X-ray fish is a small carnivorous fish that lives in the lower reaches of the Amazon River, including brackish (slightly salty) waters. It gets its name from its almost transparent body. The abdomen area is the only part covered with silvery white skin. The X-ray fish has a dark spot on the shoulder, and on the dorsal and anal fins. The tail fin has a reddish tinge. Male X-ray fish tend to be thinner than the females.