Optical parameters of euphausiid eyes as a function of habitat depth

Summary The relationships between habitat depth, eye diameter relative to body length, and the dimensions of rhabdoms and crystalline cones have been examined for 13 species of three oceanic euphausiid genera with habitats ranging from near-surface waters to the deep-sea. Rate of eye growth decreases with depth. Longer rhabdoms may increase the visual sensitivity to point and extended light sources by an eye of a particular size with depth. Larger interommatidial angles suggest that visual acuity decreases at depth. Depth-related changes in euphausiid eyes are considered with respect to the probable roles of vision and bioluminescence in the deep-sea. Unusual features of the eyes of several species are described.