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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 Müller 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. This research was supported by the University of Alicante VIGROB-186.

The environment and lifestyle of a species are known to exert selective pressure on the visual system, often demonstrating a tight link between visual morphology and ecology. Many studies have predicted the visual requirements of a species by examining the anatomical features of the eye. However, among the vast number of studies on visual specializations in aquatic animals, only a few have focused on small benthic fishes that occupy a heterogeneous and spatially complex visual environment. This study investigates the general retinal anatomy including the topography of both the photoreceptor and ganglion cell populations and estimates the spatial resolving power (SRP) of the eye of the Mediterranean triplefin Tripterygion delaisi. Retinal wholemounts were prepared to systematically and quantitatively analyze photoreceptor and retinal ganglion cell (RGC) densities using design-based stereology. To further examine the retinal structure, we also used magnetic resonance imaging (MRI) and histological examination of retinal cross sections. Observations of the triplefin’s eyes revealed them to be highly mobile, allowing them to view the surroundings without body movements. A rostral aphakic gap and the elliptical shape of the eye extend its visual field rostrally and allow for a rostro-caudal accommodatory axis, enabling this species to focus on prey at close range. Single and twin cones dominate the retina and are consistently arranged in one of two regular patterns, which may enhance motion detection and color vision. The retina features a prominent, dorso-temporal, convexiclivate fovea with an average density of 104,400 double and 30,800 single cones per mm2, and 81,000 RGCs per mm2. Based on photoreceptor spacing, SRP was calculated to be between 6.7 and 9.0 cycles per degree. Location and resolving power of the fovea would benefit the detection and identification of small prey in the lower frontal region of the visual field.