In fish biology, which structure helps in moving water over their gills when the fish is at rest?

The operculum is the structure that plays a crucial role in facilitating the movement of water over the gills in fish, particularly when they are at rest. It acts as a protective bony flap that covers the gills, and its movements assist in creating a pressure difference. By opening and closing, the operculum helps draw water into the mouth and push it over the gills, where gas exchange occurs, even when the fish is not actively swimming.

This efficient mechanism allows fish to maintain a constant flow of water over their gills, ensuring that they can extract oxygen from the water without needing to be in motion. The operculum effectively enables a fish to breathe quietly and efficiently in a stationary position.

In contrast, other structures mentioned do not primarily serve this purpose. The urogenital pore is involved in excretion and reproduction, while gill rakers are designed to filter food particles from the water, not to assist in water movement. The buccal pump does facilitate water flow, but it is typically associated with a more active process rather than the resting state of a fish.