Comparative effects of cadmium, mercury, zinc, copper, and lead on sperm motility parameters of the endangered sistan snowtrout (Schizothorax zarudnyi Nikolskii, 1897) (cypriniformes: cyprinidae)

Sperm motility is a critical indicator of semen quality and reproductive success in fish. This study provides a comparative assessment of the short-term in vitro effectsof five potentially toxic metals—cadmium (Cd), mercury (Hg), zinc (Zn), copper (Cu), and lead (Pb)---on sperm motility parameters of the endemic Sistan snowtrout ( Schizothorax zarudnyi ). Sperm samples from six male broodstock were exposed to 0 (control), 0.1, 1, 10, and 100 mg/L of each metal chloride. Total motility duration and the percentage of motile sperm at 80%, 40%, and 10% motility levels were recorded using light microscopy and video analysis. All metals significantly reduced motility duration and percentage in a concentration-dependent manner ( p < 0.05). At 100 mg/L of metal chloride, Cd and Hg caused complete and immediate sperm immobilization, whereas Zn, Cu, and Pb significantly reduced but did not completely abolish motility. The order of toxicity based on effective concentration was: Hg ≈ Cd > Pb > Cu > Zn. Sperm sensitivity increased over time: even at 0.1 mg/L, all metals significantly reduced the duration during which only 10% or 40% of sperm remained motile, whereas differences at 80% motility were smaller. These in vitro findings indicate that environmentally relevant concentrations of heavy metals can severely impair sperm quality in S. zarudnyi , under laboratory conditions, potentially reducing reproductive success in contaminated habitats. Our results provide essential baseline data for conservation management of this endemic species in the Hamoun International Wetland ecosystem.