Dm. Rowe et Ej. Denton, THE PHYSICAL BASIS FOR REFLECTIVE COMMUNICATION BETWEEN FISH, WITH SPECIAL REFERENCE TO THE HORSE MACKEREL, TRACHURUS-TRACHURUS, Philosophical transactions-Royal Society of London. Biological sciences, 352(1353), 1997, pp. 531-549
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.