Anterior Figure 7. Brain. Source: Ref. 3.

ample, with various feedback of important information from the cardiovascular system. It involves the coordination of gross behavior such as schooling, where inputs from visual and acoustical sensors are coordinated with gross muscular action involved in swimming, which in itself involves a balancing act involving the labyrinths, fin positions, and trunk movement.

Sensory Organs

Fish have an acute sense of smell and rely on the olfactory organ to find food, avoid predators, and to help guide migratory fish to their spawning grounds. Some fish utilize pheromones, chemical substances that can be smelled, to signal alarm to the rest of the school. Catfish, which have an excellent sense of smell, use the specific odor of the slime of individual fish to differentiate fish in a school hierarchy. As the water passes through the nostril over a transverse septum, olfactory epithelium connected to the olfactory nerve signals the smell response to the olfactory lobes in the brain.

Taste buds are the gustatory organs and can be found in the epithelium of the mouth, lips, esophagus and snout. Some fish, mainly those that are not sight feeders, may have taste buds at other places on the body. Free nerve endings from the cranial nerves appear in sensitive regions, suggesting that touch works with taste to stimulate the feeding response. External taste receptors respond to monovalent ions and, to a lesser degree, divalent ions. Internal receptors respond to amino acids, sugars, strong salts, and acids. Both taste and smell are based on organic chemoreception. It also appears that tactile, chemical, and temperature sensations are closely integrated in fish skin.

Sight is extremely important to salmonids and other fish that depend on this sense to detect movements and color changes that signal prey, predator, and mate. The eyes of most bony fishes have a spherical lens, the most powerful shape for a single lens, that in some cases can be changed in convexity and in distance from the retina, the visual layer of the eye (Fig. 8). This is how the fish focus, because very little focusing is done by the cornea. There is little difference in the refractive index, the source of light diffraction, between the cornea and water and, therefore, little focusing is necessary. The cornea does play a role in protecting the rest of the eye from physical pollutants and harm and may help prevent dehydration. The iris controls the amount of light getting through to the retina. Most fish

Figure 8. Cross section of a fish eye. Source: Ref. 2.

(eg, elasmobranchs) have a fixed iris that contains guanine and melanin. Sight feeders, such as the salmonids, have a well-developed lens muscle and focus by actually moving the lens. The eyeball is ellipsoid and lends a great depth of field, that is the sharp simultaneous focus of objects that are distant and near. This is due to a wide-angle configuration of the components: the flattened eyeball, the round lens, and the protrusion of the lens out of the iris. Light travels through the lens and is focused on the retina. Retinal layers such as the optic nerve fibers, ganglion cells, bipolar cells, and photoreceptor cells, (the cones and rods) all have a role in converting the light to sight images in the brain. The rod cells are sensitive to low light levels but not to color. The cone cells react to strong light and are color sensitive. Photochemical reactions in these cells are transmitted as electrical impulses to the optic nerve and eventually to the optic lobe in the brain. The sensitivity of the retina to particular colors of light depends on the pigments found in the rods and cones in the retina.

The small pineal organ serves a visual as well as a secretory role in fish. It is a small fingerlike organ is located just under the cranium on the dorsal midline of the brain. It serves possible functions in the detection of day length. While it is not vital to life, it may serve an important role in the adjustment of body color to the environment as well as in the synchronization of physiological changes associated with smoltification, in anadromous salmonids, with day length.

The inner ear functions in both equilibrium and hearing. It is found on either side of the medulla oblongata of the brain. The inner ear is made up of two sections, the pars superior and the pars inferior. The pars superior has three semicircular canals filled with fluid and has an ampullae and utriculus. The ampullae contains the receptor tissue, which has sensory cells with long sensory hairs similar to taste buds, and the lateral line. Displacement of the body causes the sensory cells to react and the proper righting mechanism to take place. The utriculus of the pars superior contains an otolith; a calcarious stone. This stone lies on the sensory hairs and responds to the force of gravity. This system works with the retina for maintenance of balance. The pars inferior is the structure of actual sound reception and it is composed of the two vesicles, sacculus and lagena. These vesicles, containing otoliths, are innervated by the auditory nerve. The otoliths will vibrate at a different resonance then the rest of the body when presented with sound waves. This triggers the sensory hairs to send a nervous impulse up the cranial nerve.

The lateral line is a set of sense organs found only in fishes and the aquatic stages of amphibians. The lateral line is innervated by cranial nerves and is closely associated with the auditory centers. The brain relays secondary impulses from the lateral line to the hypothalamus, thalamus, the optic tectum, and the cerebellum. The lateral line helps in the detection of prey, predators or mates by sensing hydrodynamic displacement. The lateral line receptors respond to pressure waves and low frequency vibrations and other localized disturbances.

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