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Sandra J. Raredon / Smithsonian Institution

The elongated body, characteristic long and narrow snout, and small teeth make the slender snipe eel (Nemichthys scolopaceus) easily identifiable in this X-ray image. Snipe eels live at great depths in the ocean: this specimen was collected at over 2,000 meters (more than a mile down) in the North Atlantic. They swim with their mouths open, feed on invertebrates, and have over 700 vertebrae—more than any other animal.

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.