The Operculum

by Peggy Williams

green, brown or even pink, blue or purple, making them quite pretty. Some have "pimples", some are whorled, others smooth. In many cases you can tell which species made the operculum without seeing the shell.

Some Naticids also have a calcareous operculum but these are not very thick. They are variously sculptured according to the species and help to identify the species for shell collectors.

The "conchs", or Strombidae, have a long, slim operculum which is nevertheless very strong, due to thick ridges reinforcing it. Too small to close the opening, it is pointed on one end, and the animal uses it in a unique way. Instead of gliding along the bottom on its foot as most gastropods do, the conch will stretch the foot forward with the pointed operculum ahead, stab the operculum into the sand (as a pole vaulter's pole), and push against it so the whole shell can be lifted up and moved forward in a "hopping" motion. The shell is built so that, even when lurching along in this way, the animal is completely covered by the shell and protected all the time.

Serious shell collectors value the operculum as part of the shell. When a shell is collected live, the operculum should be saved and kept with the shell, or glued to a cotton plug, which can then be glued back into the aperture (using water-soluble glue, you won't hurt the shell or the operculum). However, as you can see, you need to be sure to get the right operculum with the right shell! Also, take care to be sure the operculum is right side up (there is a bottom and a top).

Take a look at some opercula; they're remarkable constructions, intricate in design and unique in purpose.

This article is copyrighted 2006 by Peggy Williams. It may be used in shell club publications with attribution to Peggy Williams, www.Shelltrips.com

Many gastropods (mollusks with a single shell) have an operculum attached to their foot. In most cases, the operculum serves as a "trap door": when the animal withdraws completely into its shell, the operculum tightly seals the opening to the shell.

It is obvious that this protects the animal from predation by crabs, lobsters, fishes, and other mollusks, but there is another enemy that every intertidal mollusk faces daily: desiccation when the tide falls.

The operculum is manufactured by the animal in much the same way as the shell, but by the foot, not the mantle. It is formed by a disc on the upper surface of the back of the foot and grows spirally as the animal grows, from a central nucleus. Shells with an elongate aperture have elongate opercula, more oval than round.

When mollusks hatch from the egg, most spend the first part of their lives as swimming larvae, called "veligers". The operculum is first found in the late larval stage of the animal's growth, and in some groups of gastropods it is lost when the shell settles to the bottom to begin life as a crawling animal. Limpets, for instance, hug the rocks tightly, growing the shell to conform to their "home base" and returning after each feeding to the same spot where they are safest. They have no need for an operculum, and indeed it would be in the way.

Most mollusks make their opercula of horny conchiolin. These are generally brown and somewhat flexible, especially the edges, so they may seal the opening more efficiently.

Turbinid mollusks, however, have a calcareous operculum, heavy and thick. These are usually basically white, but may have suffusions of