Ray-finned fishes perceive the external environment in five major ways – vision, mechanoreception, chemoreception, electroreception and magnetic reception, and to humans several of these sensory systems are entirely alien. Many types of perception are also used by ray-finned fishes to communicate with individuals of the same (conspecifics) or other species (heterospecifics).

Vision is the most important means of communication and foraging for many ray-finned fishes. The eyes of fish are very similar to terrestrial vertebrates so they are able to recognize a broad range of wavelengths. A species’ ability to perceive various wavelengths corresponds to the depth at which it lives since different wavelengths attenuate (become weaker) with depth. In addition to the normal spectrum perceived by most vertebrates, several shallow-water species are able to see ultraviolet light; others, such as anchovies , cyprinids , salmonids and cichlids , can even detect polarized light! Many fishes also have specially modified eyes adapted for sight in light-poor environments and even outside of water (e.g. mudskippers). For example, several families of deepsea fishes (deepsea hatchetfishes , pearleyes , giganturids , barreleyes) have elongate (long and narrow), upward-pointing, tubular eyes that enhance light gathering and binocular vision, providing better depth perception. Also, several deepwater, midwater and a few shallow species actually have internally generated lights around the eyes to find and attract prey and communicate with other species (see below). Light is usually produced in two ways: by special glandular cells embedded in the skin or by harnessing cultures of symbiotic luminous bacteria in special organs.

One way fishes communicate visually is simply through their static color pattern and body form. For instance, juveniles progress through a range of color and shape patterns as they mature, and sexes are often colored differently (  sexual dimorphism). In addition, some fishes are quite good at identifying other species; the Beau Gregory damselfish is apparently able to distinguish 50 different reef fish species that occur within its territory. A second way fishes communicate visually is through dynamic display, which involves color change and rapid, often highly stereotyped movements of the body, fins, operculae, and mouth. Such displays are often associated with changes in behavioral state, such as aggressive interactions, breeding interactions, pursuit and defense. A third form of visual communication is light production, found among numerous fishes in deepsea habitats. Midwater species, such as lanternfishes , hatchetfishes and dragonfishes have rows of lights along the underside of the body, probably for mating and identification as well as foraging. Even some shallow-water species, such as pineconefishes , cardinalfishes and flashlight fish (family Anomalopidae) of the Red Sea utilize internal light sources to form nighttime feeding shoals or for other behavioral interactions.

Mechanoreception includes equilibrium and balance, hearing, tactile sensation, and a ‘distance-touch-sense’ provided by the lateral line (Wheeler, Alwyne 1985:viii). Detecting sound in water can be difficult because waves pass through objects of similar density. Therefore, ray-finned fishes have otoliths, which have greater density than the rest of the fish, in the inner ear attached to sensory hair cells. Since gas bubbles increase sensitivity to sound, many ray-finned fish (e.g. herrings , elephantfishes and squirrelfishes) have modified gas bladders and swimbladders adjacent to the inner ear. Most ray-finned fishes have keen hearing ability and sound production is common but not universal. In groups that do utilize sound for communication, the most common purpose is territorial defense (e.g. damselfishes and European croakers) or prey defense (e.g. herrings , characins , catfishes , cods , squirrelfishes and porcupinefishes). Sound production is also used in mating (for attraction, arousal, approach or coordination) and communication between shoal mates. Stridulation, which involves rubbing together hard surfaces such as teeth (e.g. filefishes) or fins (e.g. sea catfishes), or the vibration of muscles (e.g. drums), is the most common way sound is produced. Often the latter structures have a muscular connection to the swimbladder to amplify sound. Accordingly, the swimbladder itself is the source of the most complex forms of sound production in many groups (e.g. toadfishes , searobins and flying gurnards). The lateral line is composed of a collection of sensory cells beneath the scales and is able to detect turbulence, vibrations and pressure in the water, acting as a close-quarters radar. This sensation is particularly important in the formation of schools (see Behavior) because consistent positioning is essential for turbulence reduction and smooth hydrodynamic functioning. Consequently, individuals are “so sensitive to the movements of companions that thousands of individuals can wheel and turn like a single organism” (Moyle and Cech 2004:206). Experiments have shown that the lateral line sensation can even compensate for loss of sight in some species, such as trout. The fact that several naturally sightless fish occupy caves (e.g. cavefishes) and other subterranean environments, making extensive use of distance-touch sensation, provides further evidence.

Chemoreception involves both smell (olfaction) and taste (gustation), but, as in terrestrial vertebrates , olfaction is much more sensitive and chemically specific than gustation, and each has a specific location and processing center in the brain. Many fishes use chemical cues to find food. Taste buds are scattered widely around the lips, mouth, pharynx, and even the gill arches; and barbels are used for taste reception in many families (most carps , catfishes and cod). However, the use of nares (like nostrils, located on the top of the head) to detect   pheromones is probably the most important type of chemoreception in fishes. Pheromones are chemicals secreted by one fish and detected by conspecifics, and sometimes closely related species, producing a specific behavioral response. Pheromones allow fish to recognize specific habitats (such as natal streams in salmon), members of the same species, members of the opposite sex, individuals in a group or hierarchy, young, predators, etc. Some groups in dominance hierarchies even associate the scents of individuals with their particular ranking. Also, groups of closely related species, such as cyprinids , are able to detect ‘fear scents,’ which are pheromones released when the skin is broken (i.e. a predator has attacked), prompting others to adopt some type of predator avoidance behavior.

Some ray-finned fishes, usually inhabiting turbid environments, have specialized organs for electroreception. Several groups can detect weak electrical currents emitted by organs, such as the heart and respiratory muscles, and locate prey buried in sediment (catfish) or in extremely turbid waters (elephantfishes). Elephantfishes and naked-back knifefishes actually produce a constant, weak electrical field around their bodies that functions like radar, allowing them to navigate through their environment, find food, and communicate with mates. In fact, a diverse range of actinopterygian orders have developed the ability to use electricity for communication: Mormyriformes (elephantfishes and Gymnarchidae), Gymnotiformes (six families) , Siluriformes (electric catfishes), and Perciformes (stargazers). The key to electrical communication is not simply the ability to detect electrical fields, but to produce a mild electrical discharge and modify the amplitude, frequency, and pulse length of the signal. This makes electrical signals individually specific, in addition to being sex and species-specific. Consequently, “electrical discharges can have all the functions that visual and auditory signals have in other fishes, including courtship, agonistic behavior and individual recognition” (Moyle and Cech 2004:206). Finally, a few highly migratory ray-finned fishes can apparently detect earth-strength magnetic fields directly, in much the same way sensation occurs with the lateral line. While the specific mechanisms of magnetic reception are unknown, researchers have found magnetite in the heads of some tunas (e.g. yellowfin tuna) and in the nares of some   anadromous salmon (subfamily Salmoninae). Presumably, magnetic perception helps fish locate long distance migration routes for both feeding and reproduction.

Clearly ray-finned fishes display considerable complexity in their ability to perceive their environment and communicate with other individuals, yet until recently it was assumed that fish had negligible cognitive ability. Current research, however, indicates that learning and memory are integral parts of fish development and rely on processes very similar to those of terrestrial vertebrates. Experiments have shown, for instance, that individuals can remember the exact location of holes in fishing net years after exposure, and that fish in schools learn faster by following the lead other individuals. Some researcher believe that the cognitive ability of some fishes is even comparable to that of non-human primates.

Communication Channels: visual ; tactile ; acoustic ; chemical ; electric

Other Communication Modes: photic/bioluminescent ; mimicry ; duets ; choruses ; pheromones ; scent marks ; vibrations

Perception Channels: visual ; infrared/heat ; ultraviolet; polarized light ; tactile ; acoustic ; vibrations ; chemical ; electric ; magnetic