How does the gonopodium help male live-bearing fishes reproduce?

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In male live-bearing fishes (family Poeciliidae), the gonopodium is a specialized intromittent organ formed from modified anal fin rays.1

This organ is essential for reproduction because it facilitates internal fertilization. By acting as a conduit, the gonopodium allows the male to transfer sperm directly into the female's reproductive tract. This internal delivery is a key adaptation for live-bearing species, ensuring that fertilization occurs within the female's body, which subsequently allows the embryos to develop internally until they are born as fully developed young.123