The Visual Cell Morphology of Pagrus major and Its Adaptive Changes with Shift from Palagic to Benthic Habitats

Abstract

The visual cell morphology in 2-90mm long larvae and juveniles, and adult of the red sea bream Pagrus major was examined histologically. The visual system of the larvae is poorly developed at hatching, but becomes functional in 36 hours. At this time, the eyes are well pigmented, cones are formed, and optic nerves connect with the optic tectum. The visual cells of larvae smaller than 10.6mm are all single cones. At 11mm TL, the single cones fuse to form twin cones, and rods appear at the same time. Red sea bream shifts from a pelagic to a benthic habitat when about 12-15mm long. The observed increase in visual sensitivity at 11mm appears to be a preadaptation to life at greater depths. Likewise, the specialized retinal region for acute vision shifts from temporal to dorso-temporal at a length of 30mm, implying a change in visual axis from fore to lower-fore direction, and an adaptation to the change in diet from planktonic to benthic animals.