Reflexion of light by external surfaces of the herring, Clupea harengus
Denton, E.J. and Nicol, J.A.C. (1965) Reflexion of light by external surfaces of the herring, Clupea harengus. Journal of the Marine Biological Association of the United Kingdom, 45 (3). pp. 711-738.
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The orientations of the reflecting layers in the external surfaces of the herring have been found both by light-measurements on the fresh fish and by histological studies on preserved specimens. The reflecting platelets which lie under the scales are orientated, with respect to the surface of the fish, in a similar way to the platelets found in the bleak and described earlier by Denton & Nicol (1965). However on the curved dorsum of the fish, although the reflecting platelets are much more perpendicular to the sea surface than the scales on which they lie, these platelets are still inclined some 20 degrees to the perpendicular. It is shown that, in this region, the fish reflects the fraction of the light striking the platelets which is sufficient to match the background against which the fish is seen. The platelets on the curved dorsum have the property of reflecting green light well if it falls obliquely on them but refelecting it poorly when it strikes them at angles close to normal incidence. On the broad flank of the herring the scales have reflecting platelets under most of their surfaces, and the individual scale has several distinctly coloured regions. When we look at any particular region of the flank of an intact fish we are always looking at several overlapping layers differing greatly from one another in their spectral reflecting properties. It is the combination of the reflexions of several layers which gives the very bright silvery reflexions of the intact fish. A system of overlapping scales of this kind is needed even to reflect one waveband of light well over a wide range of angles of incidence. The constitution of the platelets was studied. It was shown that no matter what the colour of the platelet the individual crystals of which it is composed are white. It is deduced from observations on the crystals that they are approximately 0.07 microns thick. It is shown that the dark dorsal surface of the fish has roughly the reflectivity required to match the light field against which it is viewed by the sea. This surface has several overlapping layers of melanophores which will allow it to have a very low reflectivity. However, it contains irridescent layers which seem to be arranged so as to make the sideways scattering of light greater than the upward scattering of light. The camouflaging of a silvery fish such as the herring is compared with that of a countershaded fish and it is shown that the herring is only silhouetted against a brighter light field when viewed by an observer almost directly beneath the fish. Silvery surfaces are of greater importance to a shoaling fish like the herring than to a solitary fish.
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