Información sobre la ecología e historia de vida de varias especies de rivulidos no ha sido estudiada, por lo tanto se analiza la biología reproductiva de dos especies neotropicales (Austrolebias vandenbergi y Neofundulus ornatipinnis) que habitan pozos semipermanentes y temporales en el Gran Chaco Boliviano. Las recolectas se realizaron al inicio y al final de la estación lluviosa. Dimorfismo sexual, fecundidad absoluta, diámetro de los ovocitos y proporción sexual fue determinada para cada especie de rivulido. Machos en las dos especies presentaron mayor tamaño de longitud estándar y altura del cuerpo cuando fue comparado con las hembras. Austrolebias vandenbergi tuvo menor fecundidad por hembra y huevos más pequeños que N. ornatipinnis, pero hembras en ambas especies presentaron una relación estrecha y positiva entre fecundidad y tamaño cuerpo. La presencia de varios estadios gonadales sugiere un desove fraccionado, lo cual es una estrategia reproductiva que promueve la viabilidad y sobrevivencia de estos peces en hábitats de extremas condiciones. La proporción sexual no varió entre machos y hembras de A. vandenbergi (1:1), pero en N. ornatipinnis la proporción sexual fue dominada por las hembras (1:1.8). Este estudio representa uno de los primeros que investiga aspectos de la biología reproductiva de A. vandenbergi y N. ornatipinnis, lo cual es importante para la conservación de rivulidos en regiones tropicales.

The dry Chaco, a semiarid thorn forest, is experiencing some of the highest deforestation rates globally, coupled with the fact that small-bodied fish are at the highest risk of extinction, the killifish inhabiting this region may be some of the most threatened taxa. Yet, aspects of ecology and life history for Neotropical killifishes in the Bolivian Gran Chaco region are completely lacking, and basic life-history data is of critical importance for the design and implementation of conservation measures. Collections were conducted during the early (January 2011) and late (March-April 2011) rainy season using an area-based sampler and dip net surveys. Fish standard length and body depth were measured as well as the number of oocytes per size class, mean oocyte diameter per size class, and total fecundity for the females of each species. A total of 490 specimens of rivulids were captured; Austrolebias vandenbergi: 85 females, 105 males and 39 juveniles, and N. ornatipinnis: 62 females, 113 males, 86 juveniles. Sexual size dimorphism, absolute fecundity, oocyte developmental stages, oocyte diameter, and population sex ratios were determined for each species. Both species exhibited sexual size dimorphism. Male A. vandenbergi exhibited longer standard length (mean±SD; males: 27.07±3.89mm, females: 23.6±2.02mm) and body depth (males: 8.9±1.7mm, females: 7.2±1.1mm) as compared to females. Male N. ornatipinnis had a similar pattern for both standard length (males: 26.0±7.1mm, females: 19.1±5.83mm) and body depth (males: 5.6±1.9mm, females: 4.7±1.0mm). Austrolebias vandenbergi had fewer and smaller oocytes per female (47±31.6) than N. ornatipinnis (206±131.2). There was a positive relationship between fecundity and female body size in both species. The presence of multiple developmental stages of oocytes (immature, maturing, and mature) suggest that both species of rivulids exhibit fractional spawning, a reproductive strategy that enhances reproductive success in these extreme habitats. Neofundulus ornatipinnis exhibited a higher mean oocyte diameter for all three developmental stages (immature, maturing, and mature) as compared to A. vandenbergi. Austrolebias vandenbergi exhibited an equal adult sex ratio (males:females, 1:1), but there was a slight female biased ratio for N. ornatipinnis (males:females, 1:1.8). These results provide fundamental and valuable information for ensuring rivulid conservation in tropical regions, and also improve the knowledge on the biology and ecology of these poorly known species. To our knowledge, this study represents the first contribution on the reproductive biology of two Neotropical annual rivulid fishes (Austrolebias vandenbergi and Neofundulus ornatipinnis) inhabiting semi-permanent and ephemeral ponds in the Gran Chaco of Southeastern Bolivia.