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The opisthoproctid fish Macropinna microstoma occupies lower mesopelagic depths in Monterey Bay and elsewhere in the subarctic and temperate North Pacific. Like several other species in the family, Macropinna has upward-directed tubular eyes and a tiny, terminal mouth. This arrangement is such that in their upright position, the visual field of these highly specialized eyes does not include the mouth, which makes it difficult to understand how feeding takes place. In situ observations and laboratory studies reveal that the eyes of Macropinna can change position from dorsally-directed to rostrally-directed, which resolves the apparent paradox. The eyes are contained within a transparent shield that covers the top of the head and may provide protection for the eyes from the tentacles of cnidarians, one of the apparent sources of the food of Macropinna.
THE upward-looking, tubular eyes of some mesopelagic fishes confer distinct optical advantages within the dim light regime they inhabit. A tubular eye typically has a large-aperture lens for enhanced light gathering, coupled through a cylindrical tube to a densely structured main retina at the base. The tubular shape maximizes light intake and accommodates the increased focal length of a large lens, without the greater structural costs of a large spherical eye. A pair of adjacent tubular eyes improves sensitivity, increases contrast perception, and creates broad binocular overlap of the visual fields to provide accurate depth perception (Munk, 1966; Lythgoe, 1979; Johnson and Bertelsen, 1991; Herring, 2002; Warrant and Locket, 2004). There are also some drawbacks to this high level of specialization. The field of view is significantly reduced when compared with spherical eyes, and while accessory retinal tissues may line the tubes, images in this region are not focused.
Several fish species with tubular eyes have additional structural adaptations that help to compensate for their restricted visual fields. Some scopelarchids have a pad of fibrous light-guides that direct light from the side and below, to the accessory retinal tissue inside the tube (Locket, 1977). Opisthoproctids have retinal diverticula that may provide peripheral sensitivity to bioluminescence, but do not produce images (Munk, 1966; Frederiksen, 1973). Tubular eyes occur in 1 1 fish families and in some nocturnal terrestrial animals as well (Marshall, 1971; Lythgoe, 1979). In many such fishes the eyes are directed upward,...