We follow Alonso et al. (2023) in considering Matilebias a valid genus, rather than a synonym of Austrolebias.

Matilebias juanlangi occurs in Brazil and Uruguay. It occurs in small wetlands, generally smaller than 1,000 m². These ponds are thought to be undergoing a continuing decline in habitat quality, mostly due to draining for industrial or farming activities. As such, given a moderately small extent of occurrence (16,925-28,755 km²), the species is assessed as Near Threatened. 

Matilebias juanlangi is distributed in Patos-Mirin lagoon system and upper Negro river basin (Costa 2006, Volcan et al. 2011), in Rio Grande do Sul state, Brazil, and in Uruguay. The type locality is a temporary wetland next to Minuano wetlands, BR 153 freeway, Jaguarão river basin, Mirin lagoon tributary, Patos lagoon hydrographic system, in Uruguay (ICMBio/MMA 2018).

There is no information about any population parameter. 

Matilebias juanlangi is a seasonal killifish with a maximum size recorded of 3.1 cm in length (ICMBio/MMA 2018 and references therein). It presents considerable sexual dimorphism; males being more colourful, larger and having more rays in both dorsal and anal fins like most Cynolebiasine species. The species occurs in small ephemeral wetlands, in puddles severely fragmented, mostly by rice cultivation. The wetlands with species records present thick aquatic vegetation and are in campestral areas, completely exposed to sunlight. Some biotopes are restricted to little draining channels from resting rice farmland. Species subpopulations have been registered in syntopy with other seasonal fish species, like Acantholebias quirogai and Austrolebias univentripinnis. In the little wetlands it occurs, the females tend to be more abundant. Like most seasonal killifishes (Aplocheiloidei), it has a short life cycle, reaching sexual maturity early and dying in the drought season, when the puddles it inhabits dry out. The eggs are deposited in the muddy bottom, maintain a diapause state and hatch when the puddles fill again, in the rainy season (ICMBio/MMA 2018 and references therein).

The biggest threat to the species is the drainage and fragmentation of the wetlands for rice and soybean cultivation, as well as for forestry, whilst herbicide and pesticide release also negatively affects the viability of eggs and the growth of fish. Other threats are the possible installation of businesses with potential impact on the area, such as wind farms and silviculture to the area and general urban expansion (ICMBio 2013 and references therein). Little is known about these threats, but unregulated collection and eucalyptus plantations could also impact the species (M. Loureiro and J. Bessonart pers. comm. 2024). 

In Uruguay, there may be unregulated capture of these fishes by aquarists as there are no extraction laws in place (M. Loureiro and J. Bessonart pers. comm. 2024). 

Matilebias juanlangi is listed in the National Action Plan for Conservation of Endangered Rivulidae - PAN Rivulídeos in Brazil (ICMBio 2013). There are numerous records of annual fish of conservation importance in the occurrence area of Matilebias juanlangi. There is a need for protection mechanisms of Jaguarão river’s floodplains and its smaller tributaries, in addition to new sampling efforts to verify the existence of this species in new biotopes (ICMBio/MMA 2018). In Brazil, M. juanlangi was categorized as Endangered (EN) (ICMBio/MMA 2018). In Uruguay, one subpopulation is within protected areas (M. Loureiro and J. Bessonart pers. comm. 2024).