The sickle darter (Percina williamsi) is a recently identified 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 closely related to a well-known darter, Percina macrocephala. Discovered in 2007 in the upper Tennessee River drainage, the sickle darter is around 90.5 mm (3.56 in) in length and is a slender bodied, freshwater and benthopelagic fish that most likely feeds on small crayfish and mayflies, since it shares a large mouth and long snout with its sister species. The known distribution of the sickle darter is the upper drainage of the Tennessee River of Tennessee, North Carolina and Virginia. It is distinguished from all other Percina darters except its sister species P. macrocephala by a dark suborbital bar and a black bar subtending a medial black spot on the base of the caudal fin. It stays in mostly rocky, sandy, or silty substrates in clear creeks or small rivers in the Appalachian Mountains. The sickle darter spawns in late winter in gravel shoals. It also has large scales which make it different from macrocephala. It is known to be extirpated from most of its home habitat mainly because of agricultural practices that cause siltation and turbidity which causes most populations of the sickle darter to be widely scattered. But where it is found, it can be observed with regularity. Taking all this into consideration, the sickle darter does not have a formal conservation status under any federal or state law although its closely related species is considered a species of concern by the TWRA. Future management plans should include finding more locations of the sickle darter and decreasing further habitat destruction in known distributions by stating it as a fish of concern by state law.

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