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Vision in the southern hemisphere lamprey Mordacia mordax: Spatial distribution, spectral absorption characteristics, and optical sensitivity of a single class of retinal photoreceptor

Published online by Cambridge University Press:  01 September 2004

SHAUN P. COLLIN
Affiliation:
Department of Anatomy and Developmental Biology, School of Biomedical Sciences, The University of Queensland, Queensland, Australia
NATHAN S. HART
Affiliation:
Vision, Touch and Hearing Research Centre, School of Biomedical Sciences, The University of Queensland, Queensland, Australia
KATE M. WALLACE
Affiliation:
Department of Anatomy and Developmental Biology, School of Biomedical Sciences, The University of Queensland, Queensland, Australia
JULIA SHAND
Affiliation:
School of Animal Biology, The University of Western Australia, Western Australia, Australia
IAN C. POTTER
Affiliation:
School of Biological Sciences and Biotechnology, Murdoch University, Murdoch, Western Australia, Australia

Abstract

The dorso-laterally located eyes of the southern hemisphere lamprey Mordacia mordax (Agnatha) contain a single morphological type of retinal photoreceptor, which possesses ultrastructural characteristics of both rods and cones. This photoreceptor has a large refractile ellipsosome in the inner segment and a long cylindrical outer segment surrounded by a retinal pigment epithelium that contains two types of tapetal reflectors. The photoreceptors form a hexagonal array and attain their peak density (33,200 receptors/mm2) in the ventro-temporal retina. Using the size and spacing of the photoreceptors and direct measures of aperture size and eye dimensions, the peak spatial resolving power and optical sensitivity are estimated to be 1.7 cycles deg−1 (minimum separable angle of 34′7′′) and 0.64 μm2 steradian (white light) and 1.38 μm2 steradian (preferred wavelength or λmax), respectively. Microspectrophotometry reveals that the visual pigment located within the outer segment is a rhodopsin with a wavelength of maximum absorbance (λmax) at 514 nm. The ellipsosome has very low absorptance (<0.05) across the measured spectrum (350–750 nm) and probably does not act as a spectral filter. In contrast to all other lampreys studied, the optimized receptor packing, the large width of the ellipsosome-bearing inner segment, together with the presence of a retinal tapetum in the photophobic Mordacia, all represent adaptations for low light vision and optimizing photon capture.

Information

Type
Research Article
Copyright
2004 Cambridge University Press

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