title: Reflecting optics in the diverticular eye of a deep-sea barreleye fish (Rhynchohyalus natalensis).
abstract: We describe the bi-directed eyes of a mesopelagic teleost fish, Rhynchohyalus natalensis, that possesses an extensive lateral diverticulum to each tubular eye. Each diverticulum contains a mirror that focuses light from the ventro-lateral visual field. This species can thereby visualize both downwelling sunlight and bioluminescence over a wide field of view. Modelling shows that the mirror is very likely to be capable of producing a bright, well focused image. After Dolichopteryx longipes, this is only the second description of an eye in a vertebrate having both reflective and refractive optics. Although superficially similar, the optics of the diverticular eyes of these two species of fish differ in some important respects. Firstly, the reflective crystals in the D. longipes mirror are derived from a tapetum within the retinal pigment epithelium, whereas in R. natalensis they develop from the choroidal argentea. Secondly, in D. longipes the angle of the reflective crystals varies depending on their position within the mirror, forming a Fresnel-type reflector, but in R. natalensis the crystals are orientated almost parallel to the mirror's surface and image formation is dependent on the gross morphology of the diverticular mirror. Two remarkably different developmental solutions have thus evolved in these two closely related species of opisthoproctid teleosts to extend the restricted visual field of a tubular eye and provide a well-focused image with reflective optics.
authors.affiliation: School of Biological Sciences, University of Bristol, , Woodland Road, Bristol BS8 1UG, UK, Department of Optometry and Visual Science, City University London, , Northampton Square, London EC1V 0HB, UK, Queensland Brain Institute, University of Queensland, , St Lucia, Brisbane, Queensland 4072, Australia, Anatomisches Institut, Universität Tübingen, , Ősterbergstrasse 3, Tübingen 72074, Germany, School of Animal Biology, University of Western Australia, , 35 Stirling Highway, Crawley, Perth, Western Australia 6009, Australia.
authors.forename: J C
authors.initials: JC
authors.lastname: Partridge
authors.forename: R H
authors.initials: RH
authors.lastname: Douglas
authors.forename: N J
authors.initials: NJ
authors.lastname: Marshall
authors.forename: W-S
authors.initials: WS
authors.lastname: Chung
authors.forename: T M
authors.initials: TM
authors.lastname: Jordan
authors.forename: H-J
authors.initials: HJ
authors.lastname: Wagner
mesh_terms: D000818:Animals; D005123:Eye / Q000033:anatomy & histology*; D005399:Fishes / Q000033:anatomy & histology* / Q000502:physiology; D009792:Oceans and Seas; D055620:Optical Phenomena; D014785:Vision, Ocular; D014794:Visual Fields
keywords: Rhynchohyalus natalensis; deep-sea; mirror optics; vision