Superfetation refers to a second conception during pregnancy, leading to a mother carrying different-aged embryos. It’s rare in most creatures, but well known in Poeciliidae fish.
The least killifish, for example, found in freshwater habitats across the south-east of the USA, stores sperm in her ovaries after mating. She sequentially fertilises her eggs, resulting in cohabiting embryos at different stages of development.
A female will typically carry three to five litters (though up to seven litters have been found) of different ages, with her placenta providing each with nutrition for about four weeks before she gives birth. The fry are released over a period of 10-14 days, or longer.
It’s thought that this strategy allows her to maintain a relatively sleek physique during pregnancy, which is crucial when needing to evade predators at speed.
