Clearly, Steve was looking-his big hooded eye followed me, and a single five-foot-long arm reached out to the hand I held above the water's surface. doi: 10.1242/ biologist Roland Anderson of the Seattle Aquarium pulled back the tank's lid, I wasn't sure whether it was to let me get a look at Steve or to let Steve get a look at me. Eye-independent, light-activated chromatophore expansion (LACE) and expression of phototransduction genes in the skin of Octopus bimaculoides. Ramirez and Oakley also plan to compare opsins from the skin and eyes of different species, in order to see how they are related, and to determine whether these non-visual light responses co-opted existing opsins, or evolved independently. The fact that opsins are present in mechanically sensitive cells suggests they have a common and ancient role in these processes. It’s still not entirely clear whether octopus chromatophores act as light sensors, mechanical receptors, or both, but Ramirez and Oakley are planning to find out, in a series of new experiments designed to determine what kind of behaviours they are involved in. This is supported by recent studies showing that opsin is present in the fruit fly antenna, where it detects mechanical vibrations, and is critical for hearing. This raises the intriguing possibility that opsins, which have always been associated with vision, might also contribute to other senses. The researchers also noted that the chromatophores in their skin preparations expanded in response to light touch as well as to light, and their antibody staining experiments revealed that they are expressed in the neurons that are sensitive to mechanical pressure. In chromatophores, they are arranged loosely, and so light-sensitive skin would probably detect changes in brightness, rather than forming a detailed image. In eyes, opsins are arranged in an organized manner inside photoreceptor cells, so that they can recreate a faithful copy the visual field on the retina. Ramirez and Oakley believe that octopus skin acts in a similar way. These primitive creatures continue to avoid light after being decapitated, suggesting that the opsins found outside their brain are indeed involved in sensing light. Other research shows that the marine ragworm, a “living fossil” with primitive eyes consisting of patches of opsin-containing cells at the front of its brain, also expresses the same opsin protein in neurons located on the underside of its nerve cord, and in the hair-like appendages it uses to crawl and swim. But this study provides the first clear evidence that octopus skin is also sensitive to light, and also hints at a plausible mechanism by which chromatophores detect and respond to it. Octopuses aren’t unique in this respect, as various other species are now known to have skin that contains opsins and is sensitive to light. Sure enough, they found that sensory neurons in the skin synthesize one version of the opsin protein, along with G protein alpha and phospholipase C, two enzymes that relay signals from opsin molecules that have been activated by light to the interior of the cell, and which are needed to initiate the cellular response. To test this, they stained some skin preparations with fluorescently-labelled antibodies that recognise and bind to opsins and other proteins that interact with them. Ramirez and Oakley therefore predicted that opsins are present in octopus skin, where they might act as light sensors. This also happens to be the wavelength that some opsins, the pigmented light-sensitive proteins found in eyes, absorb best. In these, experiments, the chromatophores were most responsive to wavelengths of 480 nanometers (nm, or billionths of a meter), which corresponds to blue light. By contrast, red light caused slow, rhythmic muscle contractions, but not chromatophore expansion. They noticed that the chromatophores expanded quickly, and remained expanded, pulsating rhythmically, when exposed to continuous bright white light. Experiments performed in the 1960s showed that chromatophores respond to light, suggesting that they can be controlled without input from the brain, but nobody had followed this up until now.Įvolutionary biologists Desmond Ramirez and Todd Oakley of the University of California, Santa Barbara therefore removed patches of skin from 11 hatchling and adult bimac octopuses ( Octopus bimaculoides), mounted them onto Petri dishes with insect pins, and used light emitting diodes to shine light of different wavelengths onto the skin preparations. Despite apparently being colour blind, they use their eyes to detect the colour of their surroundings, then relax or contract their chromatophores appropriately, which assume one of three basic pattern templates to camouflage them, all within a fraction of a second. Octopuses are thought to rely mainly on vision to bring about these colour changes.
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