New Suit
  Real Audio
  28k


Log Transcript

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

The Odyssey stopped over night on Salomon Island in the Chagos Archipelago, and before lifting anchor the next day some of us took an early morning walk on a deserted white beach. Walking along the sandy shoreline, the apparent absence of life was quite striking-nothing but abandoned shells, coconuts and the occasional flock of wading birds. However, first appearances can be very deceptive, as we soon learned.

Ghost crabs, a characteristic species of tropical shores, derive their name from their sandy white coloring, which seemingly allows them to appear and disappear at will. As it turned out, a virtual army of ghost crabs was hiding by their thousands in their small burrows. But as they began to emerge to scavenge at low tide, they scampered across the sand by the thousands. Some were the size of a fist, others so tiny, their white bodies were barely visible as they dashed by. The rapid back and forth antics of these foraging ghost crabs were rather comical to watch. Their hard pale coats of armour glistened in the early morning sunlight, but even with such protective suits, the slightest hint of movement by any of us in their direction sent them scurrying for cover.

A crabs' shell affords good protection from predators. However, the possession of such a hard, crusty exterior poses a serious problem-it restricts the crabs' ability to grow. However, these adaptable crustaceans have a fascinating method that gets them around this problem.

LatestPhoto
The moulted shell of a crab. Beachcombers often mistake the perfectly intact exoskeleton for a dead animal.
Photo: Genevieve Johnson

As with all arthropods, a crab moults by casting off its hard external skin or shell and replacing it with a new, larger one, thereby growing bigger. In the same way that a growing child is constantly in need of shedding its old clothes and replacing them with bigger clothes, the crabs' solution to the problem of size gain is to periodically shed its old "clothing"-its armor, and to construct a new, and larger suit of armor.

Moulting is regulated by hormones and is essentially a full-time business for a crab. The periods between successive moults are spent concluding the last moult and preparing for the next. Just before a crab is ready to moult, it partially absorbs the calcified layer of the shell-thereby avoiding wastage. Enzymes from the outer skin are released in order to digest the protein and dissolve the calcium carbonate, which make up the shell. The rate of absorption is particularly high at the places where the shell will eventually split to allow the crab to escape it.

Once the old shell begins to separate from the underlying, newly forming shell, the crab is ready for the shedding process to begin. There are great selective pressures to have shedding occupy the shortest possible time, as this is when a crab has no armour with which to defend itself and is at it's most vulnerable to predators such as birds, fish and larger crabs. During this defenceless stage, crabs often seek shelter under a rock ledge or in a burrow. When absorption has rendered the shell sufficiently soft, fractures begin to occur along specific, pre-formed lines. These fractures quickly become big enough to allow the animal to extract itself from its old armor-its old exoskeleton. Of course, the crab must extract its legs from their long thin stocking-like armor tubes-and the antennae from their even thinner armor tubes. The new suit begins to harden at once and although the old one is sometimes left behind it is usually devoured later (again to prevent wastage-the materials in it being exactly what the crab needs to complete its moult).

Interestingly, species of crabs that are targeted and landed by commercial fisherman during this process of moulting are called 'peelers'. They are caught and sold as 'soft shells', a gourmet delicacy.

So how do crabs ensure that their new shell is going to big enough to allow room for future growth? During the moult, in order to expedite the process of shedding the old shell, the crab's body absorbs water and swells up. This enables the enlarged body to create a suit of armor bigger than is needed to accommodate the true, unswollen size of the crab's body. Then, as the new shell begins to harden, the crab slowly releases the excess water it has taken in and returns to its normal size, so that it "rattles around" so to speak in its new shell. Over the next few weeks or months, however, as the crab's body grows it fills the new shell so full that it finally becomes too snug a fit, and the entire process begins again.

LatestPhoto
A Ghost Crab racing across a beach.
Photo: Chris Johnson

Beachcombers often mistake the perfectly intact exoskeleton (the moulted shell of a crab) for a dead animal. So next time you see what you think is a dead crab lying on the beach, take a second look. On closer inspection, you may find that it is the entirely empty, clean, but discarded armor of a nearby crab that has recently traded it in for a brand new suit of armor.

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Log by Genevieve Johnson

 
 
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