Are seahorses and horses genetically related

The seahorse's secret of success deciphered

Seahorses do not necessarily look like typical fish and their swimming abilities are also rather poor compared to most other fish species - and yet they can be found in almost all tropical and temperate oceans. Researchers have now investigated the secret of the success of these strange animals, in which the males give birth to the young, using almost 360 seahorse genomes. With the help of a family tree of 21 species created from this, she was able to reconstruct the distribution paths of seahorses across the oceans and understand where and when new species emerged.

The international research cooperation with the participation of the working group of evolutionary biologist Axel Meyer from the University of Konstanz and scientists from China and Singapore succeeded in using the largest data set to date on seahorse genomes to identify factors that determine the developmental success of seahorses: rapid adaptability and large genetic variability.

Griffin tail instead of fins

Seahorse species of the genus Hippocampus originated from pipefish, their closest relatives, in the Indo-Pacific around 25 million years ago. While these can mostly swim relatively well, seahorses have lost both their pelvic and caudal fins and instead developed a prehensile tail, which they use to cling to seaweed or corals, for example. They split into two main groups early on. "One group more or less stayed on site while the other conquered the world," says Ralf Schneider, co-author of the study published in the journal "Nature Communications". In the home region in the Indo-Pacific, the species that remained there were able to diversify due to a special island landscape, while the others made their way to Africa, Europe and America and finally spread in the Pacific.

The particularly large amount of data that was collected for this study allowed the research team to create a particularly reliable seahorse family tree, whereby the relationships were determined and the spread of seahorses across the world could be traced. "If you compare the relationship with the ocean currents, you can see that seahorses were washed across the oceans by these currents," says the evolutionary biologist Schneider.

On rafts across the oceans

For example, if they are carried into the open sea by storms, they hold onto anything they can find with their prehensile tail, such as torn algae or tree trunks. The animals can survive here for a long time. The currents often carry these "rafts" hundreds of kilometers across the sea before they wash up somewhere, the seahorses dismount and find a new home.

Since seahorses have been around for 25 million years, it was important not only to consider today's flow conditions and sea corridors, as these are constantly changing due to the shifting of the tectonic plates. About 15 million years ago, for example, the Tethys Sea stretched roughly the size of the present-day Mediterranean. In the west there was a connection to the Atlantic at the point of the Strait of Gibraltar, in the east there was an exit to the Indian Ocean in what is now the Arabian Peninsula.

Just made it to the Tethys Sea

The researchers were able to confirm, for example, that the seahorses coming from the Arabian Sea just colonized the Tethys before tectonic changes lifted the land plates and thereby closed the sea in the east. With the new flow dynamics towards the Atlantic, the seahorses reached North America via the western opening. A few million years later, the western opening also closed, and the entire sea dried up. Ralf Schneider: "So far it was unclear whether the seahorses in the Atlantic were not all descended from a tribal line that migrated south from the Arabian Sea along the African east coast and past the Cape of Good Hope into the South Atlantic and at the height of South America Crossed the Atlantic. We found out that a second line did that too, but later. "

Because the research team had around 20 animal specimens available from each habitat, the genetic variation between the individuals could also be determined. The general rule here is: the higher the variation, the larger the population. "The type of variation can be used to reconstruct how old it is. This can then be used to calculate back when the population was how large," explains the evolutionary biologist. According to this modeling, the population that came to North America across the Atlantic was very small, which supports the thesis that it goes back to only a few animals that got there with the current holding onto their raft. Using the same data, it was also possible to show that seahorses from Africa continue to cross the South Atlantic and bring their genetic material into populations in South America.

Quickly changed and adaptable

Not only were seahorses spread across the oceans by the currents, they were also amazingly good at colonizing new habitats. Seahorses have heavily modified genomes and have lost many genes during their evolution, new genes have arisen or have been obtained from gene duplications. That means: Compared to other fish, seahorses changed very quickly. Presumably, therefore, "spines" evolved quickly and independently of each other in different species, which in some habitats serve to ward off predators.

In fact, some genes have been identified that are particularly modified in the sting species, but not in the same way in all species. The fact that spines were selected quickly and independently of each other several times, although the same genes play a role in the development of these skin spines, but different mutations were responsible, shows: These slow, sessile, i.e. localized seahorses can adapt particularly quickly. Scientists see this as one of the main reasons why seahorses have been so successful in colonizing new habitats. (red, 1.3.2021)