The division of labor in reproduction, embryo development, and the later rearing of offspring varies wildly among marine animals. While on the one hand, you have the female octopus dutifully laying down her life starving to death as she tends to her young (potentially as a way of ensuring she doesn't eat them), the female seahorse takes more of a backseat. For these wee fish, it’s the male seahorse who becomes pregnant.

How do male seahorses give birth?

First, a quick recap on reproduction for seahorses. After finding a mate, a female will deposit her eggs into a specialized pouch on the male’s body. Once the eggs are inserted, he will fertilize them with sperm before carrying them for around 24 days while they develop. A male seahorse giving birth is quite a sight, it has to be said. Far from the slow and strained entrance we humans make into the world, baby seahorses fling into existence like bubbles out of a bubble gun. Thousands of babies can be born from a single pregnancy, making it quite the dive into parenthood for the expectant father.

Male pregnancy is seen in seahorses and pipefish

This mode of reproduction is unique to the seahorses and their close relatives, as despite birth having evolved more than 150 times independently, theirs are the only known examples where it’s the male of the species that gets pregnant, carries, and births the offspring. Pregnancy under any circumstances is a complex process in which the body must learn to support the developing offspring without identifying it as a foreign object and getting rid of it accordingly.

While embryos are built from the parent’s genetic material, sexual reproduction means that it is a blend of themselves and their mate and so the embryo appears as an invader for intents and purposes. Mammals are able to suppress the protests of their immune systems by down-regulating two major histocompatibility complex pathways known as MHC I and II. However, the knowledge train stalls here slightly as we’re unable to observe and recreate a birth process that is so internalized. Fortunately, the syngnathids (the fish family that seahorses and pipefish sit within) represent a sliding scale of pregnancy approaches from external to internal, offering better opportunities to observe what’s going on.

Genes facilitate male seahorse pregnancy tolerance

Research recently published in the journal PNAS was able to monopolize on this, and in observing different pregnancies among male seahorses and pipefish were able to uncover the genes that enable them to carry to term without their immune system attacking the embryos. The researchers found that the MHC II pathway showed signs of alteration among pregnant syngnathids; for the pipefish, this was the deletion of several genes, whereas for seahorses the pathway incorporated a highly divergent invariant chain known as CD74. It’s their belief that this results in an evolutionary trade-off that sees a dip in immune function in favor of retaining the non-self embryos, allowing, for the first time in nature’s history (as far as we know), male pregnancy to evolve.