Anatomy of finfish and shellfish

Unit 11 - Sensory System in fishes

11.3. Eyes
The eye of a fish is basically like that of all other vertebrates, but the eyes of fishes are extremely varied in structure and adaptation. In general, fishes living in dark and dim water habitats have large eyes, unless they have specialized in some compensatory way so that another sense (such as smell) is dominant, in which case the eyes will often be reduced. Fishes living in brightly lighted shallow waters often will have relatively small but efficient eyes. Cyclostomes have somewhat less elaborate eyes than other fishes, with skin stretched over the eyeball perhaps making their vision somewhat less effective.
Most fishes have a spherical lens and accommodate their vision to far or near subjects by moving the lens within the eyeball. A few sharks accommodate by changing the shape of the lens, as in land vertebrates. Those fishes that are heavily dependent upon the eyes have especially strong muscles for accommodation. Most fishes see well, despite the restrictions imposed by frequent turbidity of the water and by light refraction. Experimental evidence indicates that many shallow-water fishes, if not all, have colour vision and see some colours especially well, but some bottom-dwelling shore fishes live in areas where the water is sufficiently deep to filter out most if not all colours, and these fishes apparently never see colours. When tested in shallow water, they apparently are unable to respond to colour differences.
The essentially lidless eyes that cannot be closed are situated in orbits, one on each side of the midline of the fish head. Most often the eyes are lateral with partially independent fields of vision and movement. In many bottom dwellers, including the skates (Rajidae), most Sculpine (Cottidae) and Goosefishes (Lophidae), the eyes are dorsal in adults of flounders and their relatives both eyes on one side of the head, the eyes are variously reduced or absent in cave fishes.
Visible through the transparent skin that covers the eye and through the transparent cornea of the eyeball are
a. The opening of the pupil of the eye and through it, the spherical crystalline lens inside the eye ball
b. The colored, washer – shaped iris surrounds the pupil.