When it comes to bending gender stereotypes, seahorses and their relatives would have to be one of the most extreme examples. These fish swap the traditional roles of mums and dads as they are the only animals where the males get pregnant.

Even though fish don’t have the external genitalia that we normally associate with males and females, we can still distinguish between them. That’s because we classify animal sexes according to the size of the gametes (sex cells) they produce. Males produce the sperm (the smallest gametes) and females produce the eggs (the biggest gametes).

But in seahorses, the sperm-producers are also the ones that get pregnant. The female transfers her eggs to the male’s abdominal pouch, made of modified skin. The male releases sperm to fertilise the eggs as they enter, before incubating them for 24 days until they are born.

Investigating pregnant dads

We’ve known for a long time that seahorse males get pregnant. But until now, we haven’t known much about what actually goes on inside the male pouch.

In new research published this week in Molecular Biology and Evolution, just in time for Father’s Day, our team investigated whether male seahorses contribute more to their offspring than just sperm and a container to gestate the embryos.

We took samples from male pouches at different stages of pregnancy and then used new DNA sequencing technologies to assess how pouch gene expression changes.

This is the first time that these technologies have been used to examine the full course of pregnancy in any animal. It allowed us to examine the genetic basis of the processes going on inside the pregnant pouch.

Rudie Kuiter, Aquatic Photographics , Author provided

We found that seahorse pregnancy is incredibly complex: more than 3,000 different genes are involved. When we examined them in detail, we found genes involved in many different processes. We even discovered genes allowing seahorse fathers to provide nutrients to their developing embryos.

In particular, fathers supply energy-rich fats and calcium to allow the embryos to build their tiny skeletons and bony body rings that sit just under the skin. Other pouch genes help the males remove wastes produced by the embryo, such as carbon dioxide and nitrogen.

Seahorse dads even seem to protect embryos from infection, producing antibacterial and antifungal molecules to ward off pathogens.

Preparing for birth

Seahorse birth is even more of a mystery than seahorse pregnancy, and we were excited to find that some of those 3,000 genes also prepare the father and the embryos for labour.

With around one week to go, instead of packing a hospital go-bag, seahorse dads start producing hatching signals. These signals cause the embryos to hatch out from their thin membranes and swim freely inside the brood pouch.

As the embryos take up more room, the pouch begins to stretch, much like the belly of a very pregnant human. The hormone oestrogen also gets involved and these combined forces produce cascading genetic signals that produce birth.

Similarities across animal pregnancies

So seahorse dads make excellent “mums”, performing many of the same functions that occur in females during mammalian pregnancy and birth. Strikingly, many of the seahorse genes are similar to those in other pregnant animals.

This is surprising because pregnant mammals, reptiles and other fish all incubate their embryos inside the female reproductive tract. Their pregnancies have evolved entirely independently of seahorse pregnancy, millions of years apart, and yet we see the same processes occurring.

Why would the genes controlling male and female pregnancies be similar? We think that this is because gestation presents the same set of complex challenges to the parent, regardless of species.

Seahorse dads, just like human mums, need to make sure they can provide oxygen and nutrients to their embryos. We do it with a placenta inside a uterus and seahorse dads do it with thickened skin inside a pouch, but we’ve used similar genetic instructions to get there.

Camilla Whittington , Author provided

Our findings raise the possibility that the same genes have been repeatedly and independently recruited for pregnancy across vertebrate animals – a remarkable display of convergent evolution.

We’ve shown how seahorse dads use thousands of genes working in concert to provide the ideal environment for embryonic growth. This is a breakthrough in our understanding of the genetics of seahorse reproduction, although much follow up work is required to definitively test the functions of every one of those genes.

But we still haven’t solved the mystery of why seahorse fathers get pregnant given that females have that responsibility in every other animal. Seahorse mums still contribute nutrient-rich egg yolks that feed developing embryos, but their responsibility for their offspring ends at mating.

So seahorses, with their bizarre reproductive strategies, still have plenty more to offer evolutionary biologists.