DIRECTIONAL LIGHT TRANSMISSION IN THE PINEAL WINDOW OF ATLANTIC SALMON (SALMO-SALAR L) MAY BE USED FOR SOLAR ORIENTATION

The pineal window of adult Atlantic salmon (Salmo salar) (fork length 45-55 cm) is highly transparent, apart from a roof-like (mean angle 12 0 degrees +/- 5 degrees), bony structure formed by the pineal plates. On each side of the bony pineal plates there is a white lining which a cts like an optical diffuser. The forward scattering of an incident na rrow laser beam by the pineal plates was observed by an optical fibre and a diode array. The direction for maximum intensity of the scattere d light below the pineal plates is refracted with respect to the incid ence angle, and the half-width of the scattered light intensity is 22 degrees. Furthermore, the maximum transmittance of a light ray, with a n incidence angle mu with respect to the pineal plates, depends upon m u according to a (1 + cos mu) function. Hence, each pineal plate of th e laminar roof grossly receives light from the corresponding hemispher e and projects and assembles the rays on to the pineal organ at an ang le of about 30 degrees relative to the vertical plane along the body a xis. In spite of the light scattering by the pineal window, a polariza tion factor of 0.60 was found at the level of the pineal organ for lin early polarized incident light. The hypothesis is proposed that these properties have ultimately evolved as a directional aid enabling the f ish to orient by the directionality of the sunlight. (C) 1994 Wiley-Li ss, Inc