A survey of reflectivity in silvery teleosts

Abstract

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 thevisibility 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 beless intense than the incident light. Thin fishes, dark above with vertically reflecting sides, are effectively camouflaged except for the ventral extremity. The lower flanks of tapering or rounded fishes are well camouflaged by reflexion from below and behind because the platelets slope inwards towards the tail, and the projected area of incidence relative to reflexion is thereby increased.