Effects of hypoxia and anoxia on the reproductive performance of the blue mussel Mytilus edulis

Coastal hypoxia poses a growing threat to marine biodiversity, significantly impacting the survival and reproductive success of sessile benthic organisms such as mussels. While much research has focused on the physiological responses of adult mussels to hypoxia, the effects on their reproductive processes remain underexplored. This study investigates how hypoxia (2 mg O₂ l^-1, with one-week and two-week exposures) and short-term anoxia (<0.1 mg O₂ l^-1, with 24-h and 72-h exposures) affect the reproductive parameters (including sperm motility and metabolism, egg size and number, and fertilization success) of the Baltic Sea blue mussel, Mytilus edulis. Our findings show that prolonged (two-week) hypoxia significantly decreases the spawning response rate of adult mussels and leads to a reduction in sperm velocity by ∼50 % six hours post-spawning. Short-term (24-72 h) anoxia had no effect on the spawning rate or the sperm velocity. However, sperm mitochondrial ATP synthesis capacity and coupling efficiency diminished after 72 h of anoxic exposure, while hypoxia exposure had no effect on these traits. Increased reactive oxygen species production in sperm mitochondria was noted under both hypoxic and anoxic conditions. Maternal investment strategies, as indicated by the relationship between egg size and number, did not show significant changes following exposure to hypoxia and anoxia. However, one week of hypoxia exposure was associated with the release of larger eggs by female mussels. Despite the negative impact of low oxygen conditions on spawning rates and sperm quality, no decrease in fertilization success was observed, possibly due to fertilization being conducted under normoxic conditions. Our data suggest that a reduced spawning rate from prolonged hypoxia could disrupt spawning synchronization, challenging natural reproduction. While sperm from males pre-exposed to hypoxia and anoxia maintained their ability to fertilize eggs under normoxia, impaired sperm quality may affect fertilization success under low oxygen conditions, warranting further investigation.