These live lightning whelks’ opercula are visible in this photo.

These live lightning whelks’ opercula are visible in this photo.

by Johnny Rader

When you feel stressed or afraid, what do you do to cope? Do you run away? Play dead? Or are you a fighter? Many people are familiar with the plethora of marine gastropod shells that accumulate on our beaches, but there is another part of the animal that frequently washes up, and it has an interesting story. It has a dry, leathery appearance with many brown or grey layers, and it’s called the operculum.

The word operculum is derived from Latin and means a cover or lid, which is exactly how marine snails use it. Although not all gastropods have this calcareous structure, nearly all of the ocean-dwelling species do. The structure is attached to the animal’s muscular foot and serves as a type of door, trapping moisture in and keeping enemies out.

When you find an operculum on the beach, it will most likely be dry and grey or brown in appearance.

When you find an operculum on the beach, it will most likely be dry and grey or brown in appearance.

The aperture, or the opening of the shell, is different in each species and therefore opercula come in many different shapes. However, they are generally circular or oval. Like most things in nature, form follows function, and that is certainly true in this case.

Many times, the most essential purpose of the operculum is to protect the live gastropod from drying up. This is especially important for snails that live in the intertidal zone, where they are exposed to air during low tides and submerged during high tides. In fresh water snails, the operculum helps during prolonged periods of drought.

A fighting conch.

A fighting conch.

The operculum also helps these animals avoid predators. Gastropods can be quite vulnerable to animals like birds, raccoons, and crabs. Closing the operculum tightly makes it much more difficult for predators to turn a live shell into their next meal.

A horse conch opens its operculum wide to enjoy a meal.

A horse conch opens its operculum wide to enjoy a meal.

In the family Strombidae, the true conchs, the operculum is claw shaped. They will dig the structure into the sand and push with force to leap across the ocean floor as a means of locomotion. Florida fighting conchs can often be observed doing this on Sanibel’s beaches during very low tides.

Opercula commonly found on Sanibel’s beaches include those of lightning whelks, horse conchs, and king’s crown conchs. If you find an operculum and would like help identifying which species it came from, we invite you to visit us at Sanibel Sea School or email a photo to info@sanibelseaschool.org.

Comment