Data from: Redder isn’t always better: cost of carotenoids in Chinook salmon eggs

Carotenoids provide animals with many fitness benefits through increased mating success, immune function, gamete quality and antioxidant capacity. Despite these benefits, carotenoids are not utilized equally by all animals, implying trade-offs associated with the pigments; although, few studies have quantified fitness costs of carotenoid pigmentation. Salmon are known for their conspicuous red coloration; however, amongst Chinook salmon (Oncorhynchus tshawytscha), a natural genetic color polymorphism exists (red and white morphs) which results in carotenoid-based color differences in eggs and other tissues. While the fitness benefit of egg carotenoid content on egg incubation survival has been demonstrated, carotenoid pigmentation also results in highly visible eggs vulnerable to predation. Therefore, although white Chinook salmon eggs experience costs in terms of viability, a potential benefit in terms of reduced predation could help explain the maintenance of the polymorphism. Here, using red and white eggs from wild Chinook salmon, we show that increased carotenoid content of salmon eggs leads to greater predation risk. We found that two populations of wild-type rainbow trout (an ecologically relevant predator) showed a significant bias for red eggs over white eggs under choice experiments, where red eggs were consumed first twice as often and significantly faster than white eggs. Our study suggests that trade-offs between red and white Chinook salmon during the egg stage provide an evolutionary mechanism promoting the maintenance of the unique Chinook salmon color polymorphism in nature, while also, for the first time, demonstrating a direct fitness cost of carotenoids in salmon.

类胡萝卜素（carotenoids）通过提升交配成功率、免疫功能、配子质量与抗氧化能力，为动物带来诸多适应性收益。尽管存在这些益处，但并非所有动物都能同等利用类胡萝卜素，这暗示这类色素存在权衡取舍；不过目前鲜有研究量化类胡萝卜素色素沉着带来的适应性成本。大麻哈鱼以其醒目的红色体色为人熟知；然而在奇努克大麻哈鱼（Oncorhynchus tshawytscha）中存在自然的体色遗传多态现象（红色与白色两种形态），这使得卵粒与其他组织出现基于类胡萝卜素的颜色差异。虽然已有研究证实卵粒类胡萝卜素含量对胚胎孵化存活率的适应性益处，但类胡萝卜素色素沉着同时会使卵粒变得极易被捕食者发现。因此，尽管白色型奇努克大麻哈鱼的卵粒会面临生存力方面的代价，但其捕食压力降低的潜在益处或许可以解释该多态现象的维持机制。本研究以野生奇努克大麻哈鱼的红色与白色卵粒为实验材料，结果表明鲑鱼卵粒的类胡萝卜素含量越高，被捕食的风险也越高。我们发现，两个种群的野生虹鳟（一种生态相关的捕食者）在选择实验中均表现出对红色卵粒的显著偏好：红色卵粒被优先捕食的频率是白色卵粒的两倍，且被摄食的速度显著快于白色卵粒。本研究表明，奇努克大麻哈鱼在卵阶段存在的红色与白色形态间的权衡，为自然界中该物种独特的体色多态现象的维持提供了一种进化机制，同时也首次证实了鲑鱼体内类胡萝卜素带来的直接适应性成本。