Birth Tale of Seahorse

Seahorses are the only animals where the males get pregnant. The dads make excellent “mums”, performing almost the same functions that occur in females during mammalian pregnancy and birth.

As seahorses do not have external genitalia, they are classified according to the size of gametes they produce. Males produce smaller gametes, sperm and females produce bigger gametes, eggs. The female transfers her eggs to the male’s abdominal pouch. The male releases sperm to fertilise the eggs and incubate them for 24 days till birth. But, the minute details of the pregnancy are still unclear.

To find out the contribution of male seahorses to their offspring, researchers from University of Sydney (USYD) analysed the alteration of pouch-gene-expression at different stages of gestation. They collected samples from male pouches at different stages of pregnancy and used modern DNA sequencing technologies to find the involvement of more than 3,000 different genes, engaged in various processes:

  • Delivering nutrients like, energy-rich fats and calcium to build tiny skeletons.
  • Helping in waste-removal and gas exchange.
  • Providing immunity to the developing embryos.

The team was excited to find that some genes also prepare the father and embryos for labour by involving oestrogen (a common female hormone) and producing signals one week before birth.

Remarkably, many of male-seahorse-pouch-genes are quite similar to genes of other pregnant animals. However pregnancies of mammals, reptiles and fishes have evolved completely independently of seahorse pregnancy, millions of years apart. Yet the similar gene expression in all the creatures, shows that gestation presents the same set of complex challenges to the parent, regardless of species.

Seahorse dads, just like human mums, deliver oxygen and nutrients to their embryos through the thickened skin, unlike through placenta in case of mammals. Thus the researchers raise the possibility of the recruitment of same genes across vertebrate animals: a remarkable display of convergent evolution.

Experts think that this study is a breakthrough in the understanding of the seahorse reproduction genetics, although much follow-up is required to conclude the functions of each and every gene.

But why male seahorses get pregnant where as females in other species has that responsibility is still a mystery. So seahorses, with their weird reproductive strategies, still have plenty more to offer to evolutionary biologists.

Their research has recently been published in Molecular Biology and Evolution

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Arunima Maiti

Arunima Maiti

Biomedical scientist with special interest in reproductive biology.

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