Sensory systems in fish

Most fish possess highly developed sense organs. Nearly all fish that are primarily active during daylight have colour vision that is at least as good as a human's. Many fish also have chemoreceptors responsible for extraordinary senses of taste and smell. Their hearing is adapted to the underwater environment, using bone conduction, the swim bladder, and the inner ear. Most fish have sensitive receptors that form the lateral line system, which detects gentle currents and vibrations, and senses the motion of nearby fish and prey. Sharks can sense frequencies in the range of 25 to 50 Hz through their lateral line. Fish orient themselves using landmarks and may use mental maps based on multiple landmarks or symbols. Fish behavior in mazes reveals that they possess spatial memory and visual discrimination.

Vision

Vision is an important sensory system for most species of fish. Fish eyes are similar to those of terrestrial vertebrates like birds and mammals, but have a more spherical lens. Their retinas generally have both rod cells and cone cells (for scotopic and photopic vision), and most species have colour vision. Some fish can see ultraviolet and some can see polarized light. Amongst jawless fish, the lamprey has well-developed eyes, while the hagfish has only primitive eyespots. Fish vision shows adaptation to their visual environment, for example deep sea fishes have eyes suited to the dark environment. Fish and other aquatic animals live in a different light environment than terrestrial species. Water absorbs light so that with increasing depth the amount of light available decreases quickly. The optic properties of water also lead to different wavelengths of light being absorbed to different degrees, for example light of long wavelengths (e.g. red, orange) is absorbed quite quickly compared to light of short wavelengths (blue, violet), though ultraviolet light (even shorter wavelength than blue) is absorbed quite quickly as well. Besides these universal qualities of water, different bodies of water may absorb light of different wavelengths because of salts and other chemicals in the water.

Hearing, motion, and pressure

Hearing is an important sensory system for most species of fish. For example, in the family Batrachoididae, males use their swim bladders to make advertisement calls which females use to localize males. Hearing threshold and the ability to localize sound sources are reduced underwater, in which sound travels faster than in air. Underwater hearing is by bone conduction, and localization of sound appears to depend on differences in amplitude detected by bone conduction. As such, aquatic animals such as fish have a more specialized hearing apparatus that is effective underwater. Fish sense sound primarily through their otoliths (inner ears). In some fishes, such as some species of carp and herring, hearing is enhanced by a connection between their swim bladder and inner ear. The lateral line consists of a series of receptors along the side of a fish's body, open to the environment via a series of openings called lateral line pores. The lateral line can also play a role in hearing. Fishes can be characterised into six hearing groups for assessing their susceptibility to underwater sound, depending on the involvement of the swim bladder in hearing. For three groups (denoted P1, P2 and P3), the swim bladder is involved in hearing, which is thus sensitive to sound pressure. For the other three groups (A2, A3 and A4), the swim bladder is either not present or not involved in hearing and the hearing responds not to sound pressure but to sound particle acceleration. Fish hearing groups sensitive to sound pressure are P1 (swim bladder without adaptation such as the cod or haddock), P2 (swim bladder with a mechanical connection to the inner ear such as the Weberian ossicle), e.g., goldfish and zebrafish and P3 (swim bladder with ultrasonic adaptation, bubble near ear, or a standard ear such as shad, menhaden). Hearing group A4 comprises bony fishes with a swim bladder not involved in hearing such as sturgeon and salmon. Fish hearing groups without a swim bladder are A2 (cartilaginous fishes such as shark and skate) and A3 (bony fishes without a swim bladder, such as dab and mackerel).

Pressure detection uses the organ of Weber, a system consisting of three appendages of vertebrae transferring changes in shape of the gas bladder to the middle ear. It can be used to regulate the buoyancy of the fish. Fish like the weather fish and other loaches are also known to respond to low pressure areas but they lack a swim bladder.

Current detection

The lateral line in fish and aquatic forms of amphibians is a detection system of water currents, consisting mostly of vortices. The lateral line is also sensitive to low-frequency vibrations. It is used primarily for navigation, hunting, and schooling. The mechanoreceptors are hair cells, the same mechanoreceptors for vestibular sense and hearing. Hair cells in fish are used to detect water movements around their bodies. These hair cells are embedded in a jelly-like protrusion called cupula. The hair cells therefore can not be seen and do not appear on the surface of skin. The receptors of the electrical sense are modified hair cells of the lateral line system. Fish and some aquatic amphibians detect hydrodynamic stimuli via a lateral line. This system consists of an array of sensors called neuromasts along the length of the fish's body. Neuromasts can be free-standing (superficial neuromasts) or within fluid-filled canals (canal neuromasts). The sensory cells within neuromasts are polarized hair cells contained within a gelatinous cupula. The cupula, and the stereocilia which are the "hairs" of hair cells, are moved by a certain amount depending on the movement of the surrounding water. Afferent nerve fibers are excited or inhibited depending on whether the hair cells they arise from are deflected in the preferred or opposite direction. Lateral line neurons form somatotopic maps within the brain informing the fish of amplitude and direction of flow at different points along the body. These maps are located in the medial octavolateral nucleus (MON) of the medulla and in higher areas such as the torus semicircularis.

Chemoreception (smelling)

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