Echinoderm sperm swimming and fertilization

In an era of climate change, impacts on the marine environment include warming and ocean acidification. These effects can be amplified in shallow coastal regions where conditions often fluctuate widely. This type of environmental variation is potentially important for many nearshore species that are broadcast spawners, releasing eggs and sperm into the water column for fertilization. We conducted two experiments to investigate: 1) the impact of water temperature on sperm swimming characteristics and fertilization rate in sand dollars (Dendraster excentricus) and sea urchins (Mesocentrotus franciscanus) and; 2) the combined effects of multiple stressors (water temperature and pH) on these traits in sand dollars. We quantify thermal performance curves showing that sand dollar fertilization rates, sperm swimming velocities, and sperm motility display remarkably wide thermal breadths relative to red urchins, perhaps reflecting the wider range of water temperatures experienced by sand dollars at our field sites. For sand dollars, both temperature (8, 16, 24°C) and pH (7.1, 7.5, 7.9) affected fertilization but only temperature influenced sperm swimming velocity and motility. Although sperm velocities and fertilization were positively correlated, our fertilization kinetics model dramatically overestimated measured rates and this discrepancy was most pronounced under extreme temperature and pH conditions. Our results suggest that environmental stressors like temperature and pH likely impair aspects of the reproductive process beyond simple sperm swimming behavior.

在气候变化背景下，海洋环境受到的影响包括海水增温和海洋酸化。此类效应在浅海沿岸区域会被进一步放大，而该区域的环境条件往往波动剧烈。这类环境波动对诸多体外产卵物种（broadcast spawners）具有潜在重要意义：此类物种通过将卵子与精子释放至水体中完成体外受精。本研究开展了两项实验以探究：1）水温对沙钱海胆（sand dollars，Dendraster excentricus）和海胆（sea urchins，Mesocentrotus franciscanus）的精子运动特征与受精率的影响；2）多重环境胁迫因子（水温与pH）对沙钱海胆上述性状的联合效应。本研究量化得到的热性能曲线显示，相较于海胆，沙钱海胆的受精率、精子游动速度与精子活力展现出显著更宽的热耐受宽度，这或许反映了沙钱海胆在本研究野外采样区域所经历的水温跨度更广。对于沙钱海胆，水温（8、16、24℃）与pH（7.1、7.5、7.9）均会对其受精率产生影响，但仅水温会影响精子游动速度与活力。尽管精子游动速度与受精率呈正相关，但本研究的受精动力学模型显著高估了实测受精率，且这种偏差在极端水温与pH条件下最为显著。本研究结果表明，水温与pH等环境胁迫因子可能会损害生殖过程中的多个环节，而非仅影响单纯的精子运动行为。