Seawater carbonate chemistry and fertilization rate of Mytilus galloprovincialis

Ocean acidification (OA) poses a major threat to marine organisms, particularly during reproduction when externally shed gametes are vulnerable to changes in seawater pH. Accordingly, several studies on OA have focused on how changes in seawater pH influence sperm behavior and/or rates of in vitro fertilization. By contrast, few studies have examined how pH influences prefertilization gamete interactions, which are crucial during natural spawning events in most externally fertilizing taxa. One mechanism of gamete interaction that forms an important component of fertilization in most taxa is communication between sperm and egg‐derived chemicals. These chemical signals, along with the physiological responses in sperm they elicit, are likely to be highly sensitive to changes in seawater chemistry. In this study, we experimentally tested this possibility using the blue mussel, Mytilus galloprovincialis, a species in which females have been shown to use egg‐derived chemicals to promote the success of sperm from genetically compatible males. We conducted trials in which sperm were allowed to swim in gradients of egg‐derived chemicals under different seawater CO2 (and therefore pH) treatments. We found that sperm had elevated fertilization rates after swimming in the presence of egg‐derived chemicals in low pH (pH 7.6) compared with ambient (pH 8.0) seawater. This observed effect could have important implications for the reproductive fitness of external fertilizers, where gamete compatibility plays a critical role in modulating reproduction in many species. For example, elevated sperm fertilization rates might disrupt the eggs' capacity to avoid fertilizations by genetically incompatible sperm. Our findings highlight the need to understand how OA affects the multiple stages of sperm‐egg interactions and to develop approaches that disentangle the implications of OA for female, male, and population fitness.

海洋酸化（Ocean Acidification, OA）对海洋生物构成重大威胁，在繁殖过程中这一威胁尤为凸显——此时体外释放的配子极易受海水pH变化的影响。因此，诸多针对海洋酸化的研究聚焦于海水pH变化如何影响精子行为及体外受精速率。相较之下，鲜有研究探讨pH如何调控受精前配子互作，而这一过程在多数体外受精分类群的自然产卵事件中至关重要。在多数分类群中，作为受精关键环节的配子互作机制之一，是精子与卵源化学信号间的通讯交流。这些化学信号及其所诱导的精子生理反应，极有可能对海水化学环境的变化高度敏感。本研究以蓝贻贝（Mytilus galloprovincialis）为实验对象，已有研究证实该物种的雌性可通过卵源化学物质，促进遗传相容性匹配雄性的精子成功受精。我们设置了不同海水CO₂（进而影响pH）的处理组，在各组中让精子在卵源化学物质的浓度梯度中游动并开展实验。结果显示：与环境pH（8.0）海水相比，在低pH（7.6）环境中经卵源化学物质诱导游动后的精子，其受精速率显著提升。这一观测到的效应可能对体外受精生物的繁殖适合度产生重要影响，而在众多物种中，配子相容性对繁殖调控发挥着关键作用。例如，精子受精速率的提升可能会破坏卵子对遗传不相容雄性精子的受精规避能力。本研究结果表明，我们亟需厘清海洋酸化如何影响精子-卵子互作的多个阶段，并开发相关研究方法以阐明海洋酸化对雌性、雄性以及种群适合度的潜在影响。