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We evaluate the evidence of cryptic speciation in Larimus breviceps, a species widely distributed in the western South Atlantic, from the Greater Antilles to Santa Catarina in Brazil. Mitochondrial (COI, Cyt b, and Control Region) and nuclear (IGF1 and Tmo-4C4) sequences were obtained from populations in the western South Atlantic. The analysis revealed two genetically distinct, sympatric lineages with no gene flow, with L. breviceps lineage II (LII) being closer to Larimus pacificus than to the L. breviceps lineage I (LI). The most recent common ancestor (TMRCA) for the L. breviceps LI and L. pacificus/L. breviceps LII clade dates from 12.3 Ma, whereas TMRCA for the L. pacificus and L. breviceps LII dates from 3.4 Ma, indicating that speciation processes may be related to the rise of the Isthmus of Panama. Despite these profound genetic differences, morphometric analyses found only subtle differences between lineages, with specimens of the LI being slightly larger than those of the LII, suggesting the existence of cryptic species. Within each lineage there is a pattern of panmixia within the study area. Therefore, we suggest that it is necessary a taxonomic revision of Larimus from the western Atlantic to validate the species status of such lineages., Competing Interests: Declarations. Competing interests: The authors declare no competing interests. Ethical approval: The approval by the ethics committee was not requested because the fish were purchased from artisanal fishermen and were already dead at the time of collection., (© 2024. The Author(s).)

The definition of species boundaries constitutes an important challenge in biodiversity studies. Cynopoecilus Regan, 1912 encompasses several endangered species of annual fish, occurring in temporary ponds in a restricted area of Southern Brazil and Uruguay. Divergences about the taxonomic status of Cynopoecilus species highlight the importance of species delimitation studies. Therefore, we address here the phylogenetic structure of Cynopoecilus, while assessing its taxonomic implications. For this, fragments of the mitochondrial COI and nuclear RAG1 genes were characterized and analyzed for a set of 275 and 280 specimens, respectively. DNA barcoding and phylogenetic analyses detected subdivision of these specimens in 8–10 clusters, which comprise the six previously described species, and suggest one invalid taxon and at least 3–5 putative new species. The phylogenetic structure also suggests that the Jacuí River and the Patos Lagoon historically acted as effective barriers to gene flow between populations, although some isolated dispersal events across these water bodies could be evidenced, especially for C. melanotaenia Regan, 1912. In general, the results highlight the need of independent conservation strategies within the distribution area of each of the endemic allopatric killifish clusters, while questioning several taxonomic boundaries and distribution data. [ABSTRACT FROM AUTHOR]

• DNA barcoding analyses were conducted on Mansonia species collected from Jirau hydroelectric dam, Rondônia State, Brazil; • The results indicated five Mansonia species in the study area; • The results also pointed to the occurrence of cryptic speciation in the analyzed samples; • COI sequences were useful tools to both estimate phylogenetic relationships and identify morphologically similar species in the subgenus Mansonia. Previous studies have linked the construction of hydroelectric dams with increases in the density of mosquitoes, especially Mansonia. In Brazil, Mansonia mosquitoes are still poorly studied at the taxonomic, biological, ecological and epidemiological levels, and nothing is known about the genetic diversity and the cryptic speciation of the group. The current study analyzed the molecular taxonomy of Mansonia species captured in the area surrounding the Jirau hydroelectric dam, Rondônia state, Brazil. Samples were collected from fifteen locations between 2018 and 2019. Genomic DNA of the specimens was extracted, and the DNA barcode region of the Cytochrome Oxidase, subunit I gene was amplified with PCR and both DNA strands were sequenced. The dataset was analyzed using MEGA, Mr. Bayes and DnaSP software. The results provided COI sequences for 100 specimens collected in the area surrounding from Jirau hydroelectric dam. These belonged to five species of the Mansonia subgenus, identified morphologically as Mansonia humeralis, Mansonia amazonensis, Mansonia titillans, Mansonia dyari and Mansonia indubitans. Findings showed that the COI gene is an effective and accessible DNA barcode that provides a high-resolution tool for delimiting species within the subgenus Mansonia , with the tree construction (Bayesian Inference) well supported and non-overlapping intraspecific and interspecific (K2-P) genetic distance values. These findings also indicate the occurrence of cryptic speciation within M. dyari and near of M. titillans. This is the first study to apply molecular tools to the taxonomy of Mansonia species from Brazil. [Display omitted] [ABSTRACT FROM AUTHOR]