Chris Moffit, of Feilding Intermediate School, asks :-

How do shells get their colour?

Ian Stringer, a zoologist in the Ecology Department at Massey University, responded.

A mollusc shell protects the animal within. The shell is laid down mostly at its edge by a fleshy fold of the body called the mantle. The rest of the mantle beneath the shell slowly adds more shell underneath to increase the shell's thickness.

Pigments may be deposited continuously or intermittently and in different places resulting in the patterns. Usually such pigments are associated with an outer protective layer, the periostracum. This consists of tanned protein (a bit like your fingernails). It is particularly thick in land snails and mussels. The shell beneath consists of crystals of calcium carbonate stuck together with very small amounts of protein. The latter may also have pigments associated with it. Calcium carbonate crystals laid down by the mantle edge are orientated at right angles to the shell surface - this is termed the prismatic layer of the shell. Additional calcium carbonate crystals laid down by the rest of the mantle are thin plate-like crystals. These react with light to give the pearly or sometimes even iridescent appearance to the inside of some shells. This inner layer is known as the nacreous layer. Pearls are formed when foreign particles get under the shell and irritate the mantle so it secretes nacreous shell around them.

Shells can contain a wide variety of pigments. These may be simply excretory deposits (such as porphyrins) or they may play a useful role. Such pigments include glaucobilin (green), aplysiopurpurin and dibromoindigotin (purples), and melanins (blacks and browns). The latter are especially common in land snails. In many cases the colours and patterns of shells are probably protective by disrupting their outline or by camouflaging them. In one case, the different dark bandings of a species of English garden snail were shown to be associated with different habitats. The different patterns helped hide the snails from thrushes. On the other hand many bivalves burrow into sand, mud or even soft rock for additional protection and these usually lack colour patterns.