The overall goal of this research was to verify the involvement of membrane progestin receptor alpha (mPRα) in mediating progestin-stimulated sperm hypermotility in the Atlantic croaker and southern flounder. Sperm motility in Atlantic croaker and southern flounder were tested with both the endogenous progestin, 17,20β,21-trihydroxy-4-pregnen-3-one (20β-S) or the selective mPRα agonist, 10-ethenyl-19-norprogesterone (Org OD 02-0). In croaker, the Pi3k/Akt/Pde and ErbB2/Mapk intracellular signaling pathways were examined. The role of mPRα in mediating sperm hypermotility and fertility in southern flounder was also studied. The effects of seasonal hypoxia on sperm motility in croaker were investigated in a field study in the northern Gulf of Mexico in the fall of 2010. Finally, the effects of acidified activator solution (simulating ocean acidification) were studied in the laboratory. In vitro, Org OD 02-0 mimicked the stimulatory actions of 20β-S in inducing sperm hypermotility and intracellular signaling cascades in croaker and flounder sperm, indicating that mPRα is the mediator of progestin signaling in the sperm of these species. In croaker sperm, both the Pi3k/Akt/Pde and ErbB2/Mapk intracellular signaling pathways were shown to be important mediators of progestin-induced sperm hypermotility, suggesting novel functions of G [subscript olf] βγ-subunits in teleost sperm. In flounder sperm, mPRα was shown to be important in mediating sperm hypermotility as only high motility sperm with high expression of mPRα were responsive to progestin stimulation, resulting in higher fertilization success compared to low motility sperm. A single LHRHa injection resulted in increased sperm motility and fertility, associated with an increase in mPRα expression in the sperm plasma membrane. The results also suggest that the mPRα/Acy/cAMP pathway first described in croaker sperm is present in flounder sperm. Field studies of male Atlantic croaker exposed to chronic seasonal hypoxia showed that hypoxia exposure resulted in smaller gonads, lower spermatogenesis, reduced testicular mPRα expression, and in some sites, reduced sperm motility. Studies with croaker sperm using acidified activator solution to simulate ocean acidification indicated that croaker sperm were sensitive to environmental insult. Furthermore, the results suggested that the progestin signaling mechanism is more sensitive to changes in ocean pH levels than the mechanism that controls sperm motility. / text