In all of the vast animal kingdom spanning the planet, seahorses (and their pipefish and sea dragon relatives) are the only species whose male members give birth to young. This underwater photograph shows a male Korean seahorse (Hippocampus haema) releasing juveniles into the water off the coast of Japan. The tiny species, which was only recognized in 2017, generally grows to between about five and nine centimeters long and is native to the Korea Strait and the seas to the east of the Korean peninsula and to the south and west of Japan. After an elaborate courtship “dance,” females deposit their eggs into a male’s brood pouch, where he fertilizes them. As the embryos grow, the male’s abdomen becomes distended, just as in a human pregnancy. When he is ready to give birth, the abdomen opens, and contractions expel the juvenile seahorses. Some newborns resemble miniature versions of adults, while others may still be curled up and covered by some of their egg membrane. A male and female seahorse pair can have multiple broods. It is unknown why seahorses have this sex reversal when it comes to procreation, but one idea is that having the male bear young leaves the female free to start producing the next batch of eggs.

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Science in Images