Hatchling Fish At 25 °C, embryos hatch at about 264 hours post fertilization. The hatchling is a larval stage and differs from the adult in many ways. The reddish organ on the embryo's left side is the spleen. Pectoral fin movement is important in ventilating the gills. Hatchlings consume protists of various types. As they grow, the size of their preferred food increases. Adults in the wild specialize in eating mosquito larvae.