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P.E. Malkki, E. Löfblad, R.H. Kröger; Species-Specific Differences in the Optical Properties of Crystalline Lenses of Fishes . Invest. Ophthalmol. Vis. Sci. 2003;44(13):3483.
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Purpose: Observations by Matthiessen (1882) had suggested that the optical properties of most fish lenses are similar. Recently (Kröger et al., 1999) it was found that the lens of the African cichlid fish Haplochromis burtoni has three focal lengths. Each focal length creates a well-focused image for one of the spectral cone types, despite considerable longitudinal chromatic aberration of the lens. Since spectral sensitivity varies widely among fish species, we studied in detail the optical properties of another fish species, the South American cichlid fish Aequidens pulcher, which also has a trichromatic cone system. Methods: We used eccentric infra-red photorefractometry to investigate the optical properties of the eyes in living fish. We furthermore measured longitudinal spherical aberrations in excised A. pulcher lenses by scanning thin laser beams through meridional planes. The results from laser scans were used in model calculations to determine the focusing properties of the lenses. Results: The lenses of 15 fish were studied by photorefractometry and laser-scanning. Photorefractive images from H. burtoni and A. pulcher eyes suggested differences in the optical properties of the lenses. The results from laser scans and subsequent model calculations revealed that the lens of A. pulcher also has three focal lengths. However, the spacings between the focal points is different in H. burtoni lenses. Furthermore, about equal amounts of light are concentrated on each focal point of H. burtoni lenses. In contrast, A. pulcher lenses focused considerably more light on the central focal point than on the other two. The H. burtoni lens has three focal lengths because of negative longitudinal spherical aberration of complex shape. The longitudinal spherical aberration of the A. pulcher lens is equally complex in shape, but positive. The mean focal length of the lens is shorter in A. pulcher than in H. burtoni. Conclusions: Photorefractometry was a reliable indicator for diffences in the lenses of H. burtoni and A. pulcher. The multiple focal lengths of fish lenses seem to be specifically adapted to the needs of each species. There seem to be few evolutionary limitations on how several focal lengths of fish lenses can be achieved by specifically tuned longitudinal spherical aberration.
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