Tropical hatchetfish (Argyropelecus lychnus), like the one shown in this X-ray photograph, live in the dark depths of the ocean; this specimen was collected at about 2,789 feet (850 meters) in the Pacific. As with many other deep-sea fishes, their eyes are large and their bellies have numerous small cream-colored spots (or “light organs”) that are bioluminescent. Each species emits its own unique pattern of light. Scientists in the Division of Fishes at the Smithsonian's National Museum of Natural History use X-ray images, like the one shown, to study the complex bone structure and diversity of fish without having to dissect or damage the specimen.
In 2012, the National Museum of Natural History hosted "X-Ray Vision: Fish Inside Out," a temporary exhibit that showcases fish evolution and diversity through 40 black and white X-ray images prepared for research purposes. See more photos from the exhibit.