Regulation of sperm motility in Eastern oyster (Crassostrea virginica) spawning naturally in seawater with low salinity.

Oyster aquaculture is expanding worldwide, where many farms rely on seed produced by artificial spawning. As sperm motility and velocity are key determinants for fertilization success, understanding the regulation of sperm motility and identifying optimal environmental conditions can increase fertility and seed production. In the present study, we investigated the physiological mechanisms regulating sperm motility in Eastern oyster, Crassostrea virginica. Sperm motility was activated in ambient seawater with salinity 4-32 PSU with highest motility and velocity observed at 12-24 PSU. In artificial seawater (ASW) with salinity of 20 PSU, sperm motility was activated at pH 6.5-10.5 with the highest motility and velocity recorded at pH 7.5-10.0. Sperm motility was inhibited or totally suppressed in Na+, K+, Ca2+, and Mg2+-free ASW at 20 PSU. Applications of K+ (500 μM glybenclamide and 10-50 mM 4-aminopyridine), Ca2+ (1-50 μM mibefradil and 10-200 μM verapamil), or Na+ (0.2-2.0 mM amiloride) channel blockers into ASW at 20 PSU inhibited or suppressed sperm motility and velocity. Chelating extracellular Ca2+ ions by 3.0 and 3.5 mM EGTA resulted in a significant reduction and full suppression of sperm motility by 4 to 6 min post-activation. These results suggest that extracellular K+, Ca2+, and Na+ ions are involved in regulation of ionic-dependent sperm motility in Eastern oyster. A comparison with other bivalve species typically spawning at higher salinities or in full-strength seawater shows that ionic regulation of sperm motility is physiologically conserved in bivalves. Elucidating sperm regulation in C. virginica has implications to develop artificial reproduction, sperm short-term storage, or cryopreservation protocols, and to better predict how changes in the ocean will impact oyster spawning dynamics.