Environmental adaptations in the retina of the Magellanic Penguin: Photoreceptors and outer plexiform layer

The retina of the Magellanic Penguin (Spheniscus magellanicus) was studied with light and electron microscopy. Rods, double and single cones were obse rved. Rods showed a uniform density over the entire retina, whereas cones g radually decreased from center to periphery. Cone-to-rod ratio was 2.0 in t he center and 1.1 at the margin of the retina. Rods had a targe outer segme nt (26.6 +/- 2.8 mu m long, and 2.1 +/- 0.4 mu m wide). About 80% of the co nes were double. Principal members had a yellowish-green oil droplet and th eir outer segment was larger than that of rods (27.2 +/- 2.5 mu m, and 3.3 +/- 0.6, tapering to 2.2 +/- 0.3 mu m wide). The accessory member had a sho rt outer segment and was devoid of an oil droplet. Single cones had either a transparent or orange oil droplet and a very long and thin outer segment. The large size of rod and principal cone outer segments might be correlate d with increased sensitivity of the retina, perhaps as an adaptation to low illumination levels in deep oceanic waters. Their large diameter also impl ies that image focusing in the Magellanic Penguin eye would not be very sha rp. However, the structure of the outer plexiform laver suggests that the p enguin retina would be adapted to the extreme changes in illumination and c ontrast conditions encountered by diving penguins.