Question

How is a mollusc shell formed? Discuss the creation, formation, and lifetime of the shell. What environmental factors can affect shell growth, shape, longevity? What can molluscs do to combat these environmental pressures?

Answer 1

Mantle is a soft spongy layer of skin that forms a thin and delicate covering over the visceral hump. It is a characterstic feature of mollusc and secreates a calcareous shell over the body which protects the mollusc and acts like an exoskeleton. It is absent in Octopus.

The edge of mantle has two components, the organic part is made up majorly of glycoproteins and polysaccharides which may vary in composition. This organic frame controls calcium carbonate crystal formation as in when and where these crystals will start and stop growing and the rate of its growth. The shell is deposited in the extrapallial space which has an outer layer periostacum at the shell’s rim and caps extrapallial space. It acts as a frame for the outer carbonate layer and it also seals the compartment for crystal accumulation. The calcium ions required for shell formation comes from the surroundings via gills, epithelium and gut and pumped in via ion pumps and then transported to the required location via haemolymph. Scaffold in the organic matrix directs the crystallization. The matrix impedes the deposition of calcium and plays a role of nucleating point for crystals controlling their shape. Shell formation is controlled by various hormones and has a genetic basis.

The formation of shell of mollusc constrains by the ecology of the organism. In certain molluscs who changes ecology as they shift from larva to adult phase the shell morphology also changes during metamorphosis. In mollusk with indeterminate type of growth the growth of shell is steady over lifetime of the organism by calcium carbonate addition and so the shell in these organisms become long and wide in a spiral shape which keeps on increasing to accommodate the growing organism.

If there are any injuries or any unfavorable conditions arise then that is reflected in the form, shape and color of shell. In times of harsh environmental conditions then there is dormancy and the production of shell ceases until there are favorable conditions again.