The fish of the genus Petrotilapia, contain the largest mbuna in Lake Malawi. The fish have mainly tricuspid teeth, that they use to comb through the aufwuchs, pulling off diatomous algae, microorganisms, and loose particles. Plankton is also eaten by Petrotilapia. Territorial males will protect are area that varies between 7 to 20 meters squared against conspecifics, though heterospecifics are mostly ignored.
Males are a blue-black to black, female grey to brown, with a chessboard like melanistic pattern. Males of "nigra" group are less aggressive than many of the males that inhabit shallower waters.