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Livebearers have already featured in this series with their ability to allow their offspring to develop internally. But that’s not their only remarkable ability, although this next ability isn’t fully understood. The swordtail (Xiphophorus helleri) is somewhat well known for being able to change its sex during its life cycle, from female to male. The physical change from reproductive female to a male, complete with gonopodium and ‘sword’ is well documented, but the consequences are not so clear. Even as early as the 1920s, this fish has been put under the microscope to show that even internally, changes occur leading to the conclusion that the transformed male fish is fully functional. Others refute this idea, hypothesising that as the female is no longer capable of reproduction due to age, she will change to a male appearance to discourage the energetically costly advances of other male fish. It’s remarkable that despite being such popular fish, there are still things we don’t know about them.

Image credit: fishkeepingadvice.com

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