Large litters are necessary because only about 0. Many, if not all, of the 47 known seahorse species—14 of which were identified only in the 21st century—are in decline worldwide. Because seahorses generally live in shallow, near-coastal waters, human activities including development, pollution, fisheries, and traditional medicine have reduced their numbers. At the same time, their universal appeal has worked against them; until recently, wild seahorses were often captured for the aquarium trade.
The delicate creatures tend to fare poorly in aquaria, however. 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. 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.
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.
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. 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 , 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. 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. Science in Images is a new category of articles featuring photographs and videos from all the disciplines of science.
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