Pleistocene sea level variation and changes in the Sao Francisco river drove the evolution of Atlantic sabretooth anchovy

During the Pleistocene, the world underwent cyclic climatic oscillations that had significant impacts on terrestrial and aquatic landscapes. These changes caused fragmentation, reduction, and/or expansion in species distribution, resulting in demographic and population variations. In the marine environment, historical climatic transformations influencing sea level fluctuations emerge as a crucial process of diversification. In this study, we investigated the effects of historical sea level changes on the population history of the Atlantic sabretooth anchovy (Lycengraulis grossidens), an anadromous species widely distributed in the Southwest Atlantic Ocean, occupying marine, estuarine, and freshwater environments. Using a mitochondrial gene (cytochrome c oxidase subunit I) and a nuclear gene (rhodopsin), we analyzed the population history of L. grossidens along the South American coast. Our phylogenetic and population structure analyses revealed the existence of three clusters that diversified during the Late Pleistocene. The oldest cluster, located in cold waters at the southernmost extent of the species distribution, diverged approximately 1.05 million years ago, while the more recent clusters split about 80,000 years ago. One of these clusters exhibits a widespread distribution from the northern to the southeastern coast of Brazil, while the other is restricted to the lower Sa similar to o Francisco River in northeastern Brazil. The origin of this latter cluster is possibly associated with historical changes at the mouth of the Sa similar to o Francisco River, combined with the narrowing of the continental shelf in the region. Although this taxon is found in marine, estuarine, and freshwater environments, we did not identify genetic differentiation related exclusively to a single habitat. Our study provides a comprehensive understanding of the population history of the sabretooth anchovy, highlighting how historical climate changes and geographical processes shaped its diversification during the Pleistocene. These findings offer valuable insights for future studies on the conservation and management of these clusters.