Ontogenetic changes in retinal ganglion cell distribution and spatial resolving power in the brown-banded bamboo shark

1 2 The development of the visual system in anamniotic vertebrates is a continual process, 3 allowing for ontogenetic changes in retinal topography and spatial resolving power. We 4 examine the number and distribution of retinal ganglion cells in wholemounted retinae 5 throughout the protracted embryonic development (approx. five months) of a chondrichthyan, 6 the brown banded bamboo shark, Chiloscyllium punctatum from the beginning of retinal cell 7 differentiation (approximately half way through embryogenesis) to adulthood. We also 8 identify and quantify the number of apoptosed cells within the ganglion cell layer to evaluate 9 the contribution of apoptosis to changes in retinal topography. C. punctatum undergoes rapid 10 changes in ganglion cell distribution during embryogenesis, where high levels of apoptosis 11 especially around the retinal periphery, result in relative increases in ganglion cell density in 12 the central retina, which progressively extend nasally and temporally to form a meridional 13 band at hatching. Post-hatching, C. punctatum forms and maintains a horizontal streak, 14 showing only minor changes in topography during growth, with basal levels of apoptosis. 15 The total number of retinal ganglion cells reaches 547,881 in adult sharks, but the mean 16 (3,228 cells mm) and peak (4,983 cells mm) retinal ganglion cell densities are highest 17 around the time of hatching. Calculated estimates of spatial resolving power, based on 18 ganglion cell spacing (assuming a hexagonal mosaic) and assessment of the focal length from 19 cryosections of the eye, increase from 1.47 cycles degree during embryogenesis to 4.29 20 cycles degree in adults. The increase in spatial resolving power across the retinal meridian 21 would allow this species to hunt and track faster, more mobile prey as it reaches maturity. 22