Brazil is internationally famous by its tropical weather, beautiful landscapes, along with its unique coastal line. However, Brazil harbours some other natural features that are as unique as its renowned-beaches and forests but do not hold the same fame. Have you ever heard of the Caatinga biome?
The Caatinga is the largest dry forest in South America, and the only exclusively Brazilian biome (da Silva et al. 2018). Putting in a map, the Caatinga biome is situated in the semiarid portion of northeastern Brazil, and is characterised by its seasonally dry tropical forest, low rainfall regime, with most its aquatic regime being intermittent (Lima et al. 2017) (Fig. 1) The complex past climate events (e. g. once thought to be a continuous rainforest connecting the Amazonian and the Atlantic forests, see Auler et al., 2004; Werneck et al., 2011) on the region have contributed to its high endemism across major groups (da Silva et al. 2018).
Fig. 1. Caatinga biome in northeastern Brazil. In detail, its typical dry forest and intermittent water bodies.
The region can also represent an unique opportunity for evolutionary studies. When I was decided to pursue a career on science (particularly on evolutionary biology), I started wondering what evolutionary adaptions have evolved on Caatinga's aquatic fauna to cope with such extreme seasonality. With this in mind, I decided to go deeper into Caatinga and study the evolutionary history of freshwater fish of the region.
We have soon realised that the task would be harder than we initially thought. Although representing a fascinating system, Caatinga has been historically neglected on freshwater fish studies (for reasons why, see Lima et al. 2017), being considered one of the least known regions of the world (Léveque et al. 2008; Collen et al. 2013). Within the Caatinga biome, an specially worrying situation was observed in a region called Mid-Northeastern Caatinga (MNCE), where a problematic social/environmental context (mainly due to the extreme dry conditions) have hampered biodiversity studies (da Silva et al. 2018).
So before embarking into more complex eco-evolutionary studies, we have realised the urgent need to understand the basics. In biology, the first step for everything else starts with questions like: "which species are we going to be working with? Where are the species distributed? Are we using the right parameters to distinguish species?". These questions are the core of taxonomy and systematic studies, a whole field in biology exclusively designated to identify and delimit species, as well as classify how species are related to each other based on the most varied traits (morphological, molecular, behavioural traits, geographical distribution) (Padial et al. 2013). Taxonomic studies are specially important for megadiverse countries (such as Brazil), where a substantial part of its flora and fauna is still not described (Lewinsohn & Prado 2005).
Fig. 2. Map of individuals sequenced at Mid-Northeastern Caaatinga (MNCE) from Berbel-Filho et al. (2018) . In detail, some of MNCE species: top right corner: the ciclhid Cichlasoma orientale; in bottom left corner, the armoured catfish: Hypostomus pusarum; in the bottom right corner the pirana, Serrasalmus rhombeus.
Correctly assign species is not an easy task, and the rule of thumb when delineating species is: the more the merrier (Carstens et al. 2013). Preferably, as many traits as possible should be included on species delimitation, with the level of agreement among traits being used as how reliable the species delimitation is. Aware of all the historical taxonomic problems in MNCE, we combined an morphological and molecular approach to provide an updated checklist of MNCE, as well as used DNA-barcode based species delimitation to investigate how reliably our a priori morphologically-assigned individuals represent distinct genetic units. This combined approach estimated a total of 91 native species, revealing potential new species, new records for MNCE and some taxonomic uncertainties. Although a finalized species list for the MNCE stills awaits additional taxonomic revisions and field surveys, this study provides the most up-to-date species checklist for the MNCE and a molecular reference database for identifying MNCE fishes with DNA barcodes (Berbel-Filho et al. 2018). (Fig 2).
As counter intuitive as it may sound, I recommend people to get to know a bit more of the Caatinga biome. Its harsh climate has created particular conditions where life struggles and flourishes, seasonally. On the raining season, the rapid transition from dry to green vegetation is simple magic, one of the most beautiful natural cycles I have ever seen (Fig. 3). This natural cycle seems to have reflected on Caatinga's people, who besides the extreme conditions, always believe that the good season is yet to come, bringing with it, water, hope, and happiness.
Fig. 3. The sudden change of vegetation in Caatinga during the raining season. A magic event happening every year in northeastern Brazil.
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