Seawater incubated embryos of a freshwater Aplocheilus lineatus (Aplocheilidae; Cyprinodontiformes) support the dispersal hypothesis of killifish intercontinental distribution

BACKGROUND: Aplocheiloid killifishes (Cyprinodontiformes, Aplocheiloidei) inhabiting Africa, America, and South Asia are unique in that embryos of annual species are capable of entering a state of diapause and becoming extremely stress resistant. Distribution of aplocheiloids is a biogeographical enigma: how freshwater fish managed to cross long-distance oceanic barriers to become intercontinentally distributed? As a hypothesis, we proposed their embryos to be a dispersal stage. To test it, we compared embryonal development in seawater and freshwater incubated eggs of Aplocheilus lineatus from the Aplocheilidae family, an early-diverging aplocheiloid non-annual clade with the intercontinental disjunct distribution across Madagascar and India separated by an ocean. RESULTS: Seawater incubated A. lineatus embryos were revealed to be salt tolerant and able: (i) to delay hatching and prolong embryonic period, (ii) to retard embryonic development and growth, and (iii) to reduce consumption of endogenous food reserves. CONCLUSIONS: Revealed developmental traits in principle may favor long-distance transoceanic trips of embryos. Given eggs in aplocheilids are adhesive and can readily attach to plant debris, rafting seems to be possible as a mechanism of oceanic dispersal. It is not excluded that a similar mechanism was used by some other freshwater fish clades with intercontinental distribution (e.g., cichlids).