The deep sea is the largest habitat on earth. Its three great faunal environments--the twilight mesopelagic zone, the dark bathypelagic zone and the vast flat expanses of the benthic habitat--are home to a rich fauna of vertebrates and invertebrates. In the mesopelagic zone (150-1000 m), the down-welling daylight creates an extended scene that becomes increasingly dimmer and bluer with depth. The available daylight also originates increasingly from vertically above, and bioluminescent point-source flashes, well contrasted against the dim background daylight, become increasingly visible. In the bathypelagic zone below 1000 m no daylight remains, and the scene becomes entirely dominated by point-like bioluminescence. This changing nature of visual scenes with depth--from extended source to point source--has had a profound effect on the designs of deep-sea eyes, both optically and neurally, a fact that until recently was not fully appreciated. Recent measurements of the sensitivity and spatial resolution of deep-sea eyes--particularly from the camera eyes of fishes and cephalopods and the compound eyes of crustaceans--reveal that ocular designs are well matched to the nature of the visual scene at any given depth. This match between eye design and visual scene is the subject of this review. The greatest variation in eye design is found in the mesopelagic zone, where dim down-welling daylight and bio-luminescent point sources may be visible simultaneously. Some mesopelagic eyes rely on spatial and temporal summation to increase sensitivity to a dim extended scene, while others sacrifice this sensitivity to localise pinpoints of bright bioluminescence. Yet other eyes have retinal regions separately specialised for each type of light. In the bathypelagic zone, eyes generally get smaller and therefore less sensitive to point sources with increasing depth. In fishes, this insensitivity, combined with surprisingly high spatial resolution, is very well adapted to the detection and localisation of point-source bioluminescence at ecologically meaningful distances. At all depths, the eyes of animals active on and over the nutrient-rich sea floor are generally larger than the eyes of pelagic species. In fishes, the retinal ganglion cells are also frequently arranged in a horizontal visual streak, an adaptation for viewing the wide flat horizon of the sea floor, and all animals living there. These and many other aspects of light and vision in the deep sea are reviewed in support of the following conclusion: it is not only the intensity of light at different depths, but also its distribution in space, which has been a major force in the evolution of deep-sea vision.

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Experiments on intact retinae from the eyes of the deep-sea fishDiretmus argenteusshow that the region of the retina that receives light from above and possesses very long rods has different photosensitive pigments from the region that receives light from below. In both of these regions the retina has several banks of rods. The optical densities of thephotosensitivepigments in the retina at the peaks of absorption are almost the same (about one) in these two regions. However, the region of the retina with very long rods also contains astableyellow pigment that absorbs heavily in the blue and near ultra-violet (optical density about three at the wavelength 390 nm). If, as seems certain, this stable pigment is largely in the long rods it will filter the light reaching the other layers of rods and act, for this region, as yellow lenses do for whole retinae in other fish. The lens ofDiretmuswas shown to be transparent in the visible and near ultra-violet.

Deep or convexiclivate foveas occur in some birds, including raptors, some lizards and certain deep-sea fishes. Theories on their function are reviewed. Common to raptor and deep-sea fish foveas is a radial fibre lining, dark staining so probably optically dense, adjacent to the less refractile vitreous. The foveal curvatures and size are remarkably similar in a wide taxonomic and size range of birds and fishes.Ray plotting through traced foveal outlines suggests that sharp images will be formed beneath the centre and shoulders, with the centre image enlarged enough to account for the high acuity of raptors. Deep foveas will also exaggerate eccentricity of off-centre images of a point source, such as deep-sea fishes may meet.Despite similarities in foveal shape, the receptors differ widely between species. Raptors, and notosudid fishes, have short cones. Searsid fishes have long single rods. Howella and Bajacalifornia have multiple bank rods, more in the fovea than the periphery. Those of Howella are shown to be multiple inner-outer segment complexes rather than interrupted single rods. Implications of foveal and receptor .features are discussed.