A history of the biogeography of Amazonian fishes

Mapping the hidden diversity of the Geophagus sensu stricto species group (Cichlidae: Geophagini) from the Amazon basin

South American freshwater ichthyofauna is taxonomically the most diverse on the planet, yet its diversity is still vastly underestimated. The Amazon basin alone holds more than half of this diversity. The evidence of this underestimation comes from the backlog of morphologically distinct, yet undescribed forms deposited in museum collections, and from DNA-based inventories which consistently identify large numbers of divergent lineages within even well-studied species groups. In the present study, we investigated lineage diversity within the Geophagus sensu stricto species group. To achieve these objectives, we analyzed 337 individuals sampled from 77 locations within and outside the Amazon basin representing 10 nominal and six morphologically distinct but undescribed species. We sequenced the mitochondrial cytochrome c oxidase subunit I (COI) and delimited lineages using four different single-locus species discovery methods (mPTP-15 lineages; LocMin-14 lineages; bGMYC-18 lineages; and GMYC-30 lineages). The six morphologically distinct but undescribed species were also delimited by the majority of the species discovery methods. Five of these lineages are restricted to a single collection site or a watershed and their habitats are threatened by human activities such as deforestation, agricultural activities and construction of hydroelectric plants. Our results also highlight the importance of combining DNA and morphological data in biodiversity assessment studies especially in taxonomically diverse tropical biotas.

Climate vulnerability of South American freshwater fish: Thermal tolerance and acclimation

Freshwater fish are restricted by their physiology to rivers and lakes, and are generally limited in their capacity to disperse across basins. As a result, there is often a close match between the evolutionary history of river basins and their natural history. Thus, the regional landscape and ecological features, such as temperature, have shaped the evolution and adaptation of local fish assemblages. Climate change is expected to affect fish diversity and increase extinction, especially in low latitudes, and it has been suggested that species that inhabit low latitude species are more susceptible since they live close to their maximum thermal limits and have low capacity for acclimation. To understand the mechanisms of variation in thermal tolerance across a broad-scale of South American fishes is fundamental to be able to assess the vulnerability of species and habitat to global warming. Herein, we present the first attempt to analyze the vulnerability of South American freshwater fish species, based on the review of upper thermal limits of 106 species from a broad range of latitudinal habitats. Our findings show that upper thermal limits decrease with latitude, while the thermal safety margin (TSM) increase. Furthermore, the latitude has little effects on the acclimation response ratio, and the TSM decreased with rising temperatures. These data suggest that thermal phenotypic acclimation has low potential for mitigating global warming. These results indicate that South American fish species living in tropical areas are more susceptible to global warming since they are already living close to their maximum habitat temperature.

A Review on Fish Sensory Systems and Amazon Water Types With Implications to Biodiversity

The Amazon has the highest richness of freshwater organisms in the world, which has led to a multitude of hypotheses on the mechanisms that generated this biodiversity. However, most of these hypotheses focus on the spatial distance of populations, a framework that fails to provide an explicit mechanism of speciation. Ecological conditions in Amazon freshwaters can be strikingly distinct, as it has been recognized since Alfred Russel Wallace’s categorization into black, white, and blue (= clear) waters. Water types reflect differences in turbidity, dissolved organic matter, electrical conductivity, pH, amount of nutrients and lighting environment, characteristics that directly affect the sensory abilities of aquatic organisms. Since natural selection drives evolution of sensory systems to function optimally according to environmental conditions, the sensory systems of Amazon freshwater organisms are expected to vary according to their environment. When differences in sensory systems affect chances of interbreeding between populations, local adaptations may result in speciation. Here, we briefly present the limnologic characteristics of Amazonian water types and how they are expected to influence photo-, chemical-, mechano-, and electro-reception of aquatic organisms, focusing on fish. We put forward that the effect of different water types on the adaptation of sensory systems is an important mechanism that contributed to the evolution of fish diversity. We point toward underexplored research perspectives on how divergent selection may act on sensory systems and thus contribute to the origin and maintenance of the biodiversity of Amazon aquatic environments.

Diversification of Neotropical Freshwater Fishes

Neotropical freshwater fishes (NFFs) constitute the most diverse continental vertebrate fauna on Earth, with more than 6,200 named species compressed into an aquatic footprint <0.5% of the total regional land-surface area and representing the greatest phenotypic disparity and functional diversity of any continental ichthyofauna. Data from the fossil record and time-calibrated molecular phylogenies indicate that most higher taxa (e.g., genera, families) diversified relatively continuously through the Cenozoic, across broad geographic ranges of the South American platform. Biodiversity data for most NFF clades support a model of continental radiation rather than adaptive radiation, in which speciation occurs mainly in allopatry, and speciation and adaptation are largely decoupled. These radiations occurred under the perennial influence of river capture and sea-level oscillations, which episodically fragmented and merged portions of adjacent river networks. The future of the NFF fauna into the Anthropocene is uncertain, facing numerous threats at local, regional, and continental scales.