Intertidal Sciences blog | Dr Mike Kent | April 23rd 2019

PISCES BLOG April 23rd

Crab growth and renewal

The smelly and ugly discarded remains of a shore crab was an appropriate find for our Easter Saturday rocky shore survey. It looked like a dead or dying crab, but close examination revealed that there was “no one at home”; it was an empty shell. The remains were not due to death but to a process essential for the continued life of the crab.

Shore crabs are crustaceans whose soft body parts are constrained by a hard calcareous shell. In order to grow, a crab has to get rid of its body armour by a process which scientists call ecdysis, but which normal people refer to as a moult.

Moulting is a complex and precise process hormonally controlled. It may take place twenty or more times in the life of a crab.

Before a crab moults, it reabsorbs calcium into the body and stores it internally. Then the crab dissolves the inner layer of the shell, making it thinner and weaker. At the same time, the crab lays down the soft foundations of a new shell, complete in every detail, under the old shell.

At moulting, the softened old shell cracks along a rupture line on the animal’s back. The crab then extricates itself from the old shell, taking along all of its mouthparts, gills, legs, eyestalks and other body parts. As Judith Weis writes in Walking Sideways: the remarkable world of crabs, “Imagine yourself in a suit of armour that cracks down the back, from which you have to get out without using your hands to pull- quite a daunting process.”

On emergence, a renewed crab has very soft, flexible and thin body parts which are inflated with seawater. This watery expansion allows for tissue growth before the shell hardens. Hardening may take several days, a risky period during which the crab is extremely vulnerable to predators. Therefore recently emerged crabs are usually found hiding in deep crevices or taking refuge under a boulder.

It comes as no surprise that on seeing a squeaky clean “renewed crab” emerge from a barnacle-encrusted old shell some Christians have adopted the crab as a symbol of rebirth or resurrection. Perhaps for those well-versed in the New Testament, it will have brought to mind the words of Paul to the Ephesians:

You were taught to put away your former way of life,

your old self, corrupt and deluded by its lusts,

and be renewed in the spirit of your minds,

and to clothe yourselves with the new self,

created according to the likeness of God

in true righteousness and holiness.

Ephesians 4:22-24