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

Abstract

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. 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 coloration (dark morph), but individuals carrying the dominant "gold" allele (c. 10%) lose their melanophores later in life, revealing the underlying orange coloration (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. 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 coloration and depth on attack rate. Second, we tested the interaction of fish size and coloration. 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. 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 towards dark models. This resulted in a significant size-by-colour interaction. 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.

Increased occurrence of the rare golden color morph of Pacific chub Kyphosus sandwicensis in a no‐take marine reserve

“…While the presence of xanthochroism has been noted previously for a number of species, most studies examining the effect of gold color morphs on ecological interactions and evolutionary processes in fish have been limited to the Midas cichlid ( Amphilophus citrinellus [Günther 1864] and Amphilophus spp.) in freshwater lakes of Nicaragua (Henning et al, 2013; Torres‐Dowdall et al, 2017).…”

“…This study presents quantitative estimates of a change in the frequency of occurrence of the golden color morph for a marine fish. It is unknown if the individuals with the golden color morphs undergo an ontogenetic color change, similar to the Midas cichlids (Torres-Dowdall et al, 2017), or if they are golden throughout their lives. We observed three total individuals with partially golden colouration in 2012 and 2017, but it was unclear if they were transitioning to a golden phase or had a mixed colouration throughout life.…”

Beyond morphs: Inter‐individual colour variation despite strong genetic determinism of colour morphs in a wild bird

“…correlational selection), each corresponding to alternative peaks across the fitness landscape (Sinervo et al, 2001 ; Willink et al, 2013 ), maintained under balancing selection or under spatio‐temporal variation in selection. Frequency dependent selection could also favour the coexistence of multiple colour variants, if rare variants are at an advantage with regards to predator avoidance (Torres‐Dowdall et al, 2017 ) or sexual and social selection (Dijkstra et al, 2010 ; Hughes et al, 2013 ). Likewise, social selection can maintain extensive colour variation through advantages of signalling subtle variation in information (Chaine et al, 2011 ; Kraaijeveld et al, 2004 ).…”

Heterogeneity across Neotropical aquatic environments affects prokaryotic and eukaryotic biodiversity based on environmental DNA

“…Most crater lakes are surrounded by high rims that completely isolate them from each other and from the great lakes. Besides their age and size, the lakes also have different chemical and physical characteristics, including turbidity (Torres‐Dowdall et al, 2017), pH (7.33–8.69), levels of dissolved oxygen (0.1%–97.3%), and salinity (0.12–4.77 ppt; Table 1). These highly variable aquatic environments located within a small geographic range represent a rather unique setting and allow testing whether biodiversity is affected by a range of abiotic factors.…”