Vision in fish
Vision is an important sensory system for most species of fish. Fish eyes are similar to the eyes of terrestrial vertebrates like birds and mammals, but have a more spherical lens. Birds and mammals (including humans) normally adjust focus by changing the shape of their lens, but fish normally adjust focus by moving the lens closer to or further from the retina. Fish 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 are sensitive to polarised light.
Among jawless fishes, the lamprey[1] has well-developed eyes, while the hagfish has only primitive eyespots.[2] The ancestors of modern hagfish, thought to be the protovertebrate,[3] were evidently pushed to very deep, dark waters, where they were less vulnerable to sighted predators, and where it is advantageous to have a convex eye-spot, which gathers more light than a flat or concave one. Fish vision shows evolutionary adaptation to their visual environment, for example deep sea fish have eyes suited to the dark environment.
The retina[edit]
Within the retina, rod cells provide high visual sensitivity (at the cost of acuity), being used in low light conditions. Cone cells provide higher spatial and temporal resolution than rods can, and allow for the possibility of colour vision by comparing absorbances across different types of cones which are more sensitive to different wavelengths. The ratio of rods to cones depends on the ecology of the fish species concerned, e.g., those mainly active during the day in clear waters will have more cones than those living in low light environments. Colour vision is more useful in environments with a broader range of wavelengths available, e.g., near the surface in clear waters rather than in deeper water where only a narrow band of wavelengths persist.[5]
The distribution of photoreceptors across the retina is not uniform. Some areas have higher densities of cone cells, for example (see fovea). Fish may have two or three areas specialised for high acuity (e.g. for prey capture) or sensitivity (e.g. from dim light coming from below). The distribution of photoreceptors may also change over time during development of the individual. This is especially the case when the species typically moves between different light environments during its life cycle (e.g. shallow to deep waters, or fresh water to ocean).[5] or when food spectrum changes accompany the growth of a fish as seen with the Antarctic icefish Champsocephalus gunnari.[12]
Some species have a tapetum, a reflective layer which bounces light that passes through the retina back through it again. This enhances sensitivity in low light conditions, such as nocturnal and deep sea species, by giving photons a second chance to be captured by photoreceptors.[7] However this comes at a cost of reduced resolution. Some species are able to effectively turn their tapetum off in bright conditions, with a dark pigment layer covering it as needed.[5]
The retina uses a lot of oxygen compared to most other tissues, and is supplied with plentiful oxygenated blood to ensure optimal performance.[5]
Accommodation[edit]
Accommodation is the process by which the vertebrate eye adjusts focus on an object as it moves closer or further away. Whereas birds and mammals achieve accommodation by deforming the lens of their eyes, fish and amphibians normally adjust focus by moving the lens closer or further from the retina.[5] They use a special muscle which changes the distance of the lens from the retina. In bony fishes the muscle is called the retractor lentis, and is relaxed for near vision, whereas for cartilaginous fishes the muscle is called the protractor lentis, and is relaxed for far vision. Thus bony fishes accommodate for distance vision by moving the lens closer to the retina, while cartilaginous fishes accommodate for near vision by moving the lens further from the retina.[13][14][15]
There is a need for some mechanism that stabilises images during rapid head movements. This is achieved by the vestibulo-ocular reflex, which is a reflex eye movement that stabilises images on the retina by producing eye movements in the direction opposite to head movements, thus preserving the image on the centre of the visual field. For example, when the head moves to the right, the eyes move to the left, and vice versa. The human vestibulo-ocular reflex is a reflex eye movement that stabilises images on the retina during head movement by producing an eye movement in the direction opposite to head movement, thus preserving the image on the center of the visual field. In a similar manner, fish have a vestibulo-ocular reflex which stabilises visual images on the retina when it moves its tail.[16] In many animals, including human beings, the inner ear functions as the biological analogue of an accelerometer in camera image stabilization systems, to stabilise the image by moving the eyes. When a rotation of the head is detected, an inhibitory signal is sent to the extraocular muscles on one side and an excitatory signal to the muscles on the other side. The result is a compensatory movement of the eyes. Typical human eye movements lag head movements by less than 10 ms.[17]
The diagram on the right shows the horizontal vestibulo-ocular reflex circuitry in bony and cartilaginous fish.
Polarised light[edit]
It is not easy to establish whether a fish is sensitive to polarised light, though it appears likely in a number of taxa. It has been unambiguously demonstrated in anchovies.[29] The ability to detect polarised light may provide better contrast and/or directional information for migrating species. Polarised light is most abundant at dawn and dusk.[5] Polarised light reflected from the scales of a fish may enable other fish to better detect it against a diffuse background,[30] and may provide useful information to schooling fish about their proximity and orientation relative to neighbouring fish.[31] Some experiments indicate that, by using polarization, some fish can tune their vision to give them double their normal prey sighting distance.[9]
Double cones[edit]
Most fish have double cones, a pair of cone cells joined to each other. Each member of the double cone may have a different peak absorbance, and behavioural evidence supports the idea that each type of individual cone in a double cone can provide separate information (i.e. the signal from individual members of the double cone are not necessarily summed together).[32]
Most deep-sea fish cannot see red light. The deepwater stoplight loosejaw produces red bioluminescence so it can hunt with an effectively invisible beam of light.[45]
![Most deep-sea fish cannot see red light. The deepwater stoplight loosejaw produces red bioluminescence so it can hunt with an effectively invisible beam of light.[45]](http://upload.wikimedia.org/wikipedia/commons/thumb/b/b2/Malacosteus.JPG/179px-Malacosteus.JPG)
When the larvae of a flatfish grows, the eye on one side rotates to the other side so the fish can rest on the seafloor.
The European plaice is a flatfish with raised eyes, so when it buries itself in sand for camouflage it can still see.
Fishes that live in surface waters down to about 200 metres, epipelagic fishes, live in a sunlit zone where visual predators use visual systems which are designed pretty much as might be expected. But even so, there can be unusual adaptations. Four-eyed fish have eyes raised above the top of the head and divided in two different parts, so that they can see below and above the water surface at the same time. Four-eyed fish actually have only two eyes, but their eyes are specially adapted for their surface-dwelling lifestyle. The eyes are positioned on the top of the head, and the fish floats at the water surface with only the lower half of each eye underwater. The two halves are divided by a band of tissue and the eye has two pupils, connected by part of the iris. The upper half of the eye is adapted for vision in air, the lower half for vision in water.[35] The lens of the eye changes in thickness top to bottom to account for the difference in the refractive indices of air versus water. These fish spend most of their time at the surface of the water. Their diet mostly consists of the terrestrial insects which are available at the surface.[36]
Mesopelagic fishes live in deeper waters, in the twilight zone down to depths of 1000 metres, where the amount of sunlight available is not sufficient to support photosynthesis. These fish are adapted for an active life under low light conditions. Most of them are visual predators with large eyes. Some of the deeper water fish have tubular eyes with big lenses and only rod cells that look upwards. These give binocular vision and great sensitivity to small light signals.[37] This adaptation gives improved terminal vision at the expense of lateral vision, and allows the predator to pick out squid, cuttlefish, and smaller fish that are silhouetted against the gloom above them. For more sensitive vision in low light, some fish have a retroreflector behind the retina. Flashlight fish have this plus photophores, which they use in combination to detect eyeshine in other fish.[38][39][40]
Still deeper down the water column, below 1000 metres, are found the bathypelagic fishes. At this depth the ocean is pitch black, and the fish are sedentary, adapted to outputting minimum energy in a habitat with very little food and no sunlight. Bioluminescence is the only light available at these depths. This lack of light means the organisms have to rely on senses other than vision. Their eyes are small and may not function at all.[41][42]
At the very bottom of the ocean flatfish can be found. Flatfish are benthic fish with a negative buoyancy so they can rest on the seafloor. Although flatfish are bottom dwellers, they are not usually deep sea fish, but are found mainly in estuaries and on the continental shelf. When flatfish larvae hatch they have the elongated and symmetric shape of a typical bony fish. The larvae do not dwell on the bottom, but float in the sea as plankton. Eventually they start metamorphosing into the adult form. One of the eyes migrates across the top of the head and onto the other side of the body, leaving the fish blind on one side. The larva loses its swim bladder and spines, and sinks to the bottom, laying its blind side on the underlying surface.[43] Richard Dawkins explains this as an example of evolutionary adaptation
Prey usually have eyes on the sides of their head so they have a large field of view, from which to avoid predators. Predators usually have eyes in front of their head so they have better depth perception.[46][47] Benthic predators, like flatfish, have eyes arranged so they have a binocular view of what is above them as they lie on the bottom.
The foureye butterflyfish has false eyes on its back end, confusing predators about which is the front end of the fish.
Fish have evolved sophisticated ways of using colouration. For example, prey fish have ways of using colouration to make it more difficult for visual predators to see them. In pelagic fish, these adaptations are mainly concerned with a reduction in silhouette, a form of camouflage. One method of achieving this is to reduce the area of their shadow by lateral compression of the body. Another method, also a form of camouflage, is by countershading in the case of epipelagic fish and by counter-illumination in the case of mesopelagic fish. Countershading is achieved by colouring the fish with darker pigments at the top and lighter pigments at the bottom in such a way that the colouring matches the background. When seen from the top, the darker dorsal area of the animal blends into the darkness of the water below, and when seen from below, the lighter ventral area blends into the sunlight from the surface. Counter illumination is achieved via bioluminescence by the production of light from ventral photophores, aimed at matching the light intensity from the underside of the fish with the light intensity from the background.[48]
Benthic fish, which rest on the seafloor, physically hide themselves by burrowing into sand or retreating into nooks and crannies, or camouflage themselves by blending into the background or by looking like a rock or piece of seaweed.[49]
While these tools may be effective as predator avoidance mechanisms, they also serve as equally effective tools for the predators themselves. For example, the deepwater velvet belly lantern shark uses counter-illumination to hide from its prey.[50]
Some fish species also display false eyespots. The foureye butterflyfish gets its name from a large dark spot on the rear portion of each side of the body. This spot is surrounded by a brilliant white ring, resembling an eyespot. A black vertical bar on the head runs through the true eye, making it hard to see.[51] This can result in a predator thinking the fish is bigger than it is, and confusing the back end with the front end. The butterflyfish's first instinct when threatened is to flee, putting the false eyespot closer to the predator than the head. Most predators aim for the eyes, and this false eyespot tricks the predator into believing that the fish will flee tail first.
The John Dory is a benthopelagic coastal fish with a high laterally compressed body. Its body is so thin that it can hardly be seen from the front. It also has a large dark spot on both sides, which is used to flash an "evil eye" if danger approaches. The large eyes at the front of the head provide it with the bifocal vision and depth perception it needs to catch prey. The John Dory's eye spot on the side of its body also confuses prey, which is then sucked into its mouth.[52]