Annual shes or annual killishes (Cyprinodontiformes; Aplocheiloidei) are freshwater teleosts exposed to an extremely variable environment that have a short lifespan. They inhabit temporary ponds in South America and Africa that dry out during the dry season, leading to the death of the entire adult population. During the rainy season the adults reproduce and generate desiccationresistant embryos that remain buried in the muddy bottom of the dry ponds and hatch in the next rainy season once the ponds are ooded. The resulting juveniles reach sexual maturity in a few weeks, and a new reproductive cycle begins (Wourms, 1964, 1967; Arezo et al., 2005). Therefore, the survival of the species becomes entirely dependent upon buried embryos (Figure 2.1). The annual condition is dened by two characteristics: (1) a special and unique stage during early development, the dispersion-reaggregation of deep blastomeres that separates epiboly to embryonic axis formation and (2) the possibility to enter into developmental arrests or diapauses before hatching (Myers, 1952; Wourms, 1972a,b,c). The lifespan of annual shes and the time they reach sexual maturity is variable among species. Among the African annual genus Nothobranchius, two species (N. kadleci and N. furzeri) have a median lifespan of 9 weeks, and juveniles attain sexual maturity at 17-19 days posthatching under laboratory conditions. Both species were reported as having the most rapid sexual maturation and minimum generation time of any reported vertebrate species

CONTENTS

2.1 Introduction ............................................................................................................................ 33 2.2 Sexual Strategy ....................................................................................................................... 35 2.3 Gonads: Morphology and Function ........................................................................................ 35 2.4 Gametes Morphology .............................................................................................................36 2.5 Fertilization ............................................................................................................................ 38 2.6 Development ........................................................................................................................... 38

2.6.1 Diapauses ....................................................................................................................40 2.7 Conclusions and Perspectives ................................................................................................. 42 Acknowledgments ............................................................................................................................ 42 References ........................................................................................................................................ 42

(Blažek et al., 2013). Among Neotropical species the genus Austrolebias has a longer lifespan, about 9 months, and sexual maturity is attained 8-12 weeks after spawning under laboratory conditions (Vaz-Ferreira et al., 1964; Volcan et al., 2013). Furthermore, adults of Austrofundulus limnaeus, an annual killish endemic to northern South America (Taphorn and Thomerson, 1978), remain fertile and produce viable embryos for more than 2 years in captivity (Podrabsky, 1999).