Data from: Mouth asymmetry in the textbook example of scale-eating cichlid fish is not a discrete dimorphism after all

Individuals of the scale-eating cichlid fish, Perissodus microlepis, from Lake Tanganyika tend to have remarkably asymmetric heads that are either left-bending or right-bending. The ‘left’ morph opens its mouth markedly towards the left and preferentially feeds on the scales from the right side of its victim fish and the ‘right’ morph bites scales from the victims’ left side. This striking dimorphism made these fish a textbook example of their astonishing degree of ecological specialization and as one of the few known incidences of negative frequency-dependent selection acting on an asymmetric morphological trait, where left and right forms are equally frequent within a species. We investigated the degree and the shape of the frequency distribution of head asymmetry in P. microlepis to test whether the variation conforms to a discrete dimorphism, as generally assumed. In both adult and juvenile fish, mouth asymmetry appeared to be continuously and unimodally distributed with no clear evidence for a discrete dimorphism. Mixture analyses did not reveal evidence of a discrete or even strong dimorphism. These results raise doubts about previous claims, as reported in textbooks, that head variation in P. microlepis represents a discrete dimorphism of left- and right-bending forms. Based on extensive field sampling that excluded ambiguous (i.e. symmetric or weakly asymmetric) individual adults, we found that left and right morphs occur in equal abundance in five populations. Moreover, mate pairing for 51 wild caught pairs was random with regard to head laterality, calling into question reports that this laterality is maintained through disassortative mating.

栖息于坦噶尼喀湖（Lake Tanganyika）的食鳞丽鱼（scale-eating cichlid）物种微侧带丽鱼（Perissodus microlepis）的个体普遍拥有高度不对称的头部，可分为左弯型与右弯型两种形态。“左弯型”个体的口部显著朝向左侧张开，优先取食猎物鱼类右侧的鳞片；“右弯型”个体则啃食猎物左侧的鳞片。这种鲜明的二态性使其成为生态特化程度惊人的经典教科书案例，同时也是已知少数几例作用于不对称形态性状的负频率依赖选择（negative frequency-dependent selection）的实例之一，即该物种种群内左、右两种形态的个体频率保持均等。本研究针对微侧带丽鱼头部不对称性的程度与频率分布形态展开调查，以验证其变异是否如普遍假设的那样符合离散二态性。无论是成体还是幼体，其口部不对称性均呈连续单峰分布，未发现离散二态性的明确证据；混合模型分析亦未发现离散甚至显著二态性的相关佐证。上述结果对教科书中所载的“微侧带丽鱼头部变异属于左、右弯型离散二态性”这一先前论断提出了质疑。基于针对成体的广泛野外采样（排除了对称或弱不对称的模糊个体），我们发现五个种群中左、右两种形态的个体丰度完全均等。此外，对51对野外捕获配对个体的分析显示，其配偶选择与头部偏侧性无关，这也对“该偏侧性通过非选型交配（disassortative mating）维持”的相关报道提出了质疑。