PORTLAND, OREGON—The tiny anemone shrimp (Ancylomenes pedersoni) has nearly perfected the art of invisibility: Despite having the organs, blood, and other body fluids that make most of us opaque, they have achieved an uncanny, superherolike translucence (above). But they may lose these superhero powers when it matters most, report physiologists today at the annual meeting of the Society of Integrative and Comparative Biology. Researchers had assumed that transparent animals are always clear as glass. But while collecting anemone shrimp, a graduate student on the team noticed that after a few close calls, one shrimp’s body turned cloudy. The more tail flips it did, the more opaque it became. However, the cloudiness went away after a couple of hours. Suspecting that altered blood flow from exercise might be to blame, the researchers measured how much blood was getting into the shrimp’s muscles. They also used electron microscopy to look for structural changes between transparent and opaque muscle. They found that whenever the shrimp made sudden movements, blood flow increased and caused light to scatter, thus making the shrimp visible. Researchers described the difference as akin to that of packed ice and snow. Both substances are made of frozen water and lack any light-absorbing pigment. In ice cubes, each layer bends the light at exactly the same angle, allowing it to pass through easily. But in a snowman, air spaces between the snowflakes cause light to scatter in many different directions at many different angles. All the colors in the incoming light bounce off the snowman, making him look white. The shrimp, when resting, circulates just a small amount of blood to its tail through one major vessel. But when startled, it opens up more blood vessels, allowing blood to surround the muscle fibers. Differences between the way the blood and the fiber scatter light create the “snowman look.” The researchers also discovered that other stresses, such as altering salt levels in the shrimp tanks, cause them to become cloudy as well. Aside from exposing a weakness in the shrimp’s camouflage strategy, the work drives home how challenging it is to achieve transparency in whole animals, as opposed to single tissues like eye lenses.