Description: E. nana is an aquatic salamander. It does not leave the water to change into a terrestrial form, but rather matures in the water. As a neotenic form, E. nana retains its gills for its lifetime. The San Marcos salamander has a narrow head with a round snout, large eyes, and a dark iris. External gills are developed and pigmented. 2 to 6 palatopterygoid and 7 to 13 premaxillary teeth are present. The species is uniformly light brown in color, with a series of seven to nine irregular light spots present along its midline. The trunk is flattened above with a dorsal furrow that extends from head to tail. The venter, the bottom of a salamander, is white in color, and males have larger vents than females do. There are 16 to 17 costal grooves present, with 6 to 7 occurring between limbs. It has four toes on its forefeet and five on its hind feet. The size of the species was measured to be 2.2 inches total in length.

Habitat: Clear, flowing spring water of the San Marcos River makes a well-delineated hydrologic system for the San Marcos Salamander. The springwater maintains a temperature of 70–72 °F; the salamanders appeared to be stressed at waters over 86 °F. Critical thermal maximum temperatures of the species show a lower threshold for juveniles than adults. E. nana are often found along the river substrates, such as rocks and vegetation. The uppermost shallow portion of Spring Lake features sand, gravel, and large limestone boulders that provide habitat. Further down, concrete banks and boulders provide space for aquatic moss such as L. riparium and blue-green algae. A variety of aquatic macrophytes, including S. platyphylla, M. brasiliense, L. repens, and V. americana are also present. Substrates without vegetation and muddy slit areas are unsuitable for E. nana, as those serve as protective covers against predation from larger fish, turtles, and aquatic birds.