ONTOGENIC CHANGES IN RETINAL STRUCTURE AND VISUAL-ACUITY - A COMPARATIVE-STUDY OF CORAL-REEF TELEOSTS WITH DIFFERING POSTSETTLEMENT LIFE-STYLES

Changes in retinal structure during settlement were investigated in fo ur species of tropical reef-associated teleost fishes with differing p eriods of planktonic duration and post-settlement lifestyles. They wer e: Apogon doederleini (Apogonidae), a nocturnal planktivore; Stethojul is strigiventer (Labridae), a diurnal microcarnivore; Upeneus tragula (Mullidae), a carnivore which uses chin barbels to disturb invertebrat es from the sediment; and Pomacentrus moluccensis (Pomacentridae), a d iurnal herbivorous planktivore. The densities of cones, rods, cells in the inner nuclear layer and cells in the ganglion cell layer were est imated in a size range of each species. Visual acuity was calculated u sing cone densities and lens diameter. The ontogenetic sequence of cha nges in cell density was similar in all species but interspecific vari ation in the timing and rates of change was found and could be related to lifestyle. For example, cone densities decreased and rod densities increased most rapidly in the nocturnal species, A. doederleini, duri ng settlement. In contrast, high cone densities were maintained in the species adopting a diurnaI lifestyle. Theoretical visual acuity was f ound to increase rapidly as lens size increased, but was similar for a ll species at similar lens sizes, indicating the importance of larger eye size as a means for improving resolution during early stages of ey e growth. It was concluded that for the species undergoing abrupt:life style changes at settlement, structural re-organisation of the retina is important for the survival of the fish as they leave the pelagic en vironment and take up their reef-associated lifestyle.