Functional consequences of prey acclimation to ocean acidification for the prey and its predator

Ocean acidification is the suite of chemical changes to the carbonate system of seawater as a consequence of anthropogenic carbon dioxide (CO2) emissions. Despite a growing body of evidences demonstrating the negative effects of ocean acidification on marine species, the consequences at the ecosystem level are still unclear. One factor limiting our ability to upscale from species to ecosystem is the poor mechanistic understanding of the functional consequences of the observed effects on organisms. This is particularly true in the context of species interactions. The aim of this work was to investigate the functional consequence of the exposure of a prey (the mussel Brachidontes pharaonis) to ocean acidification for both the prey and its predator (the crab Eriphia verrucosa). Mussels exposed to pH 7.5 for >4 weeks showed significant decreases in condition index and in mechanical properties (65% decrease in maximum breaking load) as compared with mussels acclimated to pH 8.0. This translated into negative consequences for the mussel in presence of the predator crab. The crab feeding efficiency increased through a significant 27% decrease in prey handling time when offered mussels acclimated to the lowest pH. The predator was also negatively impacted by the acclimation of the prey, probably as a consequence of a decreased food quality. When fed with prey acclimated under decreased pH for 3 months, crab assimilation efficiency significantly decreased by 30% and its growth rate was 5 times slower as compared with crab fed with mussels acclimated under high pH. Our results highlight the important to consider physiological endpoints in the context of species interactions.

海洋酸化（Ocean Acidification）指人为二氧化碳（CO₂）排放所引发的海水碳酸盐系统（carbonate system）发生的一系列化学变化。尽管越来越多的研究证据表明海洋酸化会对海洋物种产生负面影响，但该过程在生态系统层面的影响仍不明确。限制我们从物种层面向上推演至生态系统层面的关键因素之一，是对观测到的生物体效应背后的功能机制认知不足。在物种相互作用（species interactions）的研究背景下，这一问题尤为突出。本研究旨在探究猎物（贻贝Brachidontes pharaonis）暴露于海洋酸化环境后，对其自身及其捕食者（蟹Eriphia verrucosa）所产生的功能层面影响。与适应pH 8.0环境的贻贝相比，暴露于pH 7.5环境超过4周的贻贝，其条件指数（condition index）与力学性能（mechanical properties）均出现显著下降（最大断裂载荷（maximum breaking load）降低65%）。这一变化在捕食者蟹存在的情况下，会对贻贝产生负面影响。当提供适应低pH环境的贻贝作为猎物时，蟹的摄食效率（feeding efficiency）显著提升，这是因为其猎物处理时间（prey handling time）缩短了27%。猎物的酸化驯化也会对捕食者产生负面影响，这可能是由于食物质量下降所致。与喂食适应高pH环境的贻贝的蟹相比，喂食经低pH环境驯化3个月的猎物的蟹，其同化效率（assimilation efficiency）显著下降30%，生长速率（growth rate）仅为前者的1/5。本研究结果凸显了在物种相互作用的研究框架中考虑生理终点指标的重要性。