How are shells formed?

How are shells formed?

The secret of shell formation lies in specialized mollusc cells, which are able to build solid shells using proteins and minerals

(photo: M-Rishal on Unsplash) Iridescent, opaque, transparent, shiny. In addition to being one of the wonders of the beaches, shells are a small miracle of biology and chemistry: they constitute the exoskeleton of molluscs such as sea snails, clams and oysters, necessary to protect the internal organs of their body. Francis Horne, a biologist expert in shell formation at Texas State University, explained to Scientific American that shells are made up of three distinct layers, and composed primarily of calcium carbonate and, to a small extent, proteins. Unlike most animal structures, shells are not made up of living cells: the body of molluscs is covered by a layer of external tissue, similar to our skin, called the mantle, responsible for the production of the shell. br>
In particular, the mantle contains specialized cells that "stretch" the shell, as if it were a layer of asphalt or concrete, using proteins and minerals. These substances are initially released into the space immediately above the mantle, and later specialized proteins create a sort of "scaffolding" that forms the support where the actual shell will grow, and determine which minerals will be used in the different parts of the shell.

Calcium carbonate (present, among other things, also in the egg shell) binds to proteins, with a process very similar to that which occurs when the cement is poured onto an iron scaffold , and forms two different types of crystals. The first is calcite, a very common crystal, found, for example, in gypsum, marble and coral; the other is aragonite, in which the calcium carbonate is arranged in a different way.

Each layer of the shells is composed of similar substances: it is the different combinations of arrangement of these substances that give each shell its unmistakable and unique look both to the eye and to the touch. The outermost layer, usually very rough and sometimes spiny, is mostly made up of proteins. The intermediate one is the most robust, and it is to him that the shells are quite difficult to break. The innermost layer is the one directly in contact with the mantle: it is the so-called mother of pearl and it is completely different from the others, since its protein composition is different.

As the animal's body grows, obviously the shell must also grow in size. And this happens starting from the outermost layer, by continuous growth, along different layers of rings, just as happens with the trunks of trees. So much so that, similarly to what is done with trees, counting the rings of the shell allows us to measure the age of the mollusk. Finally, when the animal dies, the shell is gradually eroded by rocks and sand, until it becomes sand itself. And the cycle continues.

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