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稀缺是鱼类在极地纬度持续生存的关键瓶颈，但对鱼类冬季反应的详细机制仍不清楚。鱼类越冬和冷适应的研究主要集中在冬季活动物种(例如。鲑鱼、鳕鱼)；对经历冬季休眠的许多生态重要物种(例如温带乌贼、太阳鱼)知之甚少。冬季休眠是一种不活动、遮蔽、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，可包括多达一半的鱼类寿命，温度对活动和能量收支的限制可能对鱼类种群的健康和持久性产生重大影响。这项拨款的目的是为了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，我认为这是由某些暖性适应鱼类谱系进化出的一种新的策略，以扩大它们向极地的地理范围，为研究冷适应的生理限制和地理限制提供了一个理想的模型。为了评估这一总体假设，我们将调查三个主要问题。(1)冬眠背后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冬眠鱼类特有的低代谢率方面的作用。(2)冬眠为什么会发生？首先，我们将测试冬季休眠鱼类的关键有氧(心肺和游泳)能力是否在寒冷中受到很大限制，从而进入休眠状态，可能是因为无法觅食或逃离捕食者。然后，通过测量活体摄食率、生长过程和休眠鱼类的潜在机制，我们还将测试冬季休眠是一种临时行为，以避免在食物稀缺时能量效率低下的觅食，还是温度限制对食物同化的强制性后果。(3)发育条件、表型可塑性或对当地环境的适应是如何在纬向分离的种群中为冬季休眠设定不同的季节温度阈值的，这是种群间热性能差异的结果吗？拟议的研究将解决关于与生态相关的压力因素(例如温度、食物)如何塑造鱼类的季节性生理和行为的基本知识空白。获得的信息将提高我们对生理学如何决定物种分布的理解，并将有助于预测鱼类种群是否会随着气候变化而改变或扩大范围。

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.