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 T rachurus trachurus and some other fish are described. These are relate d to the hypothesis that such structures are useful, especially to sch ooling fish, for communicating information on relative positions, orie ntations, and movements between neighbours. In addition to the silvery layers on the main body surfaces, there are: (a) highly silvered patc hes on the tail, the pectoral fins and the jaws which, in the sea, wil l become much brighter or darker with any movement such as a tailbeat or mouth opening which changes their orientations in the ambient light field, and (b) structures such as the dorsal lateral line which, in th e sea, will only appear bright from certain directions. To us, the col ours of the ventral flanks change from bright red to blue with directi on of viewing. These changes are given by two superposed layers of ref lecting platelets which differ in their orientations and have spectral reflectivity curves close to those predicted by A. F. Huxley for inte rference reflectors which are 'ideal' lambda/4 stacks of guanine cryst als and cytoplasm. The wavebands best reflected by such platelets move to shorter wavelengths with increasing angle of incidence, also in ac cord with these equations. At normal incidence, the outer layer of pla telets reflects maximally for far-red light which penetrates only a sh ort distance in the sea. Such layers can, however, be useful at obliqu e angles where they reflect maximally in the yellow and blue. The inne r layer of reflectors reflects very strongly in the blue at normal inc idence, but reflects in the ultra-violet at oblique angles. Some theor etical studies are made on the ways in which the patterns of reflectiv ity by single and superposed layers of lambda/4 stacks could signal a fish's movements or its position relative to its neighbours.