Evolution of body shape in sympatric versus non-sympatric Tropheus populations of Lake Tanganyika

Abstract

Allopatric speciation often yields ecologically equivalent sister species, so that their secondary admixis enforces competition. The shores of Lake Tanganyika harbor about 120 distinct populations of the cichlid genus Tropheus, but only some are sympatric. When alone, Tropheus occupies a relatively broad depth zone, but in sympatry, fish segregate by depth. To assess the effects of competition, we studied the partial co-occurrence of Tropheus moorii ‘Kaiser' and ‘Kirschfleck' with Tropheus polli. A previous study demonstrated via standardized breeding experiments that some observed differences between Tropheus ‘Kaiser' living alone and in sympatry with T. polli have a genetic basis despite large-scale phenotypic plasticity. Using geometric morphometrics and neutral genetic markers, we now investigated whether sympatric populations differ consistently in body shape from populations living alone and if the differences are adaptive. We found significant differences in mean shape between non-sympatric and sympatric populations, whereas all sympatric populations of both color morphs clustered together in shape space. Sympatric populations had a relatively smaller head, smaller eyes and a more anterior insertion of the pectoral fin than non-sympatric populations. Genetically, however, non-sympatric and sympatric ‘Kaiser' populations clustered together to the exclusion of ‘Kirschfleck'. Genetic distances, but not morphological distances, were correlated with geographic distances. Within- and between-population covariance matrices for T. moorii populations deviated from proportionality. It is thus likely that natural selection acts on both phenotypic plasticity and heritable traits and that both factors contribute to the observed shape differences. The consistency of the pattern in five populations suggests ecological character displacement.

Asymmetric dominance and asymmetric mate choice oppose premating isolation after allopatric divergence

“…“black” is most abundant in depths between 3 and 5 m (Kerschbaumer et al. 2014). However, in contrast to the closely related color morphs within T. moorii , which were investigated in the present study, T. polli and T.  sp.…”

Diet disparity among sympatric herbivorous cichlids in the same ecomorphs in Lake Tanganyika: amplicon pyrosequences on algal farms and stomach contents

“…This was due in part to habitat depth segregation of cichlids, as the dissimilarity of algal farms increased significantly when the difference in habitat depth increased. Segregation of habitat range by depth among cichlid species has been described in several species pairs of the same ecomorphs [10,23,30,33]. Additionally, the stomach contents of these cichlids were segregated by at least 0.75 in the Canberra dissimilarity index, even between species in the same feeding ecomorph inhabiting the same depth range.…”

“…Grazers and browsers utilise species-specific depth ranges, and species in the same depth range choose the substratum types on which they feed [10,23,30]. In particular, territorial cichlids of the same ecomorph exhibit separate habitat depths [29,31,32], and ecological character displacement in habitat depth and associated body shape were reported in sympatric populations of Tropheus moorii and Tropheus polli, compared to isolated populations of the two species [33].…”

Depth and substratum differentiations among coexisting herbivorous cichlids in Lake Tanganyika

“…In the central eastern section of Lake Tanganyika, Tropheus polli coexists with T. moorii ‘Kirschfleck’ at Mabilibili point and with T. moorii ‘Kaiser’ at Kekese point. At these sites, they live separately in neighbouring depths, but at Ikola point T. moorii ‘Kaiser’ lives alone and occupies the entire depth range [71]. Depth differentiation by sympatric Tropheus species and consequent morphological differentiations adapting to each habitat depth seem to be triggered by competition.…”