Environmental salinity-induced shifts in sperm motility activation in Fundulus grandis

Motility activation of fish sperm typically responds to levels of specific ions or osmotic pressure differences between the surrounding water and body tissues. In general, the sperm of marine fishes are activated by an increase in osmotic pressure (hypertonic salinity), and that of freshwater species by a decrease (hypotonic salinity). These stenohaline species exist in relatively stable environments, however, estuarine fishes are exposed to rapidly changing and broad salinity ranges, often resulting in external osmotic pressures that include those of the body (isotonic). To assess the ability of Fundulus grandis sperm to adapt to changes in salinity, adult males were acclimated to salinities of 0, 5, 10, 20, 35, or 50 ppt and held for 30 d. The testes were dissected from the fish and sperm were activated with deionized water, various osmolalities (100–1000 mOsmol/kg) of Hanks' balanced salt solution (HBSS), calcium-free HBSS (Ca2 +-Free HBSS), and sodium chloride solution (NaCl). The deionized water did not activate sperm motility regardless of the acclimated salinity. Compared to HBSS, Ca2 +-Free HBSS and NaCl activated sperm motility with a significantly lower percentage at the same osmolalities. The osmolality eliciting the highest motility activation was significantly different (P < 0.01) among acclimated groups and shifted from 300 mOsmol/kg (ranging from 200 to 500) for sperm collected from 5 ppt, 500 mOsmol/kg (ranging from 200 to 800) for sperm collected from 10 ppt, 600 mOsmol/kg (ranging from 400 to 700) for sperm collected from 20 ppt, 800 mOsmol/kg (ranging from 200 to 900) for sperm collected from 35 ppt, and 900 mOsmol/kg (ranging from 600 to 1000) for sperm collected from 50 ppt. Motility peaked after 30 s exposure to HBSS, and decreased over 10 min. Motility exhibited a similar initial pattern when exposed to Ca2 +-Free HBSS, however, the sperm gained motility at lower osmolalities over 10 min, exhibiting multiple peaks. These results indicate that environmental salinity can significantly influence sperm behavior in adult males of F. grandis with substantial changes after only 30 d of acclimation. As such, this should be considered as a major unrecognized variable in sperm research in this species and can be considered for use in optimizing protocols addressing in-vitro fertilization and cryopreservation. Whether this phenomenon is unique to Fundulus or is a characteristic of euryhaline fishes remains unresolved.