Brazil is one of the two South American countries to have records ofthe four genera of freshwater cnidarians, being pioneers in the study of these organisms. Nevertheless, these studies were discontinued, resulting in a significant lack of information about these organisms. Hydra is the best known of these genera, and from the first report in Brazil to the present, it has four records with somewhat detailed morphological descriptions. Three of these records describe species endemic to Brazil: Hydra iheringi, Hydra salmacidis, and Hydra intermedia. However, these findings are outdated and very isolated, and they require immediate taxonomic corroboration. To achieve this objective, a sampling plan was implemented between April and June 2023 in seven freshwater bodies in São Paulo State, which also included a sample from Mafra, Santa Catarina State.

Hydra specimens were found to be adhered to different species of floating and submerged vegetation. All polyps were morphologically described in detail, their cnidocysts and sexual structures were measured and typed, and the cytochrome oxidase I (COI) marker was extracted, amplified, and purified using a protocol that was applied for subsequent sequencing. Morphological and molecular analyzes indicated the presence of two species: the cosmopolitan Hydra vulgaris and the green species Hydra sinensis, first described in China. This is the first time that both species have been described in Brazil, based on more exhaustive identification methodologies.

These results are the first of this type obtained for Brazil, and they reveal that Hydra is a frequent and abundant genus in the freshwater fauna from this study region. Continuing these analyses in other states would provide valuable data for the phylogeny of the group, as well as the possibility of discovering new species. Considering that the presence or absence of these organisms reflects the conservation status of the ecosystem, and that they are also bioindicators of hydrographic conditions, our studies are a valuable tool for the sustainable management of continental aquatic ecosystems.