Context-dependent relationships between swimming, terrestrial jumping and body composition in the amphibious fish Kryptolebias marmoratus

Understanding the mechanisms that create phenotypic variation within and among populations is a major goal of physiological ecology. Variation may be a consequence of functional trade-offs (i.e. improvement in one trait comes at the expense of another trait) or alternatively may reflect the intrinsic quality of an organism (i.e. some individuals are simply better overall performers than others). There is evidence for both ideas in the literature, suggesting that environmental context may mediate whether variation results from trade-offs or differences in individual quality. We tested this overarching ‘context dependence’ hypothesis by comparing the aquatic and terrestrial athletic performance of the amphibious fish Kryptolebias marmoratus captured from two contrasting habitats, a large pond and small burrows. Overall, pond fish were superior terrestrial athletes but burrow fish were better burst swimmers, suggestive of a performance trade-off at the population level. Within each population, however, there was no evidence of a performance trade-off. In burrow fish, athletic performance was positively correlated with muscle content and body condition, consistent with the individual quality hypothesis. In pond fish, there was only a relationship between glycolytic white muscle and aquatic burst performance. Notably, pond fish were in better body condition, which may mask relationships between condition and athletic performance. Overall, our data highlight that population-level trends are insufficient evidence for the existence of phenotypic trade-offs in the absence of similar within-population patterns. Furthermore, we only found evidence for the individual quality hypothesis in one population, suggesting that patterns of phenotypic covariance are context dependent.

解析种群内部及种群间表型变异（phenotypic variation）的产生机制，是生理生态学（physiological ecology）的核心研究目标之一。表型变异可能源于功能权衡（functional trade-off，即某一性状的优化会以另一性状的牺牲为代价），也可能反映有机体的内在品质（即部分个体的综合表现本就优于其他个体）。现有文献中两类假说均有相关佐证，这提示环境背景可能介导表型变异的来源：究竟源于功能权衡，还是个体品质差异。我们以两种截然不同生境（大型池塘与小型洞穴）中捕获的两栖鱼类隐丽鳉（*Kryptolebias marmoratus*）为研究对象，通过比较其水生与陆生运动表现（athletic performance），对这一核心的“环境依赖性”（context dependence）假说开展验证。整体而言，池塘种群的个体陆生运动能力更优，而洞穴种群的个体爆发力游泳能力更强，这提示种群水平上存在运动性能的权衡。但在每个种群内部，均未发现运动性能权衡的相关证据。洞穴种群的运动表现与肌肉含量及体况（body condition）呈正相关，这与个体品质假说（individual quality hypothesis）相符。池塘种群中仅发现糖酵解型白肌（glycolytic white muscle）与水生爆发性能存在关联。值得注意的是，池塘种群的体况更佳，这可能掩盖了体况与运动性能之间的关联。综上，我们的研究结果表明：若种群内部未出现对应的关联模式，仅依靠种群水平的趋势不足以证明表型权衡的存在。此外，我们仅在一个种群中找到了支持个体品质假说的证据，这提示表型协变（phenotypic covariance）的模式具有环境依赖性。