Evolution of Vertebrate Eyes: a study on lens suspension and optical proterties

Abstract: Many fishes have well-developed visual systems with color vision capabilities. The crystalline lens is the major refractive element in a fish eye since the cornea is optically inactive underwater. A typical fish eye has a large pupil and a lens of short focal length, features that increase light gathering ability. They lead also to short depth of focus, which makes the eye particularly susceptible to defocus. The focal length of the crystalline lens depends on the wavelength of light. Multifocal lenses that compensate for the resulting chromatic defocus are common in teleosts and tetrapods. Such a lens focuses different wavelengths by specific zones. The refractive powers of the lens zones are matched to the spectral sensitivity of the retina, such that the lens acts as a matched filter. My results indicate that this kind of lens is present in the most basal vertebrates with well-developed eyes, the lampreys (Paper II), and all other basal fish groups investigated, such as lungfishes (Paper II), sharks, rays, sturgeons (Paper III), bichirs, and gars (Paper IV). Detailed vision undisturbed by body movements requires exact positioning and robust stabilization of the lens. Accommodative adjustments of focus are particularly beneficial if the eye has short depth of focus. Different types of lens suspension and accommodative mechanism have been described in vertebrates. However, the information on phylogenetically basal vertebrates available in the classical literature is incomplete and has recently been shown to be erroneous with regard to teleosts. I have identified three types of lens suspension and accommodative mechanism. The lamprey/sarcopterygian type was found in lampreys and lungfishes (Paper II). It has evolved further in the terrestrial sarcopterygians (tetrapods) that are able to accommodate by changing lens shape. The chondrichthyan type of lens suspension and accommodation by lens protraction occurs in sharks and rays (Paper III). Surprisingly, it was found also in sturgeons (Paper III) that are usually classified as Actinopterygii. All other actinopterygians investigated so far (bichirs, gars, and teleosts) possess a different type of lens suspension and accommodate by lens retraction (Paper IV). Except for sturgeons, the distribution of lens suspension types and accommodative mechanisms is consistent with the most generally accepted phylogeny of vertebrates. It is as yet unresolved whether sturgeons (Chondrostei, Acipenseriformes) have undergone an evolutionary reversion with respect to these characters or are incorrectly classified as Actinopterygii.