The periostracum is a thin organic coating or "skin" which is the outermost layer of the shell of many shelled animals, including molluscs and brachiopods. Among molluscs it is primarily seen in snails and clams, i.e. in gastropods and bivalves, but it is also found in cephalopods such as Allonautilus scrobiculatus . Periostracum is an integral part of the shell, and it forms as the shell forms, along with the other shell layers.
Periostracum is visible as the outer layer of the shell of many molluscan species from terrestrial, freshwater and marine habitats, and may be seen in land snails, river mussels and other kinds of freshwater bivalves, as well as in many kinds of marine shelled molluscs.
The word "periostracum" means "around the shell", meaning that the periostracum is wrapped around what is usually the more calcareous part of the shell.
The formation of a shell requires certain biological machinery.
The periostracum is secreted from a groove in the mantle, termed the periostracal groove.
Periostracum is often yellowish or brownish in color.
In the shells of species which have periostracum, this shell layer is quite often physically worn away or chemically eroded in the parts of the shell that are older, thus it may only still be visible in the more recently formed areas of the shell.
Periostracum can in some cases be quite thin, smooth, glossy and transparent, such that it looks almost like a thin yellow varnish, or it can be thicker and more or less opaque. When it is thick it is often relatively rough in texture and dull. In some species the periostracum is tufted, or forms hair-like growths which in some cases can give the fresh shell a velvety feel, see. In some species the periostracum adheres very tightly to the underlying shell surface, in others the periostracum is less firmly attached.
In certain marine species, such as for example certain species of cone snails, a heavy periostracum obscures the color patterns that exist on the calcareous layer of the shell.
In many aquatic species, once a shell has been removed from the water and has had time to completely dry out, then the periostracum may become brittle and start to flake or peel off of the surface of the shell.
It is not uncommon for shell collectors to deliberately remove a periostracum layer (using household bleach) if they feel that a shell is more attractive without it.
Haired shells occur in gastropods in several species of the Stylommatophoran families Polygyridae, Helicidae and Hygromiidae. These families are only distantly related, suggesting that this features has evolved several times independently. Haired shells are almost exclusively observed in species living in moist microhabitats, like layers of fallen leaves, broad-leaved vegetation, damp meadows or wet scree. Such a correlation suggests an adaptive significance of the trait in such a habitat; it was thus speculated that the hydrophobic hairs facilitate the movement in wet environments by relieving surface tension. [[CITE|-1|http://doi.org/10.1186/1471-2148-5-59]]
These hairs can reach varying densities (up to 20 per squaremilimetre) and lengths (up to three millimetres).
Experiments by Pfenninger et al. (2005) [[CITE|-1|http://doi.org/10.1186/1471-2148-5-59]] on genus Trochulus showed an increased adherence of haired shells to wet surfaces. Haired shells appeared to be the ancestral character state, a feature most probably lost three times independently. The possession of hairs facilitates the adherence of the snails to their herbaceous food plants during foraging when humidity levels are high. The absence of hairs in some Trochulus species could thus be explained as a loss of the potential adaptive function linked to habitat shifts. [[CITE|-1|http://doi.org/10.1186/1471-2148-5-59]]
The periostracum of brachiopods is made of chitin. Relatively new cells on the edges of the brachiopod mantle secrete material that extends the periostracum, but are displaced on the upper side of the mantle by more recent cells, and switch to secreting the mineralized material of the shell valves.