Epulopiscium fishelsoni, bacteria that live in the gut of the brown surgeonfish (Acanthurus nigrofuscus), are visible to the naked eye. It was first identified in 1985 and mistakenly classified as a protozoan (mostly aquatic, single-celled organisms; for more about protozoa, see this chapter). Later studies analyzed the organism’s genetic material that showed it to be a bacterium of unprecedented size: 0.015 inches (0.38 millimeter) in diameter—about the size of a period at the end of this sentence.