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Life can be tough when you’re only 3cm long, and when you’re one of the one of the smallest and most visually unassuming fish around, you’ve got to pack a lot of potential into a tiny package. Thankfully, the Least Killifish (Heterandria formosa) manages to do just that.

Despite its name, it isn’t a Killifish at all but rather a livebearer. At a maximum of 3cm long, it’s one of the smallest fish, the smallest livebearers and the smallest vertebrates known to science and lives primarily in slow-moving or standing freshwater in North America, happily nibbling on worms, plants and crustaceans.

Despite their size, each male boasts an impressive reproductive appendage known as a “gonopodium”. This modified anal fin is used to impregnate the females during mating and reaches nearly the full length of the male’s body. When the males engage in intermittent disputes for territory, they use their gonopodia to “sword-fight” and thus establish dominance.

The females, however, are even more remarkable, exhibiting a form of reproductive behaviour known as “superfoetation”. Put simply, this means that the female carries its young at varying stages of growth, from newly-fertilised eggs to fry that are ready for birth. Rather than releasing all its live fry in a single brood as most livebearers do, H. formosa instead produces its fry on a continuous “conveyor belt”, popping a new one out nearly every day for almost the entirety of the female’s adult life. This seemingly exhausting process does have an evolutionary benefit though, as it allows the female to bear larger fry, thus giving each individual fish a better chance of survival.