Shallow-water fishes generally have smaller eyes. The pupils of some species of bony fishes, such as eels, contract and dilate depending on how much light is needed. In most species of bony fishes, however, pupils can't contract or dilate. Pleuronectiformes have one eye that migrates across the top of their skull during development to lie adjacent to the eye on the other side. The half of the pupil above the waterline focuses on images above the water, while the half of the pupil below the waterline focuses on images under water.
Most bony fishes have a single pair of gill openings.
In most bony fishes, the gill openings are covered by a flexible plate called an operculum that protects the gills. Oxygenated water enters through the fish's mouth and is forced over the gills. It exits behind the operculum. The nostrils of most bony fishes have no internal connections with the oral cavity. In some bony fishes such as eels, the nostrils' incurrent and excurrent openings are widely separated.