The Maryland darter (Etheostoma sellare) is a species of freshwater ray-finned fish, a darter from the subfamily Etheostomatinae, part of the family Percidae, which also contains the perches, ruffes and pikeperches. It is considered one of the rarest freshwater fish species in the world, due in part to its incredibly limited geographic range and difficulty of detection. The last sighting of one was in 1988. The Maryland darter is named after the only state in which it is known to occur. The species was long known only by two specimens until being "re-discovered" in 1962. From 1965 into the 1980s, the species was believed to have been confined to a single riffle in Deer Creek. Possible explanations for the decline of the species center around widespread habitat degradation and reduction in water quality resulting from increasing rates of urbanization within the watershed. While the IUCN has declared the species extinct, the United States Fish and Wildlife Service has not, and keeps it on the Endangered Species List. History The Maryland darter was originally discovered in 1912 in Swan Creek, a tributary of the larger Susquehanna River drainage basin located near Havre de Grace, Maryland. The two biologists responsible for the discovery, Lewis Radcliffe and William W. Welsh, promptly published a description of the newly discovered species in an early scientific journal in 1913. Despite initial detection of the species in Swan Creek, it was never found there again. However, in 1962 the Maryland darter was "re-discovered" after an adult female specimen was collected in nearby Gasheys Run, a tributary of Swan Creek. Perhaps the most significant detection of the Maryland darter occurred in 1965, when over 70 individuals were collected from Deer Creek. Additional sightings of the species in Deer Creek occurred irregularly between 1974 and 1988, where between 1 and 10 individuals were detected by seining sampling or snorkeling. However, the last significant sighting of the Maryland darter occurred in 1988. It has not been seen since, and is believed to be extinct. This species is the only member of the subgenus Mooreichthys. Characteristics The Maryland darter is a member of the genus Etheostoma and a member of the darter family of fishes. A relative of the yellow perch (Perca flavescens) and walleye (Sander vitreus), the Maryland darter is a bottom-dwelling fish. Ideal habitat consists of highly oxygenated, swiftly flowing portions of streaming containing moderate amounts of vegetation and