Hatchetfish are deep-sea fish found in the Pacific, Atlantic, and Indian oceans. They use a form of bioillumination to mimic light intensity from the surface, effectively cloaking it from poor-sighted predators. Creepy, but cool.
Larval Leaf Scorpionfish (photograph by Chris Newbert, Minden Pictures). Lacking any other defense, many larval fish have adapted transparency as a method of camouflage—such as this tiny, see-through larval leaf scorpionfish in Hawaii.