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

Abstract

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-hand side of its victim fish, and the 'right' morph bites scales from the victims' left-hand 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.

Towards understanding the genetic basis of mouth asymmetry in the scale‐eating cichlidPerissodus microlepis

“…; Kusche et al . ), rather than the two clear discrete states originally described (Hori ). These findings challenged also the single gene determination model (Hori ; Hori et al .…”

“…This appears to be the case of P. microlepis, whose equal abundance of both morphs observed within populations (Hori ; Kusche et al . ) is considered maintained by the advantage of the less frequent morph (known as negative frequency‐dependent selection; Hori ; Nakajima et al . ).…”

“…) and wild‐caught (Kusche et al . ) fish and analysis of stomach contents (Takeuchi et al . ) all suggest that this trait can be influenced by external factors such as predation mode and feeding experience.…”

On the Measurement of Ecological Novelty: Scale-Eating Pupfish Are Separated by 168 my from Other Scale-Eating Fishes

“…Third, scale-eater populations always remain much smaller than their prey populations. Lastly, lateral jaw asymmetry has evolved in scale-eating specialists at least four times independently across a wide range of foraging strategies and habitats (from Amazonia to Lake Tanganyika to the mesopelagic ocean: [31], [32], [34], [36]; but also see [40]). This suggests that laterally asymmetric jaws may be a universally adaptive trait for scale-eaters by allowing lateral attacks while pursuing prey, whereas symmetrical jaws may require perpendicular alignment with the prey.…”

Analyzing Fluctuating Asymmetry with Geometric Morphometrics: Concepts, Methods, and Applications

“…Different studies found either bimodal [184,185] or unimodal [186,187] distributions of measures of asymmetry in the mouth region. Also, there is evidence that the distribution of asymmetry in larvae is unimodal [188] and that both genetic factors [188,189] and plasticity [186] influence the asymmetry of the mouth in adults.…”