[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.

[11] Within the retina, rod cells provide high visual sensitivity (at the cost of acuity), being used in low light conditions.

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.

In a similar manner, fish have a vestibulo-ocular reflex which stabilises visual images on the retina when it moves its tail.

[22] A wide range of fish species has developed and maintained this visual trait throughout evolution, suggesting it is advantageous.

In experimental settings, female guppies spent significantly more time inspecting males with UV-reflective colouring than those with UV reflection blocked.

The prominent role of UV light detection in fish mate choice has allowed the trait to be maintained over time.

For example, juvenile brown trout live in shallow water where they use ultraviolet vision to enhance their ability to detect zooplankton.

[22] The two stripe damselfish, Dascyllus reticulatus, has ultraviolet-reflecting colouration which they appear to use as an alarm signal to other fish of their species.

[31] Some experiments indicate that, by using polarization, some fish can tune their vision to give them double their normal prey sighting distance.

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.

[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.

[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.

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.

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 ...bony fish as a rule have a marked tendency to be flattened in a vertical direction....

[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.

The large eyes at the front of the head provide it with the bifocal vision and depth perception it needs to catch prey.

[52] Barreleyes are a family of small, unusual-looking mesopelagic fishes, named for their barrel-shaped, tubular eyes which are generally directed upwards to detect the silhouettes of available prey.

The dome is tough and flexible, and presumably protects the eyes from the nematocysts (stinging cells) of the siphonophores from which it is believed the barreleye steals food.

[53][54][55] Another barreleye species, the brownsnout spookfish, is the only vertebrate known to employ a mirror, as opposed to a lens, to focus an image in its eyes.

However, some species, including the great white shark (Carcharodon carcharias), do not have this membrane, but instead roll their eyes backwards to protect them when striking prey.

[58] A micro-spectrophotometry study of 17 species of shark found 10 had only rod photoreceptors and no cone cells in their retinas giving them good night vision while making them colourblind.

"[63] The "many eyes effect" is based on the idea that as the size of the group increases, the task of scanning the environment for predators can be spread out over many individuals, a mass collaboration presumably providing a higher level of vigilance.

However, some oceanic predatory fish, such as swordfish and some shark and tuna species, can warm parts of their body when they hunt for prey in deep and cold water.

The highly visual swordfish uses a heating system involving its muscles which raises the temperature in its eyes and brain by up to 15 °C.

The warming of the retina improves the rate at which the eyes respond to changes in rapid motion made by its prey by as much as ten times.

Eyeshine allows fish to see well in low-light conditions as well as in turbid (stained or rough, breaking) waters, giving them an advantage over their prey.

The top part of the iris descends to form a loop which can expand and contract called an iris operculum; when light levels are high, the pupil reduces in diameter and the loop expands to cover the center of the pupil giving rise to a crescent shaped light transmitting portion.

[74] Other senses which can also provide data about location or distant objects include hearing and echolocation, electroreception, magnetoception and chemoreception (smell and taste).