Senses of Fish Like all vertebrates, fish have sense organs that tell them what is happening in their environment. The organs enable them to see, hear, smell, taste, and touch. In addition, almost all fish have a special sense organ called the lateral line system, which enables them to "touch" objects at a distance. Fish also have various other senses that help them meet the conditions of life underwater.

Sight A fish's eyes differ from those of land vertebrates in several ways. For example, most fish can see to the right and to the left at the same time. This ability makes up in part for the fact that a fish has no neck and so cannot turn its head. Fish also lack eyelids. In land vertebrates, eyelids help moisten the eyes and shield them from sunlight. A fish's eyes are kept moist by the flow of water over them. They do not need to be shielded from sunlight because sunlight is seldom extremely bright underwater. Some fish have unusual adaptations of the eye. For example, adult flatfish have both eyes on the same side of the head. A flatfish spends most of the time lying on its side on the ocean floor and so needs eyes only on the side that faces up-ward. The eyes of certain deep-sea fish are on the ends of short structures that stick out from the head. These structures can be raised upward, allowing the fish to see overhead as well as to the sides and front. A few kinds of fish are born blind. They include certain species of catfish that live in total darkness in the waters of caves and the whalefish, which lives in the ocean depths. Some of these fish have eyes but no vision. Others lack eyes completely.

Hearing All fish can probably hear sounds produced in the water. Fish can also hear sounds made on shore or above the water if they are loud enough. Catfish and certain other fish have a keen sense of hearing. Fish have an inner ear enclosed in a chamber on each side of the head. Each ear consists of a group of pouches and tubelike canals. Fish have no outer ears or eardrums to receive sound vibrations. Sound vibrations are carried to the inner ears by the body tissues.

Smell and taste All fish have a sense of smell. It is highly developed in many species, including catfish, salmon, and sharks. In most fish, the olfactory organs (organs of smell) consist of two pouches, one on each side of the snout. The pouches are lined with nerve tissue that is highly sensitive to odours from substances in the water. A nostril at the front of each pouch allows water to enter the pouch and pass over the tissue. The water leaves the pouch through a nostril at the back. Most fish have taste buds in various parts of the mouth. Some species also have them on other parts of the body. Catfish, sturgeon, and a number of other fish have whiskerlike feelers called barbels near the mouth. They use the barbels both to taste and to touch. Touch and the lateral line system are closely related. Most fish have a well-developed sense of touch. Nerve endings throughout the skin react to the slightest pressure and change of temperature. The lateral line system senses changes in the movement of water. It consists mainly of a series of tiny canals under the skin. A main canal runs along each side of the trunk. Branches of these two canals extend onto the head. A fish senses the flow of water around it as a series of vibrations. The vibrations enter the lateral line through pores and activate certain sensitive areas in the line. If the flow of water around a fish changes, the pattern of vibrations sensed through the lateral line also changes. Nerves relay this information to the brain. Changes in the pattern of vibrations may warn a fish of approaching danger or indicate the location of objects outside its range of vision.

Touch and the Lateral Line System Touch and the lateral line system are closely related. Most fish have a well-developed sense of touch. Nerve endings throughout the skin react to the slightest pressure and change of temperature. The lateral line system senses changes in the movement of water. It consists mainly of a series of tiny canals under the skin. A main canal runs along each side of the trunk. Branches of these two canals extend onto the head. A fish senses the flow of water around it as a series of vibrations. The vibrations enter the lateral line through pores and activate certain sensitive areas in the line. If the flow of water around a fish changes, the pattern of vibrations sensed through the lateral line also changes. Nerves relay this information to the brain. Changes in the pattern of vibrations may warn a fish of approaching danger or indicate the location of objects outside its range of vision. Other senses include those that help a fish keep its balance and avoid unfavourable waters. The inner ears help a fish keep its balance. They contain a fluid and several hard, free-moving otoliths (ear stones). Whenever a fish begins to swim in other than an upright, level position, the fluid and otoliths move over sensitive nerve endings in the ears. The nerves signal the brain about the changes in the position of the body. The brain then sends messages to the fin muscles, which move to re-store the fish's balance. Fish can also sense any changes in the pressure, salt content, or temperature of the water and so avoid swimming very far into unfavourable waters.

Other Senses Other senses include those that help a fish keep its balance and avoid unfavourable waters. The inner ears help a fish keep its balance. They contain a fluid and several hard, free-moving otoliths (ear stones). Whenever a fish begins to swim in other than an upright, level position, the fluid and otoliths move over sensitive nerve endings in the ears. The nerves signal the brain about the changes in the position of the body. The brain then sends messages to the fin muscles, which move to re-store the fish's balance. Fish can also sense any changes in the pressure, salt content, or temperature of the water and so avoid swimming very far into unfavourable waters.