The eyes of deep-sea fishes and the changing nature of visual scenes with depth.

The visual scenes viewed by ocean animals change dramatically with depth. In the brighter epipelagic depths, daylight provides an extended field of illumination. In mesopelagic depths down to 1000 m the visual scene is semi-extended, with the downwelling daylight providing increasingly dim extended illumination with depth. In contrast, greater depths increase the prominence of point-source bioluminescent flashes. In bathypelagic depths (below 1000 m daylight no longer penetrates, and the visual scene consists exclusively of point-source bioluminescent flashes. In this paper, I show that the eyes of fishes match this change from extended to point-source illumination, becoming increasingly foveate and spatially acute with increasing depth. A sharp fovea is optimal for localizing point sources. Quite contrary to their reputation as 'degenerate' and 'regressed', I show here that the remarkably prominent foveae and relatively large pupils of bathypelagic fishes give them excellent perception and localization of bioluminescent flashes up to a few tens of metres distant. In a world with almost no food, where fishes are weak and must swim very slowly this range of detection (and interception) is energetically realistic, with distances greater than this physically beyond range. Larger and more sensitive eyes would give bathypelagic fishes little more than the useless ability to see flashes beyond reach.