Phylogenomics of pike cichlids (Cichlidae: Crenicichla): the rapid ecological speciation of an incipient species flock.

The rapid rise of phenotypic and ecological diversity in independent lake-dwelling groups of cichlids is emblematic of the East African Great Lakes. In this study, we show that similar ecologically based diversification has occurred in pike cichlids (Crenicichla) throughout the Uruguay River drainage of South America. We collected genomic data from nearly 500 ultraconserved element (UCEs) loci and >260 000 base pairs across 33 species, to obtain a phylogenetic hypothesis for the major species groups and to evaluate the relationships and genetic structure among five closely related, endemic, co-occurring species (the Uruguay River species flock; URSF). Additionally, we evaluated ecological divergence of the URSF based on body and lower pharyngeal jaw (LPJ) shape and gut contents. Across the genus, we recovered novel relationships among the species groups. We found strong support for the monophyly of the URSF; however, relationships among these species remain problematic, likely because of the rapid and recent evolution of this clade. Clustered co-ancestry analysis recovered most species as well delimited genetic groups. The URSF species exhibit species-specific body and LPJ shapes associated with specialized trophic roles. Collectively, our results suggest that the URSF consists of incipient species that arose via ecological speciation associated with the exploration of novel trophic roles.

Ecological diversification associated with the benthic-to-pelagic transition by North American minnows.

Ecological opportunity is often regarded as a key factor that explains why diversity is unevenly distributed across life. Colonization of novel environments or adaptive zones may promote diversification. North American minnows exhibit an ancestral benthic-to-pelagic habitat shift that coincided with a burst in diversification. Here, we evaluate the phenotypic and ecological implications of this habitat shift by assessing craniofacial and dietary traits among 34 species and testing for morphology-diet covariation, convergence and adaptive optima. There were several instances of morphology-diet covariation such as correlations between mouth angle and the consumption of terrestrial insects and between relative gut length and the consumption of algae. After accounting for size and phylogenetic nonindependence, benthic species had longer heads, longer snouts, eyes positioned higher on their head, smaller mouth angles and longer digestive tracts than pelagic minnows. Benthic minnows also consumed more algae but less terrestrial insects, by volume, than pelagic minnows. Lastly, there were three distinct evolutionary regimes and more convergence in morphology and dietary characteristics than expected under a Brownian motion model of evolution. These findings indicate that colonization of the pelagic zone by minnows involved myriad phenotypic and dietary changes associated with exploitation of terrestrial subsidies. Thus, minnows exhibit phenotype-dietary covariation, an expansion of ecological roles and a burst in diversification rates in response to the ecological opportunity afforded by the colonization of a novel habitat.

Species-specific ontogenetic diet shifts among Neotropical Crenicichla: using stable isotopes and tissue stoichiometry.

Ontogenetic diet shifts were compared among five sympatric pike cichlids Crenicichla in a subtropical South American stream using stable C and N isotopes and tissue stoichiometry (C:N). Within species, stable N isotopes were positively related to body size while C:N showed negative relationships. Stable C isotopes, however, were not related to body size in any species. By modelling the switch to piscivory using gut content-isotope-body size relationships, diet shifts were shown to be species-specific with regard to both rate and degree of piscivory. Compared to other piscivorous lineages, Crenicichla appear to be unusually small-bodied (based on maximum body size). Because of their diversity, abundance and dynamic size-structured functional roles, Crenicichla may exert broad and complex predation pressures on the aquatic community.