Environmental Adaptations in the Retina of the Magellanic Penguin: Photoreceptors and Outer Plexiform Layer

Abstract

-The retina of the Magellanic Penguin (Spheniscus magellanicus) was studied with light and electron microscopy. Rods, double and single cones were observed. Rods showed a uniform density over the entire retina, whereas cones gradually decreased from center to periphery. Cone-to-rod ratio was 2.0 in the center and 1.1 at the margin of the retina. Rods had a large outer segment (26.6 ? 2.8 pm long, and 2.1 ? 0.4 pm wide). About 80% of the cones were double. Principal members had a yellowish-green oil droplet and their outer segment was larger than that of rods (27.2 ? 2.5 gtm, and 3.3 ? 0.6, tapering to 2.2 ? 0.3 pm wide). The accessory member had a short 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 correlated with increased sensitivity of the retina, perhaps as an adaptation to low illumination levels in deep oceanic waters. Their large diameter also implies that image focusing in the Magellanic Penguin eye would not be very sharp. However, the structure of the outer plexiform layer suggests that the penguin retina would be adapted to the extreme changes in illumination and contrast conditions encountered by diving penguins. Received 20 September 1998, accepted 25 January 1999.