Selection, hybridization, and the evolution of morphology in the Lake Mala\u0175i endemic cichlids of the genus Labeotropheus

Michael J Pauers,Kelsey R Fox,Robert A Hall,Kesha Patel

doi:10.1038/s41598-018-34135-x

Abstract

The cichlid fishes of Lake Malaŵi are the paramount example of adaptive radiation in vertebrates. Evidence of their astounding diversity is perhaps most visible in their adaptations for obtaining food; the genus Labeotropheus, due to their prominent snouts, are an interesting example of an extreme adaptation for feeding. Two different body types are found in this genus: a deep-bodied form (e.g., L. fuelleborni) found most often in turbulent shallow water; and a slender bodied form (e.g., L. trewavasae) found in structurally-complex deep water habitats. Here we test the hypothesis that L. trewavasae should suffer a loss in fitness, measured as growth rate, if raised in turbulence; additionally, we examined growth and morphology of L. fuelleborni and L. fuelleborni x L. trewavasae hybrids under these conditions. We did find the predicted loss of fitness in turbulent-raised L. trewavasae, but found no loss of fitness for L. fuelleborni in either condition; hybrids, due to an unusual morphology, performed better in turbulent as opposed to control conditions. Fitness in turbulent conditions was dependent upon morphology, with deeper bodies and upturned neurocrania allowing a greater growth rate under these conditions. Directional selection on morphology was crucial in the evolution of morphology in the Labeotropheus.

Highlights

The cichlid fishes of Lake Malaŵi are one of the most fascinating examples of adaptive radiation and speciation; no other group of vertebrates can match either their sheer number of species or vast diversity of form and function
In this preliminary examination of morphology, it appears that the slender L. trewavasae specimens are clustered within the positive quadrant, while the robust L. fuelleborni individuals seem to be confined to the negative quadrant
When comparisons are made within species between treatments, there is no difference between treatments for L. fuelleborni, but both L. trewavasae and the Labeotropheus hybrids do exhibit statistically significant differences in fitness (Table 3). This directional selection is made apparent when the fitness curves are superimposed on the same set of axes (Fig. 4e). The results of this experiment support the hypothesis that L. trewavasae would suffer a loss of fitness if raised in a turbulent environment; young L. trewavasae have a lower growth rate when grown in turbulent conditions than when raised in calm conditions

Summary

Introduction

The cichlid fishes of Lake Malaŵi are one of the most fascinating examples of adaptive radiation and speciation; no other group of vertebrates can match either their sheer number of species or vast diversity of form and function. The diagnostic feature shared by all members of this small genus is an enlarged snout of fibrous connective tissue that hangs over an inferior, subterminal mouth, with which all of these species scrape filamentous, epilithic algae[1,6,7,8] This snout has been the focus of many hypotheses regarding how the Labeotropheus feed, and it is generally assumed to act as a fulcrum with which the fish contacts the substrate, while the jaws work to remove the attached algae; this process is exquisitely described by Fryer[1]; see [3,8,9,10]. We test these hypotheses and predictions in L. fuelleborni, L. trewavasae, and their hybrids

Methods

Results

Conclusion