title: The eyes of the deep diving hooded seal (Cystophora cristata) enhance sensitivity to ultraviolet light.
abstract: The mammalian visual range is approximately 400-700 nm, although recent evidence suggests varying ultraviolet (UV) extensions in diverse terrestrial species. UV sensitivity may have advantages in the dim, blue light shifted environment experienced by submerged marine mammals. It may also be advantageous when seals are on land as UV is reflected by snow and ice but absorbed by fur, enhancing visual contrast. Here we show that the pelagic hooded seal (Cystophora cristata) has a highly UV permissive cornea and lens. Seals like other carnivores have a tapetum lucidum (TL) reflecting light back through the retina increasing sensitivity. The TL in this seal is unusual being white and covering almost the entire retina unlike that in other carnivores. Spectral reflectance from its surface selectively increases the relative UV/blue components >10 times than other wavelengths. Retinal architecture is consistent with a high degree of convergence. Enhanced UV from a large TL surface with a high degree of retinal convergence will increase sensitivity at a cost to acuity. UV electrophysiological retina responses were only obtained to dim, rod mediated stimuli, with no evidence of cone input. As physiological measurements of threshold sensitivity are much higher than those for psychophysical detection, these seals are likely to be more UV sensitive than our results imply. Hence, UV reflections from the TL will afford increased sensitivity in dim oceanic environments.
authors.affiliation: University College London, Institute of Ophthalmology, London EC1V 9EL, UK Moorfields Eye Hospital, London EC1V 2PD, UK.
authors.forename: Chris
authors.initials: C
authors.lastname: Hogg
authors.affiliation: University College London, Institute of Ophthalmology, London EC1V 9EL, UK Moorfields Eye Hospital, London EC1V 2PD, UK.
authors.forename: Magella
authors.initials: M
authors.lastname: Neveu
authors.affiliation: Department of Arctic and Marine Biology, University of Tromsø, 9037 Tromsø, Norway.
authors.forename: Lars
authors.initials: L
authors.lastname: Folkow
authors.affiliation: Department of Arctic and Marine Biology, University of Tromsø, 9037 Tromsø, Norway.
authors.forename: Karl-Arne
authors.initials: KA
authors.lastname: Stokkan
authors.affiliation: University College London, Institute of Ophthalmology, London EC1V 9EL, UK.
authors.forename: Jaimie
authors.initials: J
authors.lastname: Hoh Kam
authors.affiliation: Department of Optometry and Visual Science, City University London, London EC1V OHB, UK.
authors.forename: Ron H
authors.initials: RH
authors.lastname: Douglas
authors.affiliation: University College London, Institute of Ophthalmology, London EC1V 9EL, UK g.jeffery@ucl.ac.uk.
authors.forename: Glen
authors.initials: G
authors.lastname: Jeffery
keywords: Enhanced Contrast; Hooded Seal; Retina; Ultraviolet