Changes in visual morphology through life history stages of the New Zealand snapper, Pagrus auratus
Pankhurst, PM and Eagar, R, Changes in visual morphology through life history stages of the New Zealand snapper, Pagrus auratus, New Zealand Journal of Marine and Freshwater Research, 30, (1) pp. 79-90. ISSN 0028-8330 (1996) [Refereed Article]
Plastic resin histology and transmission electron microscopy were used to examine the development of the retina in snapper Pagrus auraius (Bloch & Schneider, 1801) larvae before, and at the onset of feeding. Light microscopy was used to examine changes in ocular morphology with increasing 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-old 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 were undifferentiated in fish larvae at the time of hatching. Development of the eye proceeded rapidly so that at 3 days of age, a pigmented cone retina had formed and the optic nerve connected to the optic rectum. On the fourth day after hatching, the lens had become crystalline, and synaptic ribbons and vesicles were present in the cone cell synaptic junctions, indicating that the retina was now functional. This coincided with the onset of feeding behaviour in the fish larvae. Putative rod precursor cells were present in fish at 18 days of age. Juvenile and adult fish had duplex retinae with cones arranged in a regular mosaic 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 continued to increase with increasing eye size such that theoretical spatial acuity increased from 2°1′ (Minimum Separable Angle, MSA) in a 4-day-old fish, to asymptotic values of between 3′ and 4′ (MSA) in adult fish.