Pacifastacus fortis (known as the Shasta crayfish or placid crayfish) is an endangered crayfish species endemic to Shasta County, California, where it is found only in isolated spots on the Pit River and Fall River Mills. The exact subpopulations for the Shasta Crayfish were discovered in 2004 through a genetic study that determined three different genetic clusters: JeShe, Lava, and Spring Creeks, Thousand Springs, and Big Lake Springs.