Abstract
The left–right asymmetry of scale-eating Tanganyikan cichlids is described as a unilateral topographical shift of the quadratomandibular joints. This morphological laterality has a genetic basis and has therefore been used as a model for studying negative frequency-dependent selection and the resulting oscillation in frequencies of two genotypes, lefty and righty, in a population. This study aims were to confirm this laterality in Perissodus microlepis Boulenger and P. straeleni (Poll) and evaluate an appropriate method for measuring and testing the asymmetry. Left–right differences in the height of the mandible posterior ends (HMPE) and the angle between the neurocranium and vertebrae of P. microlepis and P. straeleni were measured on skeletal specimens. Snout-bending angle was also measured using a dorsal image of the same individuals following a previous method. To define which distribution model, fluctuating asymmetry (FA), directional asymmetry (DA), or antisymmetry (AS), best fit to the lateral asymmetry of the traits, we provided an R package, IASD. As a result, HMPE and neurocranium–vertebrae angle of both species were best fitted to AS, suggesting that P. microlepis and P. straeleni showed a distinct dimorphism in these traits, although snout-bending angle of P. microlepis was best fitted to FA. Measurement error was low for HMPE comparing the snout-bending angle in P. microlepis, indicating that measuring HMPE is a more accurate method. The scale-eating tribe Perissodini showed distinct antisymmetry in the jaw skeleton and neurocranium–vertebrae angle, and this laterality remains a valid marker for further evolutionary studies.
Highlights
The bilateral asymmetry of the scale-eating Tanganyikan cichlid Perissodus eccentricus was first described as “bilateral differential growth rates of two bones in the jaw suspension and the remodeling of articular surfaces of the lower jaw joints” (Liem and Stewart 1976)
The mechanical and integrated asymmetries of the cranial skeleton of this taxon are clearly demonstrated in the figures in Liem and Stewart (1976)
The mouth asymmetry is shared by a 2013 The Authors
Summary
The bilateral asymmetry of the scale-eating Tanganyikan cichlid Perissodus eccentricus was first described as “bilateral differential growth rates of two bones in the jaw suspension and the remodeling of articular surfaces of the lower jaw joints” (Liem and Stewart 1976). A lefty opens its mouth toward to the right and attacks the left side of prey fishes when attacking from behind (Hori 1993; Takahashi et al 2007; Takeuchi et al 2012). This mouth laterality is thought to facilitate efficient scale-eating because of the increased contact area between the predator’s teeth and the flank of prey fish.
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