Certain creatures living at levels down to about 1000 m show modifications for vision in very dim illumination (Herring, 1996). The eyes of some deep-water fish are large and have exceptionally wide pupils, and, like many nocturnal land vertebrates, the light-sensitive cells of the retina are all rods. These rod cells are unusually long, and numerous rods supply each fibre of the optic nerve, an arrangement thought to form a retinal surface of extremely high sensitivity to light. The visual pigments (Bowmaker, 1976) are also different from those of shallow water fish, giving a maximum light absorption at the shorter wavelengths of the bluer light of deep ocean levels.
In coastal fish the main retinal pigments are the red-purple rhodopsins with maximum absorption at about 500 m^; but the majority of deep-sea species of the mesopelagic zone have yellow or gold chrysopsins with a maximum absorption around 480 m^ and a much increased quantity of visual pigment in the receptor cells. It is estimated that the eyes of some deep-sea fish are 60 to 120 times as sensitive as human eyes and are able to detect daylight to depths of about 1150 m. In some deep-water species the eyes are tubular structures directed either forwards (Gigantura) or upwards (Argyropelecus, Figure 4.12a). Tubular eyes have a much increased resolving power, and probably also provide a degree of binocular vision to assist perception of distance, a particular difficulty in very dim light. Below 1000 m where darkness is virtually complete, there are fish with eyes which are very small (many ceratioids), degenerate (Bathymicrops regis) or even absent (Ipnops), although in larval stages living near the surface the eyes may be well formed.
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