Where Am I? Niche constraints due to morphological specialization in two Tanganyikan cichlid fish species.

Lukas Widmer,Bernd Egger,Walter Salzburger,Adrian Indermaur

doi:10.1002/ece3.6629

Abstract

Food resource specialization within novel environments is considered a common axis of diversification in adaptive radiations. Feeding specializations are often coupled with striking morphological adaptations and exemplify the relation between morphology and diet (phenotype–environment correlations), as seen in, for example, Darwin finches, Hawaiian spiders, and the cichlid fish radiations in East African lakes. The cichlids' potential to rapidly exploit and occupy a variety of different habitats has previously been attributed to the variability and adaptability of their trophic structures including the pharyngeal jaw apparatus. Here we report a reciprocal transplant experiment designed to explore the adaptability of the trophic structures in highly specialized cichlid fish species. More specifically, we forced two common but ecologically distinct cichlid species from Lake Tanganyika, Tropheus moorii (rock‐dweller), and Xenotilapia boulengeri (sand‐dweller), to live on their preferred as well as on an unpreferred habitat (sand and rock, respectively). We measured their overall performance on the different habitat types and explored whether adaptive phenotypic plasticity is involved in adaptation. We found that, while habitat had no effect on the performance of X. boulengeri, T. moorii performed significantly better in its preferred habitat. Despite an experimental duration of several months, we did not find a shift in the morphology of the lower pharyngeal jaw bone that would be indicative of adaptive phenotypic plasticity in this trait.

Highlights

Adaptive radiation is the rapid ecological and morphological diversification of an ancestral—often generalist—species into an array of specialized descendants (Schluter, 2000)
We report a reciprocal transplant experiment under semi-natural conditions within Lake Tanganyika to test for local adaptation with respect to benthic habitat types and to examine the possible contribution of phenotypic plasticity to medium-term adaptation in two common but ecologically and behaviorally very distinct endemic cichlid species, Tropheus moorii (Boulenger, 1898) and Xenotilapia boulengeri (Poll, 1942)
In subsequent within-species canonical variance analysis (CVA), shape differences in lower pharyngeal jaw bone (LPJ) shape were detected in CV1 for the different substrate types in T. moorii (Mahalanobis distances (MD) = 2.2708, p < .001) but not in X. boulengeri (MD = 1.1637, p = .18) (Figure 3a)

Summary

| INTRODUCTION

Adaptive radiation is the rapid ecological and morphological diversification of an ancestral—often generalist—species into an array of specialized descendants (Schluter, 2000). Through common garden experiments involving different feeding regimes, it has further been shown that in some cichlid species, the morphology of the LPJ can be experimentally altered (Gunter et al, 2013; Huysseune, 1995; Meyer, 1989; Muschick, Barluenga, Salzburger, & Meyer, 2011) This suggests that LPJ diversification may initially be facilitated by adaptive phenotypic plasticity, that is, the capacity of a genotype to produce more suited phenotypes corresponding and adapting to novel environmental conditions (Schneider & Meyer, 2017; West-Eberhard, 2005). To explore the possible contribution of phenotypic plasticity to local adaptation, we compared LPJ and body shape between experimental groups and a reference sample from the wild by means of geometric morphometric analyses Both LPJ and body morphology/shape have been shown to be associated with feeding and habitat niche, respectively, in Lake Tanganyika cichlids (Muschick et al, 2012). We expected LPJ and body shape changes compared to the reference samples in experimental groups forced to live in foreign habitats

| MATERIALS AND METHODS

Findings

| DISCUSSION