Salinity and pH affect common bottlenose dolphin (Tursiops truncatus) sperm viability

The female reproductive environment modulates sperm fertilization potential and may exert numerous selective forces on sperm during fertilization. Exploring cetacean (whales, dolphins, and porpoises) sperm responses to variable extracellular salinity and pH, either from seawater exposure or from the vaginal environment, offers insight into the selective pressures exerted on sperm in aquatically-mating mammals. This study examined common bottlenose dolphin (Tursiops truncatus) sperm motility, plasma membrane integrity, morphology, and acrosome integrity upon semen exposure to biologically relevant extracellular salinities (0-40 ppt) and in situ measured dolphin vaginal pHs (5-7.4). Sperm motility and kinematics were assessed by computer-aided sperm analysis (CASA). Sperm plasma membrane integrity, morphology, and acrosome integrity were assessed post-hoc. Dolphin sperm motility and plasma membrane integrity significantly decreased when exposed to solutions ≥ average oceanic salinity (35 ppt). Vaginal pHs were highly variable among individuals (pH 5.04-7.4). Sperm motility appeared optimal at pH 7 and sperm viability decreased significantly at pHs < 6.5. There was more inter-individual variation in sperm motility and kinematics when exposed to pHs < 7 compared to any salinity. Optimal extracellular salinity (8 - 15 ppt) and pH (pH 7) ranges that improve sperm motility and viability can inform dolphin semen processing methods for cryopreservation.