The eye of the venomous marine teleost Trachinus vipera with special reference to the structure and ultrastructure of visual cells and pigment epithelium.

The benthic Trachinus vipera occurs to a depth of 50 m and migrates inshore during the summer. It lies buried in the sand with the top of its head exposed. The benthic habitat and predatory behaviour, tuned to function in twilight, are reflected in the structure and ultrastructure of the eye. The eye is well camouflaged by an iridescent cornea and a differentially coloured spectacle. The spontaneous eye movements are either coordinated, for binocularity, or independent, for monocular vision. The latter is aided by the reduction of the blind spot to a thin streak. Consequently the optic nerve is flattened. When leaving the eyeball it becomes folded, which may allow for lengthening. The cones of the duplex retina are arranged in a square mosaic, with a central single cone and identical twin cones. The square mosaic is stabilized by an intricate framework of so-called "fins", and the twin cones do not "twist" into a row pattern in the dark as observed in the diurnal guppy. The twin cones do not extend sclerally in the dark, while the single cones, rods and pigment epithelial processes undergo retinomotor movements. Regional differences are observed: The sensitivity of the dorsal retina is enhanced by a great number of rods, bundled as in deep-sea fish. The lipid granules in the dorsal pigment epithelium may act as a tapetum lucidum, reflecting light back through the retina. A cluster of triple cones in the dorso-temporal region may represent a site for increased sensitivity and/or acute vision. An unusual inclusion body, with a dense core similar to a lipid droplet, and surrounded by membranes, is observed in the dorsal pigment epithelium. The membranes are seen to be continuous with the smooth endoplasmic reticulum, nuclear membrane and myeloid bodies. This body changes diurnally and may be a type of phagosome.