澳门新永利官方网站:Science : Angry cuttlefish see one another in a different light

 作者:赵咻朊     |      日期:2019-02-28 03:01:00
By Peter Aldhous TO AN octopus or a cuttlefish, the difference between polarised and unpolarised light may be as dramatic as the difference between colour and black-and-white. Biologists Nadav Shashar and Thomas Cronin of the University of Maryland Baltimore County have found that octopuses can use the polarisation of reflected light to distinguish between objects, and that cuttlefish seem to communicate with each other using patterns of polarised light reflected from their bodies. Light consists of electromagnetic waves that can be oriented in any plane. Light that is unpolarised contains waves of all orientations, but the light reflected or transmitted by some objects is dominated by waves of a particular alignment. Biologists have shown that a range of invertebrates, plus some fish and amphibians, are sensitive to the polarisation of the light reaching their eyes. This sensitivity is used by some animals to detect the background “wash” of polarisation in the environment, which helps them to orient themselves in surroundings lacking any obvious landmarks. “The pattern of polarisation varies with the position of the Sun, even on cloudy days,” Shashar explains. However, the fact that animals are sensitive to this background polarisation does not mean that they use their polarisation sensitivity for detailed visual discrimination. Shashar and Cronin reasoned that if any group of animals was capable of detailed polarisation vision it might be the cephalopod molluscs—octopuses and their kin. Uniquely among invertebrates, cephalopods are highly intelligent predators that rely heavily on their vision. Shashar and Cronin took pieces of polarising filter measuring 4 centimetres square and cut out a circle 2 centimetres in diameter from the centre of each one. These circles of filter were replaced either in the same orientation, or rotated through 澳门新永利官方网站 to create a contrast in the polarisation of the light transmitted by the filter’s centre compared to its edges. The researchers rewarded the octopuses with pieces of shrimp each time they touched filters with contrasting centre and edges, and found that they quickly learnt to distinguish between the two kinds. Shashar and Cronin describe these experiments in next month’s Journal of Experimental Biology. What octopuses use their detailed polarisation vision for is still uncertain, but Shashar suspects that it is important for detecting prey, as some of the crabs that they hunt reflect polarised light. Octopuses themselves do not have polarised body patterns, but cuttlefish do, and in subsequent experiments Shashar and Cronin have found that these markings appear to be used in aggressive displays. When male cuttlefish meet, they typically size one another up for a while. Each animal extends a tentacle towards its opponent as part of a display during which the animals’ polarisation patterns are prominent. If neither male backs down, both cuttlefish switch off their polarisation patterns moments before the encounter escalates into violence by altering the orientation of cells in their skin called iridophores. Cuttlefish also respond to their own reflection with aggressive displays, so the Baltimore researchers used mirrors to investigate the importance of the polarisation patterns in aggressive encounters between males. If a depolarising filter was placed in front of a mirror, masking the animal’s polarisation pattern, it completely ignored its reflection. “When you take off the polarisation component, all the aggression disappears,” says Shashar. What the polarisation patterns look like to cuttlefish is unclear, but Cronin suggests that when a cuttlefish is presented with its mirror image reflected through a depolarising filter, it may be like watching a black-and-white television. “If you look at an image in colour,” he says,