Ophthalmology of Cartilaginous Fish: Skates, Rays, and Sharks

Abstract

The cartilaginous fish or Chondrichthyes are a large and successful group of aquatic organisms, comprising the Elasmobranchs, modern sharks, skates and rays (96% of existing species) and the Holocephali, chimeras and elephant fish making up the remaining 4%. These fish have a long evolutionary history with members of the lineage being present in the fossil record 450 million years ago (Sansom et al. 1996) and over that time the fish have evolved to occupy a wide range of varying ecological habitats. To cope with substantially divergent environments, they possess a sophisticated battery of sensory systems with olfaction as an exceptionally sensitive arm of their interaction with their environment. Indeed, this has led some in the past to term them ‘swimming noses’ (Aronson 1963). Investigation of their sense of smell has led to a smaller degree of attention being paid to their vision and ophthalmic anatomy and physiology with early investigators such as Walls (1942) and Rochon-Duvigneaud (1943) suggesting that their retina was populated only by rods rendering the fish scotopic in their visual behaviour with poor visual acuity and lacking colour vision. Gruber’s discovery in 1963 that the retina of the lemon shark Negaprion brevirostris possessed rods and cones opened a new field of study in chondrichthic vision and now we know that there are shark species with a high retinal cone population (Gruber and Cohen 1978) and that it is only deep-sea species which have retinae populated solely by rods (Bozzano et al. 2001). In fact, the visual system of sharks of the deep (etmopterid and dalatiid species) has developed numerous strategies for photon capture in a very dim environment, including semicircular tapeta and aphakic gaps not previously shown to exist in elasmobranchs (Claes et al. 2014). Work by Collin’s group more recently has led to a much deeper understanding of the eye in cartilaginous fish and the role of vision in their behaviour (Collin 2018).