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Thursday, April 03, 2008

A new suit of armor for Pinchy

The last time Pinchy underwent this process was on July 26, 2007. The molt on the left is from May 23, 2007 — the one and only molt I kept (gave her some sterilized eggshells as a trade). The molt on the right is from today. A growth from 8 cm to 21 cm: way to go, Yabby! When kept in clear, clean waters, these critters tend to turn blue, with gorgeous purple markings.

Yabbies grow by a process known as “moulting”. Moulting is the process of shedding the old shell (exoskeleton) and growing a new one in its place. The new shell is soft and prior to it hardening, the yabby will take up and store water within it’s body tissues thus effectively artificially expanding it’s size and ‘stretching’ the new shell. The water is expelled from the body once the new shell has hardened. The overall effect is that the yabby now has a shell bigger than it’s actual size into which it can then grow. In newly hatched yabbies, moulting may take place every couple of days. The frequency will decrease as the yabby gets older until it will only moult once or twice a year.

The hardening of the shell is achieved by drawing calcium from deposits in the body and from the surrounding water. Calcium is stored in the body by reabsorbing it from the old exoskeleton prior to moulting and depositing it in two calcareous deposits in the stomach wall known as gastroliths. Yabbies often eat their discarded exoskeleton after moulting in order to conserve calcium. The calcium is then redeposited into the new exoskeleton in order to harden it.
         (Biology of Yabbies)

Hemispherical in shape, gastroliths (stomach ossicle, stomach stone, crab's eye, or crab's stone) are resistant to weathering.

Occasionally, one finds small, round, stone-like concretions in a fish's stomach. These 'stones' are often seen in aquaria containing crayfish, and can also be found in the nests of some water birds and even in Aboriginal middens. They are gastroliths (literally 'stomach stones', sometimes called 'crabs' eyes') and are produced as a pair in the lining of the stomach of a crayfish preparing to moult. Being the hardest parts of the crayfish, they are either refused or are the last to be digested by predator. After the crayfish moults the gastroliths fall into its stomach, where the calcium of which they are composed is resorbed into the blood. In earlier times, gastroliths were used in medicine for their absorbent and antacid properties.

An entertaining article on calcium transport before and after moulting.

With a hard shell, how do yabbies grow?

The general process of growth is called moulting and involves a cycle of moult stages (A-D), which has to be repeated many times through their life. Periodically, they make a new, but soft, shell under the old hard one (stage D). Then, laying on its side, the Yabby breaks out of the old shell at the join on top between head and tail- an involved contortion called ecdysis. Amazingly, this involves all parts of the external shell (and some fore and rear gut lining): eyes, gills, legs,...). The new exposed soft shell is expanded quickly by drinking water (land insects take in air) (stage A), and then hardens so the Yabby can get mobile (stage B). During the subsequent intermoult (stage C) the Yabby feeds and replaces the water with soft body growth. In early stage C the Yabby is hungriest and most readily caught in a baited trap.

Yabbies often lose legs by fighting; they can completely replace a leg, gradually through 3 or 4 moult cycles.

Most people call the actual shedding of the shell "moulting", but this stage of the whole moult cycle is more correctly called "ecdysis".

Do they die often during moulting?

Yabbies are very vulnerable during moulting; most crayfish deaths occur while they're sluggish just before or after ecdysis or defenseless at ecdysis - they get stuck trying to emerge from the old shell or they're attacked by other crayfish or a predator. A Yabby on its side in shallow water is usually moulting; sometimes what you see is the old empty shell; if not, if you wait a few minutes the "new" Yabby will emerge. Don't disturb or handle an ecdysing yabbie; they can't breathe at this stage and any delay means they run out of puff before finishing. Small crayfish ecdyse in a few minutes but very big ones can take 20 minutes, or longer.
         (Yabby FAQ)

As before, I only borrowed Pinchy's old molt for the pictures. It's back in her tank now (she needs to consume it to reclaim the calcium).

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