An eel-like, permanently aquatic salamander with external gills, small eyes, small forelimbs with four toes, and no hind limbs at all. The 3 pairs of external gills are red or grayish red and have a bushy appearance. Body color varies from dark gray to brown to almost black. The belly is lighter than the back. Tiny dark brown or black flecks or spots are usually scattered over the back. There are 31 to 38 costal grooves (vertical grooves on the sides of the body). Sirens produce a large amount of mucus on their skin. This, plus their ability to wriggle and squirm, makes them almost impossible to hold. They do not bite and are completely harmless to humans.
Similar species: The three-toed amphiuma (Amphiuma tridactylum) lacks feathery external gills and has fore- and hind limbs; in Missouri, its range is limited to the Bootheel. Both of these salamanders can be distinguished from eels and lampreys (which are fish) by their limbs with fingers, tiny eyes, and lack of fins and scales.