The mottled sculpin may attain a length of two and one-half to three and one-half inches. The male is larger than the female. The stout body has a large head and an incomplete lateral line. The back and sides are brown with dark mottling. Body color tends to match that of the substrate, as this ﬁsh can change colors readily. The belly is white, but the chin is speckled. Two or three dark bars are present on the body under the dorsal ﬁn. Black spots can be seen at the front and back edges of the ﬁrst dorsal ﬁn. The breeding male is dark. His dorsal ﬁn is black with an orange or yellow band.
The mottled sculpin may be found in the northern one-fourth of Illinois. This ﬁsh lives in creeks, rivers and lakes. It is often found in cold-water habitats, such as streams that receive water from springs. The mottled sculpin spawns in spring. The male prepares a nest under a rock and stays with the eggs and young until they leave the nest. The female turns upside down to lay eggs on the roof of the nest chamber. More than one female may lay eggs in the same nest. The eggs hatch after three or four weeks. The mottled sculpin feeds on the bottom, eating aquatic invertebrates and small ﬁshes. It may eat members of its own species.