Male seahorses grow placentas to incubate their young

Seahorses are some of the most dazzling fish in the sea. They’re also the only group of animals in which the males, not the females, go through pregnancy and give birth.

Now, new research finds the male’s brood pouch—which can hold up to 1000 baby seahorses at a time—develops and functions like a human placenta.

“Evolution is just mind boggling,” says Camilla Whittington, an evolutionary biologist at the University of Sydney who led the new work. The study is the first to thoroughly examine how males nurture their young brood while they’re still in the pouch, says Mari Kawaguchi, an evolutionary biologist at Sophia University in Tokyo.

Kawaguchi, who has studied seahorses for some 2 decades, has long suspected pregnant seahorses develop something resembling a placenta. Now, at last, there’s proof.

Male seahorses start their path toward fatherhood with a dance. They twirl together with their chosen female under the water, changing colors and linking tails as they pirouette around a shared holdfast. Next, they align the female’s ovipositor with the male’s pouch opening so the female can deposit her eggs. Once the deed is done, the male gently sways to settle the eggs. Ten days to 6 weeks later, depending on the species, the male spends hours in labor, pumping and thrusting to force hundreds of tiny babies out into the water. There, they drift until they are grown. As for dad, he is ready for another round of courtship within hours after birth.

But during pregnancy, males have one goal: Provide the embryos with everything they need, from oxygen to nutrients to antibodies. “One of the biggest challenges that all pregnant parents have is getting oxygen to their embryos and carbon dioxide away from the embryos,” Whittington says. “That’s really what motivated our study–how do those baby seahorses actually breathe, if you will, inside the brood pouch?”

To find out how baby seahorses, called fry, get the blood and oxygen they need, Whittington and her colleagues examined big-belly seahorse (Hippocampus abdominalis) brood pouches at different points during their 34-day pregnancies.

As the embryos grew, the brood pouch became thinner and sprouted numerous blood vessels, just like a mammalian placenta during embryonic development, the researchers report in Placenta. The brood pouch reached its thinnest point in late-term pregnancy. At that point, the inner lining of the pouch was heavily wrinkled, which provided surface area for the new blood vessels to grow. Within 24 hours of giving birth, the male’s brood pouch had reverted to its prepregnancy form.

This placenta is very similar to the one that develops in pregnant humans, Whittington says. But although they share the same purpose, the tissues that make them up are very different. “In humans, the parent’s portion of the placenta is the uterus, whereas in seahorses, the parent’s portion … is essentially modified belly skin. It’s different tissues, but they are growing in the same kind of way.”

That makes sense to Sarah Foster, a conservation scientist at Project Seahorse, a research group based in Canada and the United Kingdom. “Those young need to get their oxygen and nutrients from somewhere, and the most intuitive place would be from the male’s blood, just as what happens with human fetuses.”

But what’s special about the seahorse placenta, Foster adds, is that it’s a fascinating example of convergent evolution, in which different species come up with similar biological solutions for the same problem. Despite the fact that seahorses are the only male animals to go through true pregnancy, that process looks remarkably similar to pregnancy in all other species. “Clearly, this is the best way for this to work,” Foster says. “And I just find that super, super cool.”

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