The spottail darter may attain a length of about three inches. The male is larger than the female. The lateral line is incomplete, and the cheek is without scales or with only a few scales behind the eye. The back and sides are brown, with dark saddles on the back and dark mottling on the upper sides. The belly is white. A black bar can be seen in front of, below and behind each eye. Three dark spots are present at the base of the tail fin. The second dorsal and tail fins each have black bands, and the pectoral fins have light bands. The breeding male has small, white knobs on the dorsal fin spines, a black head and a white body between the head and second dorsal fin followed by seven to eight dark bars on a gray background. This fish has a life span of about three years.
The spottail darter may be found in the southern one-third of Illinois. This fish lives in creeks and rivers. It spawns in spring. Females begin spawning at one year of age, while males rarely spawn until their second year. The eggs are placed on the bottom of a stone in the stream. Several females may spawn with the same male in one nest, but only one female is present in the nest at any time. A nest may contain nearly 1,500 eggs. The male guards the eggs during the incubation period of five to 11 days. The spottail darter eats insects and crustaceans.