ABSTRACT
Unlike the continuous and extended nature of terrestrial visual environments, the visual world of midwater plankton consists primarily of point objects, either bioluminescent sources contrasting against a dark background or dark silhouettes seen against the bright background of downwelling day-light. The eye design of planktonic invertebrates reveals a number of unusual specializations exploiting this unique visual environment. Some eyes provide large-field general vision, which is useful in bright shallow habitats and suits the detection of bioluminescence at any depth. Other eyes have narrow visual fields specialized for spotting objects against the dim downwelling day-light at considerable depths (down to 1000 m). A relation between depth and eye design is particularly evident in some crustaceans such as euphausiids and hyperiid amphipods: the deeper the habitat, the more the eye is dedicated to upward vision. The presence of upward-looking predators is the obvious reason for several types of camouflage reducing the size or contrast of the body silhouette. General transparency is one such adaptation which even involves a special type of transparent compound eye occurring in several crustacean groups.
