Some marine animals - like hermit crabs - use sea shells for protection of their soft rear body. They have an intricate array of behavior to determine which shell fits them, and is light and spacious enough for them. But the hermit crab doesn’t build its own shell, so who makes them? I have been asked this question more often, so here is the answer. Molluscs do, like some species of cephalopods (Nautilus species) and slugs for instance. Slugs are born without their shell and they actually build it as they grow. Below I will explain how they do it.
This process takes place in the mantle of the slug. The part of the slug that secretes the shell is, formed very early in the embryonic phase. Part of the slugs ectoderm (outer skin layer) thickens, and then invaginates to form the shell gland. The shape of this gland is tied to the form of the adult shell. In gastropods, it is a simple pit, whereas in bivalves, it forms a groove which will eventually becomes the hinge line between the two shells, where they are connected by a ligament. The gland then invaginates in molluscs that produce an external shell. Whilst invaginated, a periostracum - which will form a scaffold for the developing shell - is formed around the opening of the invagination, allowing the deposition of the shell when the gland is everted.
Sea shells in many forms
The inside of an abalone with the nacre layer
Shells consist of calcium carbonate and other trace elements from the environment. The shell is formed by a process called biomineralization, where the minerals are absorbed in soft protein tissue, including silk gel, acidic proteins and chitin, produced by the slug. The protein matrix is secreted by the slug and binds calcium ions (from its blood). The binding of calcium ions to the protein matrix causes crystallization in very specific arrangements. First, an uncalcified (outer) layer of conchiolin protein and chitin, a strengthening, naturally produced polymer is formed. Then comes the highly calcified prismatic layer (middle) that is followed by the final pearly layer, or nacre (inside). The outer layer is built first, then the middle and inner layer.
Proteins from the slug seem to determine whether the crystal formed is, calcite - as in the prismatic layer of the shell, or aragonite - as in the nacre of the shell.
The shell is grown at the mantle of the slug and the outer lip of the shell, as shown in the following illustration (Look for the fine blue lines in the shell). The first image shows the shell size when the slug was much smaller, the second where it grew, the last its current size. The slug keeps its shell, and only adds material over time.
Shell formation at an early stage
Shell formation after growth
With every new addition of material, the shell grows longer and wider. This way the shell can grow along with its host and grow stronger with time. The size of the shell is directly proportional to the size of the slug. Because the sea shell of slugs not only grows wider, but also in whirling spirals, it also grows stronger over time, providing adequate protection for its host.
There you go! That is how they are built.
By Peer van Spreeuwel
Spiral shaped shell for increased strength