Genetic structuring among silverside fish (Atherinella brasiliensis) populations from different Brazilian regions

Abstract Estuaries are dynamic environments, key for the survival of innumerous ecologically or economically important fish species. Among these species are Neotropical silversides ( Atherinella brasiliensis ), which are resident and abundant in Brazilian estuaries and used as a complementary source of income and food for local communities. To better understand silverside populations in Brazil, we evaluated the genetic diversity, structure and demography of fish sampled at six estuaries from the northeastern to the southern coast, using Random Amplified Polymorphic DNA and mitochondrial DNA (D-loop) markers. High haplotype diversities (h ranging from 0.75 to 0.99) were found in all populations except Carapebus, located in Southeast Brazil (h = 0.54). A total of 69 mtDNA haplotypes were found, with Itaparica (Northeast Brazil) and Carapebus presenting only exclusive haplotypes, while some were shared among populations in the South. Strong regional structure was observed, with very high differentiation between Itaparica and Carapebus, as well as among these two populations and the ones from the Southern region (Paranagua, Conceicao, Camacho and Patos). Among southern areas, low/moderate structure was detected. Most populations showed unimodal mismatch distributions indicating recent demographic expansion, while Carapebus presented a multimodal distribution characteristic of a stable or bottlenecked population. Times since possible population expansion were highest in Itaparica (32,500 ya) and Carapebus (29,540 ya), while in the Southern region longest time was observed at Conceicao (25,540 ya) and shortest at Patos (9720 ya). In a general manner, haplotype diversities were directly related to times since population expansions; again, Carapebus was the exception, displaying long time since expansion but low diversity, possibly due to a recent bottleneck caused by the isolation and human impacts this lagoon is subject to. Isolation by Distance was significant for Itaparica and Carapebus, and considering the extremely high differentiation of these populations, we suggest that they could be undergoing speciation. To adequately manage and maintain the genetic variability of silversides in Brazilian estuaries, we propose three distinct management units for this species: 1) Itaparica; 2) Carapebus; and 3) Southern Brazil.