Anatomy of finfish and shellfish

Unit 11 - Sensory System in fishes

11.1. Introduction
A fish’s eyes are adapted or modified for underwater vision, but they are not very different from human eyes. Fish do not have true eyelids. Human eyelids prevent the eyes from becoming dry and also protect against dirt. A fish’s eyes are always covered by water; therefore, they require no lids.
The metallic-looking ring, called the iris, encircling the dark center, or lens, of the fish’s eye cannot move as it does in the human eye. The human iris can expand or contract, depending upon light conditions. Because light never attains great intensity underwater, a fish needs no such adaptation. The big difference between a human eye and the eye of a fish occurs in the lens. In humans it is fairly flat or disc like; in fish, it is spherical or globular. Human eyes are capable of changing the curvature of the lens to focus at varying distances—flatter for long-range focusing and more curved for shorter range. Although the eye of a fish has a rigid lens and its curvature is incapable of change, it can be moved toward or away from the retina (like the focusing action of a camera).
Fish can distinguish colors. There are indications that some kinds of fish prefer one color to another and also that water conditions may make one color more easily distinguished than others.
Many kinds of fish have excellent vision at close range. Fish that live in the dusky or dimly lit regions of the sea commonly have eyes that are comparatively larger than the eyes of any other animal with backbones. Fish that live in the perpetual darkness of caves or other subterranean waters usually have no eyes, but those inhabiting the deep sea, far below the depth to which light rays can penetrate, may or may not have eyes. The reason that most deep-sea fish have well-developed eyes is the prevalence of bioluminescence. Deep-sea squid, shrimp, and other creatures, as well as fish, are equipped with light-producing organs. The light they produce is used to recognize enemies or to capture prey.
Many fish with poor vision have well-developed senses of smell, taste, and touch. Improbable as it may seem, a fish does possess nostrils. Four nostrils are located close to the top of the snout, one pair on each side. Each pair opens into a small blind sac immediately below the skin. Water, carrying odors, passes through the sacs, which are lined with the receptors of smell. Some fish, including sharks, possess an extremely acute sense of smell.
Fish have taste organs located in the skin of their snouts, lips, mouths, and throats. A fish’s tongue, unlike the human tongue, is flat, rigid, and cartilaginous and moves only when the base below it moves; nevertheless, it does possess taste buds that indicate to the fish whether to accept or to reject anything taken into its mouth. There is a close relationship between the senses of smell and taste in fish, just as in humans. Many types of fish are first drawn to food by its odor.
Although fish obviously do not possess outer ears as humans do, they are still capable of hearing. A fish possesses only an inner ear, found in the bones of the skull. In many fish, these ear bones are connected to their air bladders. Vibrations are transmitted to the ear from the air bladder, which acts as a sounding board.
The lateral-line system, a series of sensory cells usually running the length of both sides of the fish’s body, performs an important function in receiving low-frequency vibrations. Actually, it resembles a “hearing organ” of greater sensitivity than human ears. The typical lateral line is a mucus-filled tube or canal under the skin; it has contact with the outside world through pores in the skin or through scales along the line or in-between them. A nerve situatedmat intervals alongside the canal sends out branches to it. In some cases, the lateral line extends over the fish’s tail, and in many fish it continues onto their heads and spreads into several branches along the outer bones of their skulls, where it is not outwardly visible. The fish utilizes its lateral line to determine the direction of currents of water and the presence of nearby objects, as well as to sense vibrations. The lateral line helps the fish to determine water temperature and to find its way when traveling at night or through murky waters. It also assists schooling fish in keeping together and may help a fish to escape enemies.
Many fish are noisy creatures. They make rasping, squeaking, grunting, and squealing noises. Some fish produce sounds by rubbing together special extensions of the bones of their vertebrae. Others make noises by vibrating muscles that are connected to their air bladders, which amplify the sounds. Still other fish grind their teeth, their mouth cavities serving as sound boxes to amplify the noises. Many fish make sounds when they are caught. Grunts and croakers got their names from this habit.
Since fish have a nervous system and sense organs, it would appear that Smell receptors are located in the nostrils, and water (carrying odors) is drawn into sacs that are lined with the organs of smell. Olfactory nerves connect the nostrils and brain. they could feel pain. The fish’s brain is not highly developed, however. There is no cerebral cortex (the part of the brain in higher animals that stores impressions), and so the fish has little or no memory. It is not uncommon, for example, for an angler to hook the same fish twice within a short time. Many fish are caught with lures or hooks already embedded in their jaws. Fish are essentially creatures of reflex, rather than of action produced or developed by using the brain. In all probability, physical pain in fish is not very acute, and if any impression of pain is made in the brain, it is quickly lost.

“A number of highly specialized organs, which receive physical and chemical stimuli from the environment, are called as sense or sensory organs”. They are associated with the nervous system. Most obvious sense organs are eyes, nose, ear and skin which receive stimuli for vision, olfaction, audition and touch respectively. Less obvious are Lateral line, sensory crypts and papillae, ampullae of Lorenzini.
The visual stimuli involve changes in light intensity and quality and acoustical ones are received through the inner or lateral line. Chemical stimuli are those experienced through either small or taste organs. Pain is probably not experienced as a strong sensation by fishes, though forceful or noxious physical or chemical stimuli evoke violent reactions.