The winkle Littorina littorea (L.) has camera-type (simple) eyes. Light and electron microscopy were used to generate an accurate geometrical model of the eye, and this was used to predict the eye's visual performance. The lens is spherical with a diameter of 112 μm, and examination of images formed by isolated lenses indicates a mean focal length in water of 126 μm. These images are crisp and apparently aberration-free. This, in addition to a shorter than expected focal length, implicates the presence of a gradient of refractive index in the lens. The lens has a focal length to lens-radius ratio of 2.3, which is close to the ‘Matthiessen ratio’ of 2.5 found in the aplanatic lenses of many other marine animals. The lens is predicted to focus images within the retina only when the eye is submerged in water: in air, images are focused distal to the retina. In the central retina, the inter-receptor angle is 1.8° and the rhabdom diameter is 4 μm (5.5 times larger than the diffraction blur-circle), indicating a retinal array coarser than that necessary to sample all the information contained in the image. In addition, a low F-number (1.2) and the large possible angles of incidence of rays striking the retina (up to 40°) mean that the eye is likely to suffer substantial spreading of light between rhabdoms (which are unshielded) and further degradation of resolution. Possible behavioural roles for the eyes are discussed.

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