RESUMO
BACKGROUND: Differences in species richness among phylogenetic clades are attributed to clade age and/or variation in diversification rates. Access to ecological opportunity may trigger a temporary increase in diversification rates and ecomorphological variation. In addition, lower body temperatures in poikilothermic animals may result in decreasing speciation rates as proposed by the metabolic theory of ecology. For strictly freshwater organisms, environmental gradients within a river continuum, linked to elevation and temperature, might promote access to ecological opportunity and alter metabolic rates, eventually influencing speciation and extinction processes. To test these hypotheses, we investigated the influence of environmental temperature and elevation, as proxies for body temperature and ecological opportunity, respectively, on speciation rates and ecomorphological divergence. As model systems served two closely related gastropod genera with unequal species richness and habitat preferences - Pseudamnicola and Corrosella. RESULTS: Lineage-through-time plots and Bayesian macroevolutionary modeling evidenced that Pseudamnicola species, which typically live in lower reaches of rivers, displayed significantly elevated speciation rates in comparison to the 'headwater genus' Corrosella. Moreover, state-dependent speciation models suggested that the speciation rate increased with decreasing elevation, supporting the ecological opportunity hypothesis. In contrast, a significant effect of environmental temperature, as proposed by the metabolic theory of ecology, could not be observed. Disparity-through-time plots, models of ecomorphological evolution, and ancestral habitat estimation showed for Pseudamnicola species rapid morphological divergence shortly after periods of elevational and habitat divergence. In contrast, Corrosella species did not deviate from null models of drift-like evolution. CONCLUSION: Our finding that speciation rates are correlated with elevation and ecomorphological disparity but not with environmental temperatures suggests that differences in ecological opportunity may have played a key role in Corrosella and Pseudamnicola diversifications. We propose that Pseudamnicola lineages experienced higher ecological opportunity through dispersal to new locations or habitats in lowlands, which may explain the increase in speciation rates and morphological change. In contrast, the evolution of Corrosella in headwaters is likely less facilitated by the environment and more by non-ecological processes.
Assuntos
Organismos Aquáticos/genética , Biodiversidade , Ecossistema , Água Doce , Gastrópodes/genética , Animais , Teorema de Bayes , Especiação Genética , Modelos Biológicos , Filogenia , TemperaturaRESUMO
The aim of the present study was to evaluate factors that may favor the coexistence of three endemic species of Astyanax (Astyanax bifasciatus, Astyanax minor and Astyanax gymnodontus) in Segredo Reservoir (Iguaçu River, Paraná State, Brazil), and to test the hypothesis that these species share food resources, being the dietary variations related to morphological characteristics. Fish were collected at three samplings sites in 1993 and 1994, during the first year after the filling phase of the reservoir. The diet was assessed using stomach content analysis and ecomorphology by ecomorphological indices. Diet and ecomorphology data were ordinated by multivariate techniques (DCA and PCA, respectively), aiming to characterize trophic guilds and identify ecomorphological patterns. A MRPP was performed to check differences between diet and morphology of species. Trophic and ecomorphological patterns were recorded among the Astyanax species, and significant differences were found for both aspects. In this way, our results corroborate the presupposed hypothesis. Therefore, the trophic and ecomorphological segregation between Astyanax species from Segredo Reservoir can contribute to reduce the competition, favoring thus their coexistence.
O objetivo deste estudo foi avaliar os fatores que favorecem a coexistência de três espécies endêmicas de Astyanax (Astyanax bifasciatus, Astyanax minor e Astyanax gymnodontus) no reservatório de Segredo (rio Iguaçu, Paraná, Brasil), e testar a hipótese de que as três espécies partilham os recursos alimentares, sendo que as variações na dieta estão relacionadas às características morfológicas das espécies. Os peixes foram coletados em três estações de amostragem no primeiro ano após o fechamento da barragem, nos anos de 1993 e 1994. A dieta foi avaliada através da análise dos conteúdos estomacais, e a ecomorfologia através dos índices ecomorfológicos. Os dados de dieta e morfologia foram ordenados por meio de técnicas multivariadas (DCA e PCA, respectivamente), com a finalidade de caracterizar as guildas tróficas e identificar os padrões ecomorfológicos. Posteriormente, foi realizada uma MRPP para verificar a existência de diferenças quanto à alimentação e os padrões morfológicos entre as espécies. Padrões tróficos e ecomorfológicos foram registrados entre as espécies de Astyanax. Diferenças significativas foram encontradas para ambos os aspectos. Dessa forma, os resultados corroboraram a hipótese inicial, sendo a segregação trófica e ecomorfológica entre as espécies deAstyanax do reservatório de Segredo um fator que pode contribuir para diminuir a competição entre elas, favorecendo a sua coexistência no ambiente.
