Tiny ocean creatures known as sea sapphires perform a sort of magic trick as they swim – one second they appear in splendid iridescent hues of blue, purple or green, and the next they may turn invisible (at least the blue ones turn completely transparent).How do they get their bright colors and what enables them to “disappear”? New research at the Weizmann Institute of Science in Rehovot has solved the mystery of these colorful, vanishing creatures, which are known scientifically as Sapphirinidae. The findings, which recently appeared in the Journal of the American Chemical Society, could inspire the development of new optical technologies.Sapphirinidae belong to a subclass of crustaceans called copepods that live in fresh or salt water. These animals are barely visible to the human eye, ranging from around one to several millimeters in length. It is the male sea sapphires that display striking, iridescent colors, whereas the female is transparent. Scientists think that their unique magic trick could help the creatures escape predators when necessary, but still display their flashy colors when a female of the species – or possibly another male – is nearby.The scientists, Profs. Lia Addadi, Steve Weiner, Dvir Gur and Maria Pierantoni of Weizmann’s structural biology department; Prof. Dan Oron and Ben Leshem of the institute’s physics of complex systems department; and Dr. Viviana Farstey of the Interuniversity Institute for Marine Sciences in Eilat, investigated the makeup of a crystal layer on the backs of male Sapphirinidae of several species. They first measured the reflectance, which determines the color, and then, using a microscope technique called cryo-SEM, observed the organization of the crystals along with the cellular material holding them in place.These colors are due to iridescence – the result of light reflecting off periodic (repeating) structures.These multilayer reflectors – a type of structure known to scientists as a photonic crystal – are composed of thin, transparent crystals of guanine, which is known to geneticists as one of the nucleic acid bases found in DNA.The research group found that the guanine plates in sea sapphires are stacked in incredibly precise periodic arrays. Each species has a unique color. Their analysis revealed that the main factor determining whether an animal will be yellow, blue or purple is the spacing between plates, which is controlled by the thin layer of cellular material separating them. The researchers also showed how this complex arrangement of plates enables some Sapphirinidae to disappear from sight: When certain species of males rotate their backs to the light at a 45-degree angle as they perform a spiral swimming maneuver, the wavelength of the reflected light is shifted out of the visible light range and into the invisible ultraviolet. In contrast, when light hits straight on, the beautiful blue color returns. In the ocean’s light, which comes from above, the tiny creature can control its visibility, from neon to none, just by adjusting its rudder.The spacing between the plates acts as a sort of “tuning” for the wavelength of the light, and thus the organism’s color. The closer the plates are to one another, the shorter the wavelength, that is, the bluer the light, reflected from them. This sophisticated strategy for manipulating light, say the scientists, could be used in the design of artificial photonic crystal structures – nanoscale structures that can manipulate the flow of photons.These could have many potential uses, including adaptive or changeable reflective coatings, optical mirrors and optical displays.NEW WEBSITE CAN IDENTIFY BIRDS USING PHOTOS In a breakthrough for computer vision and bird watching, researchers and bird enthusiasts have made it possible computers to achieve a task that stumps most humans – identifying hundreds of bird species pictured in photos.The bird photo identifier, developed by the Visipedia research project in collaboration with the Cornell ornithology lab and capable of recognizing 400 of the most common birds in North America, is available for free at AllAboutBirds.org/photoID.Considering the fact that more than 500 million migratory birds pass through Israel, this technology has relevance here.Called Merlin Bird Photo ID, the identifier is capable of recognizing 400 of the mostly commonly encountered birds in North America. “It identifies the bird correctly in the top three results about 90 percent of the time, and it is designed to keep improving the more people use it,” said Jessie Barry at the lab. “That’s truly amazing, considering that the computer vision community started working on the challenge of bird identification only a few years ago.” To see if Merlin can identify the bird in your photo, you upload an image and tell Merlin where and when you took it. To orient Merlin, you draw a box around the bird and click on its bill, eye and tail; Merlin does the rest. Within seconds, it looks at the pixels and combines powerful artificial intelligence techniques with millions of data points from humans, then presents the most likely species, including photos and sounds.“Computers can process images much more efficiently than humans; they can organize, index and match vast constellations of visual information, such as the colors of the feathers and shapes of the bill,” said computer science Prof. Serge Belongie. “The state of the art in computer vision is rapidly approaching that of human perception, and with a little help from the user, we can close the remaining gap and deliver a surprisingly accurate identification.”Merlin’s success relies on collaboration between computers and humans. The computer learns to recognize each species from tens of thousands of images identified and labeled by bird enthusiasts. It also taps in to more than 70 million sightings recorded by birders in the eBird.org database, narrowing its search to the species found at the location and time of year when the photo was taken.Because the photo identifier uses machine learning techniques, it has the potential to improve the more people use it. Once it can reliably identify photos taken with smartphones, the team will add it to the Merlin Bird ID app, a free app that has helped users with more than one million bird identifications by asking them five questions.