There are four species of sirens, all found in the southern United States and northern Mexico. Sirens are fully aquatic, with external gills and streamlined bodies. They have no hind limbs. Siren species can be relatively small to large, from 10 to 90 centimeters long. They live in shallow, slow-moving waters and eat invertebrates that they suck into their mouths. Sirens are considered the most primitive group of salamanders still living.
Tanya Dewey (author), Animal Diversity Web.