Eye Design and Vision in Deep-Sea Fishes

Abstract

The three great faunal zones of the deep-sea—the twilight mesopelagic zone, the numbingly dark bathypelagic zone, and the vast flat expanses of the benthic zone—are home to an enormous variety of fishes. In each of these zones, the light environment is unique and highly predictable. In the mesopelagic zone (150–1,000 m), the downwelling daylight creates an extended scene that becomes increasingly dimmer with depth. Bioluminescent point-source flashes, on the other hand, become increasingly visible. Upon entry to the bathy-pelagic zone at 1,000 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 eyes and visual capacities of deep-sea fishes. So too has the wide and dim horizon seen by benthic fishes as they cruise over the ocean floor. An exploration of these effects is the topic of this review. The eyes of fishes living at different depths in all three zones are compared, and their eye designs related to what is known of their natural histories. This analysis leads to the conclusion that the intensity, direction, and particularly the spatial organization of light into extended and point-like sources have been the dominant players in the evolution of deep-sea visual systems.