Identification of 17,20β,21-trihydroxy-4-pregnen-3-one (20β-S) receptor binding and membrane progestin receptor alpha on southern flounder sperm (Paralichthys lethostigma) and their likely role in 20β-S stimulation of sperm hypermotility

► Show direct action of progestins to increase sperm motility for the first time in a non-Sciaenid fish. ► We present evidence that membrane progestin receptor alpha mediates this phenomenon. ► Data suggest that direct progestin actions are of widespread physiological significance in fishes.