Physiology of Finfish and Shellfish (2+1)

3.3.5. Photoreception/vision

Sight is extremely important in most fishes. The eye of a fish is the primary receptor site of light from its surroundings. 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, allowing them to absorb as much light as possible in the dark. In some, if they have specialized in some compensatory way so that another sense (such as smell) is dominant, then the eyes will often be reduced. Fishes living in brightly lighted shallow waters often will have relatively small but efficient eyes and probably have colour vision. Certain visual cells are specialized to particular wavelengths and intensities. The pupils of some species of bony fishes, such as eels, contract and dilate depending on light conditions. In most species of bony fishes, however, pupils can't contract or dilate. 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.

In some species, the eye has a reflective layer called the tapetum lucidum behind the retina. The tapetum lucidum reflects light back through the retina a second time. The mudskipper (family Periophthalmidae) and several other species of bony fishes have excellent eyesight both above and below the surface of the water. The four-eyed fishes (family Anablepidae) swim at the water's surface. Their eyes lie at the water line and are adapted for seeing in air and in water. Separate retinae and an asymmetric lens allow these remarkable fish to focus on images above the water and on images under water simultaneously.

The eyesight in some species of bony fishes may be well developed. Goldfish (Carassius auratus) have excellent visual acuity up to 4.8 m (16 ft.) away. Some species of bony fishes have no eyes. The blind cavefishes (family Amblyopsidae) have no vision perception. Other senses help them find prey. The blind goby (Typhlogobius californiensis) is born with eyes that degenerate as the goby matures.

A notable feature of the typical teleost eye is the cornea of constant thickness. This cornea imposes no optical alterations (convergence or divergence) on incoming light. Thus all the focussing of light occurs at the spherical lens which has the highest effective refractive index among the vertebrates. As in other vertebrate eyes, the lens consists of water and structural proteins.

The teleost eye lens protrudes through the pupilar opening in the iris and the eye bulges from the body surface. Therefore the field of view includes a considerable arc forward, continuing laterally to almost directly behind the fish.

Adjustments in focus for near or far vision are accomplished by movemtnt of the lens without changing its shape by muscles within the eye. The specific mudscles within the eye which are responsible for these movements differ in elasmobranchs and teleosts.

The retina is made up of a dense packing of both rod and cone cells for discrimination of light images. Incoming light must penetrate a clear layer of nerve cells and fibres to reach the photochemically active tips of the rods and cones (Fig).

As light strikes the rods or cones, it is absorbed by the light sensitive pigment (eg. rhodopsin, porphyropsin) in the cell. These pigments are located in the photoreception area of the retina giving it its characteristic purple or pink colouration. In all cases, pigmented absorption of light stimulates the retinal cells to send impulses via the optic nerve to the optic lobe on the same side of the brain. The reception and integration of the visual image in the brain would then provide the appropriate motor response (eg., to the muscles and / or fins).

The choroid coat underlies the retinal layer and primarily functions to supply nutrients and oxygen to the retina with its high metabolic demand. Most teleosts possess a pigmented, choroid projection into the posterior portion of the interior of the eye called the falciform process. The falciform process is highly vascularised and probably serves a nutritive function. In addition, a choroid gland is present in most teleosts behind the retina. The arrangement of blood vessels of the choroid makes it eminently suited for oxygen delivery to the retina.

The selerotic coat or selera or firm layer of the eyeball is variously reinforced in all groups of fishes. In elasmobranchs have cartilaginous elements and also calcified plates in this layer for additional reinforcement. Highest of the bony fishes have fibrous, flexible selerotic coats within the orbit and around the optic nerve, but there may be cartilaginous or even bony supporting elements in the outer portion of the eye which can surround the entire cornea.