Climate related size shifts in aquatic species: mechanism, prediction and mitigation

体型几乎与动物生活的方方面面都有关系，无论是新陈代谢、繁殖还是生存。同样，食物网的结构、竞争、捕食者-猎物的相互作用和种群生产力都可能受到体型的影响。由于这些原因，体型通常被描述为一种“主要特征”，一个多世纪以来，物种体型的变化一直吸引着生物学家。一个物种成熟时的大小可以根据环境而变化，气候变化导致的动物大小和生物群落大小光谱的变化可能会对全球生态和经济产生影响。在变温动物中，同一物种的个体在温暖的环境中通常比在寒冷的实验室中饲养时长得更小。这种近乎普遍的生物现象，被称为温度-尺寸规则（TSR），发生在80%以上的异温物种中，从细菌到鱼类和两栖动物。在自然界中也可以看到类似的体型模式；较大的物种通常在高纬度的寒冷地区发现，而成年的体型已经被证明在一年的周期中随着温度的变化而随季节变化，因为后代在生长和发育过程中经历了不同的环境条件。预计到本世纪末，全球平均气温将上升2摄氏度以上，体型缩小被认为是对气候变暖的第三种普遍反应。随着气候变暖，水生物种的体型缩小比生活在空气中的物种大得多。这归因于氧气的可用性，在水中比在空气中更受限制。因此，水生物种在温暖的环境中努力满足它们的代谢需求，成年后长得更小被认为是一种适应性反应。此外，与温度无关的氧可用性的降低也显示了成熟时尺寸的减小。在世界各海洋区域和淡水系统中，脱氧的地理范围和严重程度正在增加，预计在未来几十年将显著恶化。显然，气候变暖加上氧气浓度的降低对水生物种构成了双重威胁。目前迫切需要量化、了解、预测和制定缓解战略，以应对水生生态系统中由变暖和氧气引起的体型变化。我们提出的研究旨在通过解决以下关键问题来解决这些问题：温度和氧气浓度的变化如何影响生态和经济上重要但代表性不足的水生物种的体型，以及适应低氧和高氧可用性环境的水生物种是否以不同的方式调整其体型？近几十年来，水生物种的体型是如何随着温度和氧气供应而变化的？这些反应是否与在实验室、不同季节和纬度观察到的模式相似？我们不能依靠实验室和季节的估计来预测未来的规模变化。描述自然种群几十年来的体型变化是关键的下一步，重要的是，这将提高我们预测的准确性和可靠性。与温度和氧气诱导的体型变化强度相关的最重要特征（如摄食模式、繁殖策略、死亡风险）是什么？我们能否利用这些信息准确预测未来的体型变化？变暖带来的体型缩小是否能完全补偿在更高温度下增加的代谢需求？这将如何影响在更温暖的条件下从食物转移到肉的总生产力和效率？我们能否利用这些信息促进水产养殖和渔业的知情决策？

Body size is linked to nearly all aspects of an animal's life, be this metabolism, reproduction or survival. Similarly, the structure of food webs, competition, predator-prey interactions and population productivity can all be influenced by body size. For these reasons, body size is often described as a 'master trait', and variation in the size of species has fascinated biologists for over a century. The size at which a species matures can change depending on the environment, and shifts in the size of animals and size-spectra of biological communities as a result of climate change are likely to have worldwide ecological and economic impacts. In ectotherms, individuals of the same species regularly grow to a smaller adult body size in the warm than in the cold when reared in the laboratory. This near-universal biological phenomenon, known as the Temperature-Size Rule (TSR), occurs in over 80% of ectothermic species, from bacteria to fish and amphibians. Similar patterns in body size have also been seen in nature; larger species are often found at higher colder latitudes, whilst adult body size has been shown to vary seasonally with temperature over an annual cycle, as subsequent generations experience different environmental conditions during growth and development. With average global temperatures predicted to rise by more than 2 degrees Celsius by the end of this century, reduced body size has been described as the third universal response to climate warming.Size reduction with warming is much greater for aquatic species than for species living in air. This has been attributed to oxygen availability, which is much more limiting in water than in air. Consequently, aquatic species struggle most to meet their metabolic demands in the warm, and growing to a smaller adult size is thought to be an adaptive response to cope. In addition, reduced oxygen availability independent of temperature has also been shown to decrease size at maturity. Deoxygenation is increasing in geographic extent and severity in regions of the world's oceans and in freshwater systems, and is predicted to significantly worsen over the coming decades. Clearly, climate warming combined with reductions in oxygen concentrations present a double jeopardy to aquatic species. There is an urgent need to quantify, understand, predict and develop mitigation strategies to deal with warming and oxygen-induced changes in body size in aquatic ecosystems. Our proposed research aims to tackle these issues by addressing the following key questions:Q1. How do changes in temperature and oxygen concentration influence body size in ecologically and economically important but under-represented aquatic species, and do aquatic species adapted to environments with low and high oxygen availability adjust their size differently?Q2. How have body sizes changed in aquatic species in relation to temperature and oxygen availability over recent decades? Are these responses similar to patterns observed in the laboratory and across seasons and latitudes? We cannot rely on laboratory and seasonal estimates to predict future shifts in size. Describing body size changes over decades in natural populations is a critical next step, and importantly, will increase the accuracy and reliability of our predictions.Q3. What are the most important traits (e.g. feeding mode, reproductive strategy, mortality risk) associated with variation in the strength of temperature- and oxygen-induced body size change, and can we use this information to accurately predict body size change in the future?Q4. Does body size reduction with warming fully compensate for increased metabolic demand at higher temperatures, and how might this affect the total productivity and efficiency of transfer from food to flesh that can be supported in warmer conditions? Can we use this information to contribute to informed decision making in the aquaculture and fisheries industries?