Foveal shape, ultrastructure and photoreceptor composition in yellow-legged gull, Larus michahellis (Naumann, 1840)

The anatomical and histological features of the foveae in a gull, Larus michahellis, were investigated combining immunofluorescence and transmission electron microscopy. In an area of higher acuity, which extends in a band across the field of view, a central convexiclivate deep fovea was observed near of the pecten oculi. Moreover, a shallow fovea was present in the dorsonasal region. The central deep fovea was characterized by a remarkably and well-excavated pit containing extremely thin cones and a radial displacement of the cone axons and inner retinal layers. Immunofluorescence demonstrated the absence of rods and the presence of violet and green/red sensitive cone opsins. Ultrastructural analyses confirmed the lack of double cones, and specialized Muller cell processes forming a network that stabilizes the foveal structure. This deep central fovea, which provides the highest resolution and colour discrimination, may be used for monocular sideways vision of distant objects and a binocular fixation. The dorsonasal fovea was shallower and all retinal layers were present at the pit. This fovea showed the same composition of photoreceptors than central fovea and could be implied in magnification of a wide monocular part of the retinal image. The bifoveate condition of yellow-legged gull retina would allow the formation of three separate and distinct visual fields: two wide lateral monocular fields mediated by the dorsonasal and the central foveae, and a small binocular field mediated by the central deep fovea depending on ocular movements.