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.

来自坦噶尼喀湖的食鳞慈鲷（scale-eating cichlid fish）细鳞奇丽鱼（Perissodus microlepis）的个体普遍拥有显著不对称的头部，可划分为左弯型与右弯型两类。左弯型个体的口部明显向左张开，优先咬取猎物右侧的鳞片；右弯型个体则咬食猎物左侧的鳞片。这一显著的形态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针对不对称形态性状的负频率依赖选择（negative frequency-dependent selection）实例之一，在该选择机制下，物种种群内左右两种形态的频率始终保持均等。
本研究针对细鳞奇丽鱼头部不对称性的频率分布范围与形态特征展开探究，以验证其变异是否如学界普遍假设的那样符合离散二态性（discrete dimorphism）。研究结果显示，无论成体还是幼体，口部不对称性均呈现连续的单峰分布，未发现支持离散二态性的明确证据。混合模型分析（mixture analyses）亦未揭示出离散乃至显著二态性的相关迹象。上述结果对教科书中此前的论断提出了质疑——该论断认为细鳞奇丽鱼的头部变异属于左右弯型的离散二态性。
基于对成体中模糊个体（即对称或弱不对称个体）的排除性大规模野外采样，我们在五个种群中发现左右两种形态的个体丰度完全均等。此外，对51对野外捕获的配偶配对分析显示，其头部偏侧性（head laterality）的配对呈随机状态，这一结果对“该头部偏侧性通过非选型交配（disassortative mating）得以维持”的相关报道提出了质疑。