Gudgeon fish with and without genetically determined countershading coexist in heterogeneous littoral environments of an ancient lake

Countershading, characterized by a darker dorsal surface and lighter ventral surface, is common among many animals. This dorsoventral pigment polarity is often thought to be adaptive coloration for camouflage. By contrast, non-countershaded (melanistic) morphs often occur within a species due to genetic color polymorphism in terrestrial animals. However, the polymorphism with either countershaded or melanistic morphs is poorly known in wild aquatic animals. This study explored the genetic nature of diverged color morphs of a lineage of gudgeon fish (genus Sarcocheilichthys) in the ancient Lake Biwa, and propose this system as a novel model for testing hypotheses of functional aspects of countershading and its loss in aquatic environments. This system harbors two color morphs that have been treated taxonomically as separate species; S. variegatus microoculus which occurs throughout the littoral zone, and S. biwaensis which occurs in and around rocky areas. First, we confirmed that the di...