A survey of reflectivity in silvery teleosts
Denton, E.J. and Nicol, J.A.C. (1966) A survey of reflectivity in silvery teleosts. Journal of the Marine Biological Association of the United Kingdom, 46 (3). pp. 685-722.
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A survey has been made of reflecting layers in the integument of selected fishes, including silvery sea lamprey and various silvery and partially translucent teleosts. Lamprey and juvenile rockling have only a stratum argenteum. Other silvery teleosts have a layer of oriented reflecting platelets lying outside the stratum argenteum; these lie more or less parallel to the vertical to the surface of the water. In dace, herring, and salmon parr the platelets on the upper flanks are tipped slightly upwards; but in some pelagic fishes, e.g. the mackerel and gar-fish, the platelets here are tipped downwards. Smelt, sand smelt, half-beaks, found in shallow or coastal waters, are partially translucent, partially silvery. The implications of these arrangements of reflecting layers are discussed. Both in clear oceanic waters, away from the surface, and in shallow or turbid rivers or ponds, light is distributed almost symmetrically about the vertical to the surface, and the greatest intensity is directed downwards. Under these conditions the reflecting layers diminish the visibility of the fish from most fields of view because they reflect light approximately equal to the background light against which the fish is seen. On the lower flanks, when the surface is sloping, the reflected light tends to be spread and to be less intense than the incident light. This is because the projected areas of such flanks are greater from positions below than from corresponding positions above the fish. On the upper flanks, where the surface is inclined upwards, the projected area is less from the reflected than from the incident direction but, because there is loss of light between the platelets, the intensity of reflected light can be no more than that of incident light. Thin fishes, dark above with vertically reflecting sides, are effectivelycamouflaged except for the ventral extremity. The lower flanks of tapering orrounded fishes are well camouflaged by reflexion from below and behind because the platelets slope inwards towards the tail, and the projected area ofincidence relative to reflexion is thereby increased. In transparent or partially transparent fishes, opaque tissues such as eyes, gut and red muscle are camouflagedby treating each as a unit which is dark above and silvery on the sides andbelow. Downwardly inclined plates on the upper flanks of epipelagic fishes such as mackerel operate so as to render the fish less conspicuous when the light is coming from above and to the side, but the camouflage effectiveness is reduced. Migratory fish, such as salmon smolt and freshwater eels, assume the pelagic pattern, becoming darker above and more silvery on their flanks, when theyforsake the rivers for the sea. In fish where orientated reflecting platelets lie in the subdermis and not under the scales, silveriness is, as in the herring, the result of the combined reflectivities of several superposed layers which differ from oneanother in spectral reflectivity. These layers of platelets also usually differ from one another in the angles which they make with the surface of the fish and the beautiful play of colours which we see, for example on handling a freshly caught mackerel, comes about because we see, in succession, reflexions from their differently coloured layers. The platelets are most transparent at normal incidence and light transmitted by them is reflected by the underlying stratum argenteum.
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