One type of fish that starts with the letter "X" is the X-ray Tetra, known for its translucent skin that makes its backbone visible, according to OneKind, an animal protection charity. The fish also is called the Golden Pristella Tetra, in reference to its golden-colored skin.
The X-ray Tetra replicates quickly because female fish lay up to 400 eggs. The eggs hatch in as little as 24 hours, and the new fish swim freely in a few days. The X-ray Tetra's skeletal structure identifies sound waves, allowing the fish to hear what is happening around them. The fish feed on insects, worms and small crustaceans.