Video: Solving the mystery of the invisible 'sea sapphire'

Male “sea sapphires” are a study in extremes. While swimming in tropical and subtropical waters around the world, the ant-sized crustaceans, known as Sapphirina copepods, blaze between brilliant shades of color and near invisibility. Different species produce a range of iridescent hues—from a vivid blue to red to gold—when light is reflected off their transparent bodies at certain angles. Scientists had known that microscopic layers of crystal plates, which are arranged in a honeycomblike pattern, in the skin cells of the copepods’ back helped them pull off the optical trick. Now, researchers have measured the light reflectance of several different-colored male species and compared the thickness of their crystal plate layers with the cytoplasm nested in between. They found that changes in the thickness of the cytoplasm layers, not the crystal layers, determined what color was reflected. Whereas the stacks of crystal among the different shades of copepods were all about 70 nanometers thick, the cytoplasm spacing ranged between 50 and 200 nanometers, the researchers report this month in the Journal of the American Chemical Society. And for one species, they discovered that light hitting the crustacean at a 45° angle caused the reflectance to shift out of the visible range into the ultraviolet, rendering the animal almost invisible (as in video). The findings could lead to developments in optical technologies like reflective coatings and optical mirrors. Here’s hoping that includes the invisibility part, too.

(Video credit: American Chemical Society)

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