Agradecimentos: The authors are grateful to team of ICMBio/CMA, Instituto Brasileiro do Meio Ambiente e dos Recursos Naturais Renováveis/Amapá, Projeto Bicho D’água (Programa Petrobras Ambiental), Sirenia Project, United States Geological Survey (USGS), Florida University and Instituto Bicho D’água staff. We thank Rodrigo P. Sousa (UFPA) for his interesting comments and suggestions, as well as the referees for carefully reading our manuscript and for giving constructive comments which substantially helped improving the quality of the paper. This research was funded by Projeto Piatam Mar and Piatam Oceano/Petrobras, Projeto Bicho D’água/Petrobras, Projeto Inova-Fiocruz, Vale/Fundação de Amparo à Pesquisa do Estado de São Paulo/Fundação de Amparo à Pesquisa do Estado do Pará/Fundação de Amparo à Pesquisa do Estado de Minas Gerais (FAPESP/FAPESPA/FAPEMIG), grant number 038/2011, PAPQ-PROPESP/UFPA (Edital 02/2022). C.R.B. was funded by CNPq and FAPERJ (304498/2014-9, E26/201.200/2014), S.S. by CNPq (306076/2019-5), R.E.-L. by CNPq (313356/2015-7), E.H.C.d.O. by CNPq (307382/2019-2) and A.F.C. by CNPq (313522/2015-4) Ver menos

Abstract: Two species of manatees are found in Northern Brazil-the Antillean manatee (Trichechus manatus), which is found along the coast from Florida to Northeastern Brazil, and the Amazonian manatee (Trichechus inunguis), endemic to the Amazon drainage basin. These species show a sympatric distribution in the region of the Marajó Archipelago, an estuarine area surrounding the Amazon River mouth. There is evidence of the occurrence of interspecific hybrids in this area, based on mitochondrial DNA analyses, although the use of nuclear markers has not corroborated this proposal. Considering that these species show very distinct karyotypes, despite being closely related (2n = 48 in T. manatus and 2n = 56 in T. inunguis), hybrids would present distinct chromosome numbers. Based on this, we conducted cytogenetic analyses using classic and molecular techniques in three calves found stranded in the Marajó Island and Amapá coast. The results showed that one of them, morphologically classified as T. inunguis, presented the correspondent karyotype, with 2n = 56. However, the other two, which were phenotypically similar to T. manatus, showed 2n = 49. Despite the same diploid number, their G-banding patterns revealed some differences. The results of the distribution of some microsatellite sequences have also confirmed the heterozygosity of some chromosomal pairs in these two individuals. These results are the first indubitable confirmation of the occurrence of natural hybrids between T. manatus and T. inunguis, and also brings about some issues concerning the viability of hybrids, considering that these two individuals do not correspond to an F1 hybrid, but instead, both presented a possible F2 karyotype Ver menos