The operculum is a bony flap that protects the gills of bony fish. It also keeps water flowing in the right direction over the gills. In some fish it is essential for respiration, as these fish use it to help move water past their gills by changing their internal pressure.
Ratfish are not bony fish, instead having skeletons made of cartilage, but their gills are also covered by a soft operculum. Most cartilaginous fish, however, such as sharks and rays, do not have an operculum. These fish must find other ways to keep water moving over their gills for oxygen absorption.