Operculum (gastropod) - Anatomy

Anatomy

In life, the operculum is attached at the ending of the columellar muscle with an opercular disc dorsally to the upper surface of the posterior part of the foot. However in Buccinum a layer of long cylindrical epithelial cells, with mostly distinct nuclei, and long divided processes enter between the muscular fibres. The operculum, a cuticular development of these cells, is composed, as may be seen in the corneous opercula of Murex, Purpura, Triton, of very thin superimposed layers. The cylindrical cells are attached with their head to the lowermost layer.

The operculum grows in size as the shell grows, such that the operculum remains in proportion to the apertural size. In many species, when the animal is active and crawling, part of the underside of the shell rests on the outer surface of the operculum.

In many species of marine shelled snails which live subtidally, the operculum is greatly reduced in size, and no longer serves to seal the shell entrance. In a large number of families it has been eliminated completely.

In species of conches, the operculum is elongated and claw-shaped, and is used to dig into the sand to enable the conch to perform a leaping type of locomotion.

The structure of the operculum can be described as follows:

• concentric: the nucleus is central or subcentral as in Lithoglyphus and Ampullaria, and in other the nucleus is near the parietal margin of the shell.
• imbricated, or lamellar: when it grows only on one side, and the nucleus is marginal, as in Purpura, Xenophora, and Paludomus.
• claw-shaped, or unguiculate: with the nucleus apical or in front, as in Turbinella and Fusus ; it is claw-shaped and serrated in Strombus
• spiral : when it grows only on one edge, and revolves as it grows ; it is always sinistral in dextral shells.
• paucispiral or oligogyrous: with few spirals as in Littorina.
• subspiral or scarcely spiral, in Thiara
• multispiral or polygyrous: having many closely spaced spirals as in Trochus where they sometimes amount to twenty; the number of turns which the operculum makes is not determined by the number of whorls in the shell, but by the curvature of the aperture, and the necessity that the operculum should revolve fast enough to fit it constantly.

• articulated, when it has a projection, as in Nerita
• radiated is a modification of the articulated operculum in which the spiral is not so evident as in Navicella

There are two basic types of opercula in terms of their material composition:

• 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 when in its natural state but may become brittle when it is dried out. The operculum varies in shape, depending on the family of snails and the shape of the aperture of their shells.

• The other kind of operculum is restricted to a few families of gastropods including the Turbinidae. This operculum structure has a corneous base with a heavy calcareous overlay. The calcareous surface in some genera has color or ornamentation of various kinds including, for example, pustules and incised grooves.

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.