Octopus, squid and cuttlefish are renowned for rapid adaptive coloration that is used for a wide range of communication and camouflage. Structural coloration plays a key role in augmenting the skin patterning that is produced largely by neurally controlled pigmented chromatophore organs. While most iridescence and white scattering is produced by passive reflectance or diffusion, some iridophores in squid are actively controlled via a unique cholinergic, non-synaptic neural system. We review the recent anatomical and experimental evidence regarding the mechanisms of reflection and diffusion of light by the different cell types (iridophores and leucophores) of various cephalopod species. The structures that are responsible for the optical effects of some iridophores and leucophores have recently been shown to be proteins. Optical interactions with the overlying pigmented chromatophores are complex, and the recent measurements are presented and synthesized. Polarized light reflected from iridophores can be passed through the chromatophores, thus enabling the use of a discrete communication channel, because cephalopods are especially sensitive to polarized light. We illustrate how structural coloration contributes to the overall appearance of the cephalopods during intra-and interspecific behavioural interactions including camouflage.

Reflecting surfaces of fish are formed of stacks of thin, flat crystals composed of guanine, as the major component, and hypoxanthine, as the minor component. The broad surfaces of these crystals are not, in general, parallel to the surfaces in which they lie in the fish but they are orientated at angles which depend on the function which they serve. The stacks of crystals in different situations also differ in the number and thickness of crystals and in spectral reflectivity. The organization of these crystals is described, in relation to function, for the silvery surfaces of bony fish, the herring and mackerel, for the reflecting tapeta found in the shark and dogfish, for the photophores of the deep-sea hatchet fish and, finally, for the eye of the scallop.

Nautilus macromphalus Sowerby when freshly caught was close to neutral buoyancy having a weight in sea water of about 0–2% of its weight in air. The animals without their shells varied considerably in density but the volume of the shell was an approximately constant fraction of the total volume of the whole animal and whole animals were brought approximately to the same density by havingmore or less liquid inside the chambers of the shell. About 80 % of the gas space in the shell was used to support the weight of the shell itself in sea water.In an adult animal the centre of buoyancy was found to be about 6 mm above the centre of gravity, which made the animal very stable in its natural swimming position, a couple of about 350 g. cm being required to turn it through 90°. The pearly partsof the chamber walls were impermeable to sea water but the chalky and horny siphuncular tubes joining the septal necks were very porous. The most newly formed tenor so chambers were the only ones to contain liquids in appreciable volume and theydid this in diminishing amounts from the newest to the oldest. The watery liquids found within the chambers were always hypotonic to sea water and sometimes markedly so; they contained principally sodium and chloride ions. One animal was in the process of forming a new chamber, this incomplete chamber was completely full of liquidwith an osmolarity close to that of sea water but differing in composition from seawater.

The highly reflecting structures found in the integuments and eyes of fish and cephalopods were studied. In all cases they consist of alternate layers of high and low refractive index ( n ) material (in fish, guanine and cytoplasm) and the high refractive index material is in the form of discrete plates. The highest reflectivity at a given wavelength λ 0 , together with the widest waveband of high reflectivity, would be given if these alternate layers all had an optical thickness of ¼ λ 0 . We have examined the possibility that fish and cephalopods can make ‘ideal' reflectors of this kind. The thicknesses ( t ) of the discrete plates released by scratching the reflecting layers were measured by interference microscopy, and it was found that although the plates from a region of a given colour sometimes varied greatly in surface area, their thicknesses were approximately constant. With one exception the optical thicknesses ( nt ) of the plates found in all the structures studied were between 100 and 200 nm. Many of the reflecting structures are highly coloured and in these there was almost always a good correlation between the wavebands best reflected and four times the optical thick­nesses of the plates which they contained. The most ventral scales in the juvenile sprat were studied in some detail. At normal incidence these scales reflect best a waveband around 720 nm and the guanine crystals which they contain all have optical thicknesses close to one quarter of this wavelength. The changes in colour with angle of viewing, and with changes in osmotic concentration of the medium in which these scales are placed, support the idea that the spaces between the crystals are ¼ λ 0 spaces. These scales have a high reflectivity to the lights penetrating best into the sea at the oblique angles of incidence from which the strongest intensities of daylight fall in life. Qualitative observations on scales from herring and from other regions on the sprat sup­ported the hypothesis that their guanine crystals were also arranged approximately in ¼ λ 0 stacks. Similar conclusions were reached for coloured surfaces found in the skin of the horse mackerel, the iris of the neon tetra, the reflecting tapeta of the dogfish and the spurdog, and in the eye of the squid, Loligo forbesi . In the scales of the roach, Rutilus rutilus , the crystals are of thicknesses which indicate that if in ‘ideal' ¼ λ 0 stacks they would at normal incidence appear red and at oblique incidence green, whilst in fact they are very little coloured. The crystals from the eyes of Callionymus lyra had 4 nt around 300 nm (in the u. v.) yet the reflexions given by piles of these crystals were bronze coloured. Possible explanations of these facts are given. In cephalopods the high refractive index plates are lozenge-shaped, flexible and of refrac­tive index about 1·56.