Vision in Tunas and Marlins

To obtain a fundamental knowlege of the vision of tunas and marlins the brains were observed morphologically, and the retinae were examined histologically and electrophysiologically. Tunas and marlins have an extremely well developed optic tectum indicating that vision is very important to them. The S-potentials recorded from the retinae showed that tunas and marlins are probably color-blind. The retinae has cones (predominantly twin cones) in parallel arrangement, 3 layers of horizontal cells, no fovea nor well-developed area. Dense retinal tapetum was found in the bigeye tuna. Visual acuity determined from cone density ranged from 0.3 to 0.5, which is the highest among fishes. It is believed that the retinae of these fishes are advantageous for movement perception, and that their eyes are well adapted to the low light levels in the lower limits of their vertical distribution (ca. 500m). Sight feeding was confirmed by experimental fishing by longlines with inedible artificial lures