Persistence and Fluctuation of Lateral Dimorphism in Fishes

Abstract

Two morphological types ("righty" and "lefty") have been discovered in several fish species and are referred to as a typical example of antisymmetry. It has been suggested, first, that this dimorphism (called laterality) is inheritable; second, that the frequencies of laterality in each species fluctuate around 0.5; and third, that predators mainly exploit prey of the opposite laterality; that is, lefty and righty predators prey on righties and lefties, respectively. The latter is defined as "cross predation"; the antonym "parallel predation" means predation within the same laterality. We hypothesized that cross predation drives alternation of the survival and reproductive advantages between two morphological types, leading to frequency-dependent selection that maintains the dimorphism. To investigate this, we constructed mathematical models of population dynamics of one prey/one predator systems and three-trophic-level systems with omnivory. Mathematical analysis and computer simulations explained the behavior of the laterality frequency in nature well, insofar as cross predation dominated over parallel predation. Furthermore, the simulations showed that when only one of the morphological types exists in a species, the other type can invade. This suggests that dimorphism is maintained in all interacting species.

Evidence of disassortative mating in a Tanganyikan cichlid fish and its role in the maintenance of intrapopulation dimorphism

“…The lefty and righty individuals of P. microlepis attack prey of the opposite morph more frequently than prey of the same morph (M. Hori, unpublished data collected between 2005 and. This crosspredation may cause negative frequency-dependent selection (Hori 1993;Nakajima et al 2004). The combination of intra-(disassortative mating) and interpopulational effects (negative frequency-dependent selection) may stabilize lateral dimorphism at a ratio of 1 : 1 more than a single mechanism.…”

“…It is dimorphic with respect to the direction of mouth opening: in one morph, the mouth opens to the right, causing the left side of the head to face forward ('lefty'), whereas the other morph has a mouth that opens to the left ('righty';Hori 1993). Note that Hori's (1993) 'dextral' and 'sinistral' have recently been redefined as lefty and righty, respectively (Nakajima et al 2004(Nakajima et al , 2005Hori et al 2007). There are no individuals with an intermediate morph, and this species is a typical example of 'antisymmetry' (Palmer 1996), which has a bimodal distribution of signed differences on both sides.…”

“…This cross-predation may cause negative frequency-dependent selection, which presumably maintains the lateral dimorphism (Hori 1993). Several population models that assume random mating ( Takahashi & Hori 1994;Nakajima et al 2005) or asexual reproduction (Takahashi & Hori 1998Nakajima et al 2004) have been developed to support this hypothesis; however, the mating pattern of this species has never been observed. In this study, we observe the mating pattern (disassortative mating between the lefty and righty individuals or assortative mating between the individuals of the same laterality) of P. microlepis in the wild, and discuss the role of this mating pattern in the maintenance of lateral dimorphism.…”

Lateralized behavior in the attacks of largemouth bass on Rhinogobius gobies corresponding to their morphological antisymmetry

“…Therefore, we believe that the lateral biases in approach direction and in evasive response corresponding to morphological antisymmetry are the principal mechanisms causing the predominance of cross predation. These behavioral biases of each morphological type generate the predominance of cross predation, which leads to the maintenance of antisymmetric dimorphism through negative frequency-dependent natural selection (Nakajima et al, 2004;Yasugi and Hori, 2011). …”

“…Moreover, mathematical models demonstrate that this predominance of cross predation can maintain the dimorphism without fixing a population to either only lefties or only righties (Nakajima et al, 2004). However, the evasive responses of prey have not previously been studied with regard to morphological antisymmetry.…”