region. Mutations in the Edn1-pathway in zebrafish are known to lead to deformities of the opercular series' shape and size.

The opercular series is vital in obtaining oxygen. They open as the mouth closes, causing the pressure inside the fish to drop. Water then flows towards the lower pressure across the fish's gill lamellae, allowing some oxygen to be absorbed from the water. Cartilaginous ratfishes (chimaeras) possess soft and flexible opercular flaps. Sharks, rays