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. I n the brighter epipelagic depths, daylight provides an extended field of il lumination. In mesopelagic depths down to 1000 m the visual scene is semi-e xtended, 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 exclusivel y of point-source bioluminescent flashes. In this paper, I show that the ey es 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 thei r reputation as 'degenerate' and 'regressed', I show here that the remarkab ly 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 intercepti on) is energetically realistic, with distances greater than this physically beyond range. Larger and more sensitive eyes would give bathypelagic fishe s little more than the useless ability to see flashes beyond reach.