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Morphology and spectral absorption characteristics of retinal photoreceptors in the southern hemisphere lamprey (Geotria australis)

Published online by Cambridge University Press:  26 June 2003

SHAUN P. COLLIN
Affiliation:
Department of Anatomy and Developmental Biology, School of Biomedical Sciences, The University of Queensland, Brisbane 4072, Queensland, Australia
NATHAN S. HART
Affiliation:
Vision, Touch and Hearing Research Centre, The University of Queensland, Brisbane 4072, Queensland, Australia
JULIA SHAND
Affiliation:
Department of Zoology, The University of Western Australia, Nedlands, 6907, Western Australia, Australia
IAN C. POTTER
Affiliation:
School of Biological Sciences and Biotechnology, Murdoch University, Murdoch, Western Australia 6150

Abstract

The morphology and spectral absorption characteristics of the retinal photoreceptors in the southern hemisphere lamprey Geotria australis (Agnatha) were studied using light and electron microscopy and microspectrophotometry. The retinae of both downstream and upstream migrants of Geotria contained two types of cone photoreceptor and one type of rod photoreceptor. Visual pigments contained in the outer segments of these three photoreceptor types had absorbance spectra typical of porphyropsins and with wavelengths of maximum absorbance (downstream/ upstream) at 610/616 nm (long-wavelength-sensitive cone, LWS), 515/515 nm (medium-wavelength-sensitive cone, MWS), and 506/500 nm (medium-wavelength-sensitive rod). A “yellow” photostable pigment was present in the myoid region of all three types of photoreceptor in the downstream migrant. The same short-wavelength-absorbing pigment, which prevents photostimulation of the beta band of the visual pigment in the outer segment, was present in the rods and LWS cones of the upstream migrant, but was replaced by a large transparent ellipsosome in the MWS cones. Using microspectrophotometric and anatomical data, the quantal spectral sensitivity of each photoreceptor type was calculated. Our results provide the first evidence of a jawless vertebrate, represented today solely by the lampreys and hagfishes, with two morphologically and physiologically distinct types of cone photoreceptors, in addition to a rod-like photoreceptor containing a colored filter (a cone-like characteristic). In contrast, all other lampreys studied thus far have either (1) one type of cone and one type of rod, or (2) a single type of rod-like photoreceptor.

Type
Research Article
Copyright
2003 Cambridge University Press

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