Directional light transmission in the pineal window of Atlantic salmon (Salmo salar L.) may be used for solar orientation
✍ Scribed by Nordtug, Trond ;Berg, Ole Kristian ;Melø, Thor Bernt
- Publisher
- John Wiley and Sons
- Year
- 1994
- Tongue
- English
- Weight
- 903 KB
- Volume
- 269
- Category
- Article
- ISSN
- 0022-104X
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✦ Synopsis
Abstract
The pineal window of adult Atlantic salmon (Salmo salar) (fork length 45–55 cm) is highly transparent, apart from a roof‐like (mean angle 120° ± 5°), bony structure formed by the pineal plates. On each side of the bony pineal plates there is a white lining which acts like an optical diffusor. The forward scattering of an incident narrow laser beam by the pineal plates was observed by an optical fibre and a diode array. The direction for maximum intensity of the scattered light below the pineal plates is refracted with respect to the incidence angle, and the half‐width of the scattered light intensity is 22°. Furthermore, the maximum transmittance of a light ray, with an incidence angle μ with respect to the pineal plates, depends upon μ according to a (1 + cosμ) function. Hence, each pineal plate of the laminar roof grossly receives light from the corresponding hemisphere and projects and assembles the rays on to the pineal organ at an angle of about 30° relative to the vertical plane along the body axis. In spite of the light scattering by the pineal window, a polarization factor of 0.60 was found at the level of the pineal organ for linearly polarized incident light.
The hypothesis is proposed that these properties have ultimately evolved as a directional aid enabling the fish to orient by the directionality of the sunlight. © 1994 Wiley‐Liss, Inc.