THE BLUE COLORATION OF THE COMMON SURGEONFISH, PARACANTHURUS-HEPATUS - II - COLOR REVELATION AND COLOR CHANGES

Measurements of spectral reflectance from the sky-blue portion of skin from the common surgeonfish, Paracanthurus hepatus, showed a relative ly steep peak at around 490 nm. We consider that a multilayer thin-fil m interference phenomenon of the non-ideal type, which occurs in stack s of very thin light-reflecting platelets in iridophores of that regio n, is primarily responsible for the revelation of that hue. The struct ural organization of the iridophore closely resembles that of bluish d amselfish species, although one difference is the presence of iridopho res in a monolayer in the damselfish compared to the double layer of i ridophores in the uppermost part of the dermis of surgeonfish. If comp ared with the vivid cobalt blue tone of the damselfish, the purity of the blue hue of the surgeonfish is rather low. This may be ascribable mainly to the double layer of iridophores in the latter since incident lightrays are complicatedly reflected and scattered in the strata. Th e dark-blue hue of the characteristic scissors-shaped pattern on the t runk of surgeonfish is mainly due to the dense population of melanopho res, because iridophores are only present there in a scattered fashion . Photographic and spectral reflectance studies in vivo, as well as ph otomicrographic, photoelectric, and spectrometric examinations of the state of chromatophores in skin specimens in vitro, indicate that both melanophores and iridophores are motile. Physiological analyses discl osed that melanophores are under the control of the sympathetic nervou s and the endocrine systems, while iridophores are regulated mainly by nerves. The body color of surgeonfish shows circadian changes, and be comes paler at night; this effect may be mediated by the pineal hormon e, melatonin, which aggregates pigment in melanophores.