Data from: The role of rare morph advantage and conspicuousness in the stable gold-dark colour polymorphism of a crater lake Midas cichlid fish

1. Genetically based stable colour polymorphisms provide a unique opportunity to study the evolutionary processes that preserve genetic variability in the wild. Different mechanisms are proposed to promote the stability of polymorphisms, but only few empirical examples have been documented, resulting in an incomplete understanding of these mechanisms. 2. A remarkable genetically-determined stable colour polymorphism is found in the Nicaraguan Midas cichlid species complex (Amphilophus cf. citrinellus). All Midas cichlids start their life with a dark-grey colouration (dark morph), but individuals carrying the dominant “gold” allele (~10%) lose their melanophores later in life, revealing the underlying orange colouration (gold morph). How this polymorphism is maintained remains unclear. Two main hypotheses have been proposed, both suggesting differential predation upon colour morphs as the proximate mechanism. One predicts that the conspicuous gold morph is more likely to be preyed upon, but this disadvantage is balanced by their competitive dominance over the dark morph. The second hypothesis suggests a rare morph advantage where the rarer gold morph experiences less predation. Empirical evidence for either of these mechanisms is still circumstantial and inconclusive. 3. We conducted two field experiments in a Nicaraguan crater lake using wax models simulating both morphs to determine predation pressure upon Midas cichlid colour morphs. First, we tested the interaction of colouration and depth on attack rate. Second, we tested the interaction of fish size and colouration. We contrasted the pattern of attacks from these experiments to the predicted predation patterns from the hypotheses proposed to explain the colour polymorphism’s stability. 4. Large models imitating colour morphs were attacked at similar rates irrespectively of their position in the water column. Yet, attacks upon small models resembling juveniles were directed mainly toward dark models. This resulted in a significant size-by-colour interaction. 5. We suggest that gold Midas cichlids experience a rare morph advantage as juveniles when individuals of this morph are extremely uncommon. But this effect is reduced or disappears among adults, where gold individuals are relatively more common. Thus, the interaction of rare morph advantage and conspicuousness, rather than either of those factors alone, is a likely mechanism resulting in the stability of the colour polymorphism in Midas cichlids.1. 基于遗传基础的稳定体色多态性（genetically based stable colour polymorphisms）为探究野外环境中维持遗传变异的进化过程提供了独特契机。学界已提出多种机制以促进多态性的稳定维持，但目前仅记录到极少数实证案例，导致学界对这些机制的认知仍不全面。 2. 尼加拉瓜境内的迈达斯慈鲷（Midas cichlid）物种复合群（Amphilophus cf. citrinellus）中存在一种显著的遗传决定型稳定体色多态性。所有迈达斯慈鲷幼体均呈深灰色（暗色型，dark morph），但携带显性“金色”等位基因（频率约10%）的个体在生长过程中会丢失黑素细胞（melanophores），从而显现出体内的橙色体色（金色型，gold morph）。目前该多态性的维持机制仍不明晰。学界已提出两种主要假说，均认为针对不同体色型的差异化捕食是其近因机制（proximate mechanism）。其中一种假说预测，醒目的金色型个体更易被捕食，但该劣势会因金色型个体相较于暗色型个体的竞争优势得到抵消；第二种假说则提出稀有型优势假说，即稀有金色型个体所面临的捕食压力更低。目前针对这两种机制的实证证据仍属间接且未达成定论。 3. 我们在尼加拉瓜的一座火山口湖（crater lake）中开展了两项野外实验，通过模拟两种体色型的蜡制模型，探究迈达斯慈鲷体色型所承受的捕食压力。其一，我们检验了体色与水深对攻击率的交互影响；其二，我们检验了鱼体大小与体色的交互影响。我们将实验得到的攻击模式与用以解释该体色多态性维持机制的相关假说所预测的捕食模式进行了对比。 4. 模拟成体体色型的大型模型，其遭受攻击的概率不受其在水层（water column）中位置的影响，攻击率无显著差异。但针对模拟幼体的小型模型的攻击，主要集中在暗色模型上，这一结果形成了显著的体型-体色交互效应。 5. 我们提出，当金色型迈达斯慈鲷幼体极为稀有时，该类群幼体会享受到稀有型优势；但当个体发育为成体且金色型个体占比相对升高时，该优势效应会减弱乃至消失。因此，稀有型优势与体色醒目性的交互作用，而非二者其一，或是维持迈达斯慈鲷体色多态性稳定的潜在机制。