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

Matilebias alexandri is native to the Uruguay river basin, occurring in the western regions of Uruguay, Brazil, and Northeastern Argentina. There is evidence of population numbers declining, due to severe suppression of habitat by anthropogenic activities, mainly rice and soy cultures, pollution and urban expansion. The species has been registered in São Donato’s Biological State Reserve, which is being overtaken by rice cultures, and is considered a priority species for conservation in Uruguay and endangered in Rio Grande do Sul. As such, based on reduction rates of over 30% over the past three generations, the species is assessed as Vulnerable A2ce; further research is required to investigate if these declines could continue in the future. 

Matilebias alexandri distribution is limited to the middle and lower Uruguay river basin, occurring in the western regions of Uruguay and Brazil, and western Argentina (Alonso et al. 2018, GBIF 2019, CRIA 2020). 

According to results of the project “Pampa’s Annual Fish” (IPPampa), the species seems to be locally abundant. However, there is no quantification of the species' subpopulations in Brazil, being considered a rare species with restricted distribution, and has being registered in small and fragmented areas (ICMBio/MMA 2018). There is intense rural activity in its range, with mostly rice, and more recently, soybean plantations growing, which may be driving a decline in the overall population of this species (ICMBio/MMA 2018). The species is also used for ornamental purposes in parts of its range. In Uruguay, a decline has not yet been monitored exclusively. Given ongoing threats however, a decline of over 30% is suspected to have occurred in the past three generations (10 years) of this species across the Brazilian, Argentinian, and Uruguay range of this species. There is little information to know if such significant reduction rates will continue in the future.

Matilebias alexandri is a small size species, reaching up to 4 cm and, like most seasonal killifishes (Aplocheiloidei), shows strong sexual dimorphism, where males are larger than females and have a brighter coloration with an intense and elaborate colour pattern. The species inhabits temporary wetland puddles in the Pampa region. The species presents life cycles similar to its congeners, like fast initial growth, early sexual maturity and short life cycles. Annual fish present are generally the largest and most conspicuous fish predators of the environment, where they feed mostly on aquatic invertebrates (ICMBio/MMA 2018). In Southern Brazil, the natural lifespan of annual fishes, including Cynolebiasinae species, was < 8 months, and the adults may die before the total desiccation of temporary pools and puddles (Lanés et al. 2016, Volcan et al. 2019), taking 2-3 months after the eggs hatch for development and reproduction (Huber 1996). Eggs are laid into the substrate of ponds, which will then go into diapause until rain fills the pond again (M. Loureiro and J. Bessonart pers. comm 2024).

The loss and fragmentation of wet areas in the western region, mostly due to rice and soybean production, are the biggest threats for the species' overall population. The release of herbicides and pesticides from these crops also affect egg viability and fish growth. Urbanisation, the construction of water dams and the recent forestry endeavours must also be highlighted, as they have been getting more investments in the region and are activities considered potentially impactful to subpopulations of M. alexandri. Furthermore, although the exact impacts are not known, eucalyptus plantations can affect hydrological cycles which could alter the hydroperiod of ponds, and unregulated collection of the species could also cause negative impacts.

In Brazil, although eggs of this species may be sold (from aquarium bred fish), collection from the wild of this species is genuinely limited and commercial trade does not occur for this species (F. Alonso in litt. 2023). Additionally, this species is hard to keep and transport (F. Alonso in litt. 2023). However, in Uruguay, there may be unregulated capture as there are no laws for the controlled extraction of these fishes (M. Loureiro and J. Bessonart pers. comm 2024).

Matilebias alexandri is listed as a target species in the National Action Plan for Endangered Rivulidae Conservation - PAN Rivulideos (ICMBio/MMA 2013). The feasibility analysis of subpopulations and acknowledgement of bioecological aspects of the species are of utmost importance to help in plans for its conservation. The species is considered a priority for conservation in Uruguay. There is a record of a subpopulation of the species in São Donato Biological reserve, which has been degraded and suppressed by rice plantations. The mapping of new subpopulations, protection and preservation of their biotopes and the recategorization of São Donato as a conservation unit are urgent measures to protect M. alexandri. The restraint of the rice plantation in the wetlands and the mandatory search and mapping of subpopulations of Matilebias in environmental licensing studies for new economic endeavours in the western region of Rio Grande do Sul must be done to avoid the loss and suppression of the remaining habitat of the species and its subpopulations (ICMBio/MMA 2018).

In Brazil, Matilebias alexandri was listed as Critically Endangered (CR) B1ab(ii,iii), according to the Brazilian national red list (ICMBio/MMA 2018).

In Uruguay, some subpopulations overlap with protected areas (M. Loureiro and J. Bessonart pers. comm 2024).