CHANGES IN VISUAL MORPHOLOGY THROUGH LIFE-HISTORY STAGES OF THE NEW-ZEALAND SNAPPER, PAGRUS-AURATUS

Plastic resin histology and transmission electron microscopy were used to examine the development of the retina in snapper Pagrus auratus (B loch & Schneider, 1801) larvae before, and at the onset of feeding. Li ght microscopy was used to examine changes in ocular morphology with i ncreasing body size, in larval, juvenile, and adult P. auratus. There was a 96-fold increase in eye size, from 0.23 mm diameter in a 4-day-o ld larva (3.42 mm standard length), to a maximum of 22 mm diameter in an adult of 333 mm fork length. Cells of the presumptive optic cup wer e undifferentiated in fish larvae at the time of hatching. Development of the eye proceeded rapidly so that at 3 days of age, a pigmented co ne retina had formed and the optic nerve connected to the optic tectum . On the fourth day after hatching, the lens had become crystalline, a nd synaptic ribbons and vesicles were present in the cone cell synapti c junctions, indicating that the retina was now functional. This coinc ided with the onset of feeding behaviour in the fish larvae. Putative rod precursor cells were present in fish at 18 days of age. Juvenile a nd adult fish had duplex retinae with cones arranged in a regular mosa ic in which four equal double cones surrounded a central single cone. Cone ellipsoid diameter increased during eye enlargement to maintain a closely packed array in fish of all sizes. Angular density of cones c ontinued to increase with increasing eye size such that theoretical sp atial acuity increased from 2 degrees 1' (Minimum Separable Angle, MSA ) in a 4-day-old fish, to asymptotic values of between 3' and 4' (MSA) in adult fish.