Throughout the course of natural history, the struggle for existence has locked species in an evolutionary arms race to develop ever more sophisticated means of catching prey or evading predators.
For example, squid and other cephalopods, such as the octopus and cuttlefish, are capable of avoiding detection by predators by changing the colour and patterning of their skin to blend in with their visual background. Other organisms are also capable of changing colour, but none can do so as quickly as the cephalopods, in which the phenomenon occurs almost instantaneously.
Anatomical studies carried out by Lydia Mäthger and Roger Hanlon, of the Woods Hole Marine Biology Laboratory, suggests that squid use a hitherto unknown mode of visual communication. The visual signals enable squid to send messages to each other, but remain hidden from other species. The work is published in Biology Letters (full access to the paper).
Cephalopods have millions of organs called chromatophores (left) in their skin, which regulate the colour changes. Each chromatophore consist of a central cell containing pigment granules. This cell is surrounded by 15-25 radially-arranged muscle fibres, which are innervated by motor neurons whose cell bodies are located in the chromatophore lobes of the cephalopod brain.
Each individual muscle in the chromatophore is innervated by 2-6 nerves, which run along its entire length. The nerves control the degree of contraction of the muscle fibres, which determine the shape of the chromatophore and the spatial distribution of pigment; this, in turn, determines the colour of the patch of skin in which that chromatophore is located. It is now known that that the neural activity mediating the colour changes in some cephalopods is mediated by the transmitters glutamate and 5-HT (serotonin).
In amphibians and crustaceans, chromatophores have a fixed shape, and colour changes are produced by the dispersal or aggregation of pigment granules, which causes lightening and darkening of the skin, respectively. In these species, colour change is under hormonal, not neural, control. Pigment granules are dispersed throughout the cell by moving from its centre along micro- tubules using the kinesin motor; they are caused to aggregate at the centre of the cell by the actions of dynein.
This film clip shows an octopus, which is initailly camouflaged, change colour to make itself visible to the approaching cameraman, before injecting ink at the camera and escaping; the clip is then played in reverse slow motion to show the octopus blending in to its background:
Squid have two layers of skin; the superficial layer contains the chromatophores which control the squid’s colour changes. Underneath this is another layer, containing cells called iridophores, which reflect polarized light (that is, light produced by particles which oscillate in one axis only) to produce the iridescence characteristic of squid. This is accomplished by layers of intracellular proteinaceuos platelets, which are organised parallel to each other. The colour of reflected incident light depends on the distance between the platelets, their orientation, and the angle from which they are viewed.
Mäthger and Hanlon carried out their experiments on the Longfin inshore squid, Loligo pealeii (left).Using a fibre optic spectrometer fitted with polarizing filters, they investigated the reflective properties of skin samples from the squid, which they pinned down in paraffin-coated Petri dishes. They found that the polarization of light reflected by the iridophores remained unaffected while passing through the chromatophores.
Below, the image on the left shows a patch of skin in which yellow chromatophores are retracted; on the right is the same patch of skin with the chrom- atophores expanded over underlying iridophores. Expansion of the chrom- atophores over iridophores alters the wavelength of light emitted by the former, so that it appears brighter; this provides a means by which the squid can modulate its iridescence. Like the chromato- phores which produce colour changes, the iridophores are under neural control, and changes in iridescence and light polarization can occur within fractions of a second.
Most means of visual communication are conspicuous to predators. But, because the visual systems of most organisms are insensitive to polarized light, the reflection of incident light by iridophores may constitute a discrete visual communication channel which is masked from other species. This would enable squid to remain camouflaged from predators while simultaneously sending visual signals to each other. There is some evidence that cuttlefish and crustaceans use polarized light to send signals to each other, and butterfly wings are known to contain polarized patterns which are used for mate recognition.
“Whether signals could also contain information regarding the presence of predators is speculation, but it may be possible,” says Mäthger, who will continue to study how polarized light reflected by the iridophores is transmitted through the upper layer of skin.