Elasmobranch visual optics

Abstract

The teleost eye, which is commonly used as a model of the typical aquatic eye, is described as having a spherical lens of elevated equivalent refractive index and an accommodative ability involving lens movement toward the retina. The question of lens aberrations, particularly longitudinal spherical aberration, is important because the lens is virtually the only refractive element of an aquatic eye and because the iris is not mobile, at least not in most teleosts. Thus, the typical teleost eye operates at low f-stop levels at all times and peripheral portions of the lens are not masked by the iris. In comparison to the teleost eye, the optical characteristics of the elasmobranch eye have been subject to much less scrutiny. Nevertheless, the data available make it clear that the elasmobranch eye is a much more diverse optical system. First, there are wide variations in lens shape. In some species the lens is spherical or nearly spherical in shape (e.g., Squalus acanthias and Raja eglanteria) while in others the axial diameter of the lens is significantly less than the equatorial one (e.g., Dasyatis sayi and Negaprion brevirostris). Second, the elasmobranch pupil varies extensively in size, shape, and rate of change. The effect of this variation on lens aberrations, particuarly longitudinal spherical aberration, has yet to be evaluated. Third, while the question of elasmobranch accommodation is still obscure, the few studies that have addressed this point indicate that certain species can accommodate and others cannot. To summarize, current confusion concerning the optical characteristics of the elasmobranch eye is due to a diversity which makes generalization difficult. It reflects a more general uncertainty regarding the quality and importance of vision in this group.