# Low Genetic Diversity and Complex Population Structure in Black Piranha ( Serrasalmus rhombeus ), a Key Amazonian Predator

## TL;DR

Black piranhas in the Amazon show low genetic diversity and complex population structure, likely shaped by historical events rather than current environmental factors.

## Contribution

This is the first study to analyze the genetic diversity and population structure of black piranhas in the central Amazon using genomic data.

## Key findings

- Low genetic diversity and signs of inbreeding were observed in half of the study sites.
- Strong genetic differentiation and complex population structure were detected, with some sites hosting unique genetic groups.
- Population structure appears influenced by historical events rather than contemporary factors.

## Abstract

The black piranha (
Serrasalmus rhombeus
), a widely spread species in the rivers of the Amazon basin, plays a vital role as both key predator and important prey. Despite its essential contribution to ecosystem stability, there is a lack of information regarding its genetic diversity and population dynamics in the central Amazon region. As the Amazon continues to undergo environmental changes in the context of growing anthropogenic threats, such knowledge is fundamental for assist in the conservation of this species. This study is the first to analyze the genetic diversity and population structure of
S. rhombeus
in the central Amazon region using high‐resolution genomic data. We employed a Genotyping‐by‐Sequencing approach with 248 samples across 14 study sites from various tributaries, encompassing diverse water types (black, white, and clear water) and characterized by 34 physiochemical parameters. The data reveals low diversity accompanied by pronounced signs of inbreeding in half of the sites and robust genetic differentiation and variation among sites and within‐sites. Surprisingly, we also found evidence of higher dispersal capacity than previously recognized. Our analysis exposed a complex and high population structure with genetic groups exclusive to some sites. Gene flow was low and some groups presented ambiguous genealogical divergence index (gdi) signals, suggesting the occurrence of potential cryptic species. Moreover, our results suggest that the population structure of black piranha appears more influenced by historical events than contemporary factors. These results underscore the need to give greater attention to this keystone species, for which no regulatory framework or conservation strategies is presently in effect.

Using a Genotyping‐by‐Sequencing approach across tributaries in the central Amazonia, we found that the black piranha (
Serrasalmus rhombeus
) exhibits low genetic diversity, pronounced inbreeding, and significant population structure, likely shaped by historical events rather than contemporary factors.