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Gastropod shell of Viviparus contectus with operculum

The operculum (plural : opercula or operculums) of gastropods is a corneous or calcareous structure which exists in some families of snails, and which, in its most typical condition, fits perfectly into the opening or aperture of the shell. In life the operculum is attached dorsally to the posterior part of the foot.

The operculum grows as the shell grows, so that it stays in proportion. However, in many species of marine shelled snails which live subtidally, the operculum is reduced in size, and no longer serves to completely seal the shell entrance.

In those species where the operculum completely seals the shell, it can serve as a protection against predators when the snail body is retracted. Perhaps its most essential function however is to alllow snails to resist drying out. This is important in intertidal marine snails during low tide, and enables operculate pond and land snails to survive periods of drought, and periods of dry weather.

The operculum has a concentric structure, and a nucleus which may be central, or which instead may be situated near the parietal margin of the shell. There are two basic types of opercula:

• The most common kind of operculum is composed of a thin to rather thick corneous protein material, which is yellow to brownish in color and is usually somewhat translucent. This matter is supple. The operculum varies in shape, depending on the family of snails and the shape of the aperture of their shells.
• The other type is restricted to certain families of gastropods. This operculum structure has a corneous base with a heavy calcareous overlay, which in some genera has color or ornamentation of various kinds including for example, pustules and incised grooves.

Opercula may be described as multispiral (having many closely spaced spirals), paucispiral (with few spirals), and the different shapes of opercula can include ungulate (hooflike), claw-like, or ovate. The type and shape of the operculum is used to help identify and classify related groups (genera) of land operculates, and likewise some marine operculates.

A few species of shelled land Pulmonates, such as the edible land snail, Helix pomatia sometimes create an epiphragm, which is not an operculum, but in some ways serves in place of one. The epiphragm can be a calcareous septum, or a dried film of mucus, and it is used to seal off the aperture of the shell during estivation.

epiphragm of Helix pomatia

epiphragm of Helix pomatia

Operculum of certain gastropods, especially varieties from the Red Sea, has long served as an incense material in ancient Jewish tradition, as well as in Christian and Arabian Muslim faiths. The operculum of conch species Strombus tricornis and Lambis truncata sebae are most commonly used in regions near the middle east. Many believe that operculum from these conches is the "Onycha" incense material described in the Book of Exodus.

Operculum powder is also an important ingredient to Chinese and Japanese incense makers, which are called "beì xiāng" (Chinese: 貝香; lit. sea shell fragrance) or "kai kou" (Japanese: 甲香, lit. shell/armour fragrance) respectively. Incense producers in these countries use the operculum of many conches and other maine snails including those found Southeast Asia, South America, and East Africa. Operculum is traditionally treated with vinegar, alcohol and water to remove any fishy smells. The clean opercula are then ground to a powder and used as a scent fixative, in a similar technique to that used in perfumes with certain plant resins.

When burnt on its own, high quality operculum reportedly smells of castoreum or other animal musks, while that of lower quality is reminiscent of burnt hair.

• Operculum

• Burnie, D. (2001). Animal: The Definitive Visual Guide to the World's Wildlife. DK Adult Publishing. pg. 24.

• Pictures of diverse gastropod opercula. This site also provides fairly extensive material on the growth, development, and physiology of the gastropod operculum as well as hundreds of pictures of both the operculum and its host shell.