Swamp Darter

This species can reach a length of 5.9 cm (2.3 in), though most are only about 4 cm (1.6 in). It has a narrow, laterally compressed body with a rounded head and a blunt, conical snout which does not extend past the upper lips. The lateral line curves slightly upwards towards the tail. In color they are green to tan dorsally with small dark saddles and dark green and brown mottling. There are 10–12 square-shaped markings on the flanks. The underparts are white to yellow with numerous black and brown speckles. There is a narrow suborbital bar and 3 dusky black spots on the caudal fin. The rays in the fins are marked with small spots and the spiny part of the male's dorsal fin normally has a dark band at its base and another dark submarginal band. When breeding, the males become darker and tubercles develop on the soft rays of the pelvic and anal fins, the spine in the pelvic fin and frequently grow on the second spine of the anal fin.

The swamp darter has a wide distribution in the lowlands of the eastern United States, it is generally found below the fall line on the Atlantic and Gulf coastal plains from southern Maine to the Sabine River, Louisiana and the Red River of the South in southeastern Oklahoma. It is also found in the tributaries of the Mississippi River as far north as Kentucky and southeastern Missouri. An introduced swamp darter population is established in the French Broad River system in North Carolina. In New York State the swamp darter only occurs in the Peconic River and in a small number of ponds in the vicinity of that river in eastern Long Island, the swamp darter also extends up the Red River of the South drainage as far as North Texas and Southeastern Oklahoma, where it is rare. No other darter is found as far south as E. fusiforme.

The swamp darter was first formally described in 1854 as Boleosoma fusiforme by the French biologist Charles Frédéric Girard (1822–1895) with the type locality given as a tributary of Charles River at Framingham, Massachusetts. It has been placed in the subgenus Hololepsis or in the larger Boleichthys. There are two recognised subspecies, although many have been proposed in the past:

E.f. barretti: from the Pee Dee River in South Carolina and including the southern and western parts of its range E.f. fusiforme: the northern Atlantic coastal range from the Waccamaw River, North Carolina as far as Maine The specific name means "spindle shaped" or tapering at each end.