The Physical Basis for Reflective Communication between Fish, with Special Reference to the Horse Mackerel, Trachurus trachurus

Some properties of reflecting structures in the external surfaces of Trachurus trachurus and some other fish are described. These are related to the hypothesis that such structures are useful, especially to schooling fish, for communicating information on relative positions, orientations, and movements between neighbours. In addition to the silvery layers on the main body surfaces, there are: (a) highly silvered patches on the tail, the pectoral fins and the jaws which, in the sea, will become much brighter or darker with any movement such as a tailbeat or mouth opening which changes their orientations in the ambient lightfield, and (b) structures such as the dorsal lateral line which, in the sea, will only appear bright from certain directions. To us, the colours of the ventral flanks change from bright red to blue with direction of viewing. These changes are given by two superposed layers of reflecting platelets which differ in their orientations and have spectral reflectivity curves close to those predicted by A. F. Huxley for interference reflectors which are 'ideal' λ /4 stacks of guanine crystals and cytoplasm. The wavebands best reflected by such platelets move to shorter wavelengths with increasing angle of incidence, also in accord with these equations. At normal incidence, the outer layer of platelets reflects maximally for far-red light which penetrates only a short distance in the sea. Such layers can, however, be useful at oblique angles where they reflect maximally in the yellow and blue. The inner layer of reflectors reflects very strongly in the blue at normal incidence, but reflects in the ultra-violet at oblique angles. Some theoretical studies are made on the ways in which the patterns of reflectivity by single and superposed layers of λ /4 stacks could signal a fish's movements or its position relative to its neighbours.