Operculum (bryozoa)

In the context of bryozoans, the operculum is a lid-like or door-like structure that closes off the aperture of the zooid, the individual animal unit within the bryozoan colony. The operculum serves as a protective barrier, shielding the delicate polypide (the feeding structure) from the external environment when it is retracted.

The operculum can be composed of either a calcified material, offering rigid protection, or a chitinous or membranous material, providing a more flexible barrier. The specific composition and structure of the operculum are important taxonomic characters used in the identification and classification of different bryozoan species.

The mechanism of opercular opening and closing is typically controlled by muscles associated with the zooid. When the polypide needs to extend for feeding, muscles relax, allowing the operculum to open. When threatened or during unfavorable conditions, muscles contract, rapidly closing the operculum and sealing the zooid. The operculum thus plays a vital role in the bryozoan's defense against predators and protection from environmental stressors like desiccation or strong currents.