Mechanisms and significance of physiological plasticity in winter-dormant fishes: a model to understand constraints on performance in cold environments

冬季的低温和随之而来的食物短缺是鱼类在极地纬度地区生存的关键瓶颈，但对鱼类冬季反应的详细机制理解仍然难以捉摸。鱼类越冬和适应寒冷的研究主要集中在越冬活性物种(如：鲑鱼,鳕科鱼);对许多具有重要生态意义的冬眠物种（如温带濑鱼、太阳鱼）了解甚少。冬季休眠是一种不活动、躲避、不进食和低代谢的状态，以应对严酷的冬季条件。了解冬季休眠很重要：这是一个关键的季节性事件，可能占鱼类寿命的一半，温度对活动和能量预算的限制可能对鱼类种群的健康和持久性产生重大影响。这项拨款的目的是揭示鱼类冬季休眠的意义和潜在机制。基于冬季休眠的进化模式，本文认为这是某些暖适应鱼类为扩大其极地地理范围而进化出的一种新策略，为研究冷适应的生理约束和地理限制提供了理想的模型。为了评估这一总体假设，我们将调查3个主要问题。(1)冬季冬眠的生理和行为机制是什么？我们将使用一种综合的方法来阐明行为活动和细胞功能在设定低代谢率方面的作用，这是冬季休眠鱼类的特征。(2)为什么会出现冬休眠？首先，我们将测试冬季休眠鱼类的关键有氧（心肺和游泳）性能是否在寒冷中受到极大限制，从而可能由于无法觅食或逃离捕食者而被迫进入休眠状态。然后，通过测量活动鱼和休眠鱼的体内摄食率、生长过程和潜在机制，我们还将测试冬季休眠是一种兼性行为，以避免在食物稀缺时能量效率低下的觅食，还是热量限制对食物同化的强制性结果。(3)发育条件、表型可塑性或对当地环境的适应如何在纬度分离的种群中设置不同的冬季冬眠季节温度阈值，这是否是种群间热性能差异的结果？拟议的研究将解决有关生态相关压力源（如温度、食物）如何影响鱼类季节性生理和行为的基本知识空白。所获得的信息将提高我们对生理如何决定物种分布的理解，并有助于预测鱼类种群是否会随着气候变化而转移或扩大范围。

Low temperature in winter, and accompanying food scarcity, are key bottlenecks on the persistence of fishes at poleward latitudes, yet a detailed mechanistic understanding of winter responses in fishes remains elusive. Research on overwintering and cold adaptation of fishes has focused on winter-active species (eg. salmons, cods); much less is known about the many ecologically-important species that undergo winter dormancy (e.g. temperate wrasses, sunfishes). Winter dormancy is an inactive, sheltering, non-feeding, and low-metabolism state to cope with harsh winter conditions. Understanding winter dormancy is important: it is a key seasonal event that can comprise up to half the fish's lifespan and temperature constraints on activity and energy budgets can have major consequences for the health and persistence of fish populations. The objective of this grant is to reveal the significance and underlying mechanisms of winter dormancy in fishes. Based on the evolutionary patterns of winter dormancy, I propose that it is a novel strategy evolved by certain lineages of warm-adapted' fishes to extend their poleward geographic range, providing an ideal model to study physiological constraints on cold adaptation and geographic limits. To evaluate this overarching hypothesis, we will investigate 3 major questions. (1) What are the physiological & behavioural mechanisms underlying winter dormancy? We will use an integrative approach to elucidate the roles of behavioural activity and cellular function in setting the low metabolic rate that is characteristic of winter-dormant fishes. (2) Why does winter dormancy occur? First, we will test whether critical aerobic (cardiorespiratory & swimming) performance of winter dormant fishes is greatly constrained in the cold, thus obligating entry to dormancy possibly because of an inability to forage or flee predators. Then, by measuring in vivo feeding rates, growth processes, and underlying mechanisms in active and dormant fish fed variable ration or force-fed, we will also test whether winter dormancy is a facultative behaviour to avoid energetically-inefficient foraging when food is scarce, or an obligatory consequence of thermal limitations on food assimilation. (3) How have developmental conditions, phenotypic plasticity, or adaptation to the local environment set different seasonal temperature thresholds for winter dormancy among latitudinally-separated populations and is it a result of interpopulation variation in thermal performance? The proposed research will address fundamental knowledge gaps about how ecologically-relevant stressors (e.g. temperature, food) shape the seasonal physiology and behaviour of fishes. The information gained will improve our understanding of how physiology sets species distributions and will aid efforts to predict if fish populations will shift or expand ranges with climate change.