Serra do Japi, located in the southeast of São Paulo State, is considered a priority area for conservation, as it houses original Atlantic Forest cover remains. Despite the significant number of studies about vertebrates and invertebrates that were carried out in this region, the meiofauna biodiversity is completely unknown. Thus, the present study aimed to investigate for the first time freshwater Gastrotricha and limnoterrestrial Tardigrada in Serra do Japi Biological Reserve. Samples of sediments, periphyton and floating vegetation in reservoirs and natural lagoons, and mosses growing on native and non-native tree trunks were collected in May 2019. At least five gastrotrichs morphotypes were identified and three of them were formally described: Chaetonotus acanthocephalus, C. dadayi (first record in Brazil), and Heterolepidoderma mariae (first record outside the type locality). In regards to tardigrades, twelve morphotypes were identified and four of them were formally described: Pseudechiniscus juanitae, Minibiotus cf. acontistus, Echiniscus dreyfusi and Itaquascon umbellinae (last two species reported for the first time outside the type locality). This study reinforces that meiofaunal diversity and distribution have been underestimated, even in one of the five largest hotspots in the world.

Due to the small size and the fragility of their bodies, that require specific handling and laboratory instruments, tardigrades have been little studied in terms of its taxonomy. In Brazil the limno-terrestrial tardigrades were not studied since the pioneers studies in the decades of 1940 and 1950. The present study aims to describe a new species of the genus Milnesium collected from Brazil using modern morphological techniques. This tardigrade is characterized by cuticle smooth, short cuticular bars under the claws IV, claw configuration [3-3][3-3] with internal and anterior spurs larger than external and posterior spurs, and dorsal pseudoplates.