Seahorses are extremely bad swimmers. It is surprising, however, that they can be found in all the world’s oceans. Based on nearly 360 different seahorse genomes, a group of researchers studied how these particular fish could have spread so successfully around the world. Based on the evolutionary tree of 21 species, it was possible to reconstruct the ways in which seahorses spread around the world and explain where and when new species appeared. An international research collaboration involving a research team led by evolutionary biologist Professor Axel Meyer of the University of Constance and researchers from China and Singapore was able to identify the factors that led to the seahorse’s success from a developmental biology perspective: its adaptive speed, for example, the spine in the skin and its rapid genetic rates of evolution. The results will be announced on February 17, 2021 Nature Communications.
Seahorses of the genus Hippocampus appeared about 25 million years ago in the Indo-Pacific region from cunning fish, their closest relatives. And while the latter usually swim quite well, seahorses lack bowls and tail fins, and instead have developed a covered tail that can be used, for example, to hold seaweed or coral. Early on, they split into two main groups.
“One group remained largely in the same place, while another spread around the world,” says Dr. Ralf Schneider, who is now a postdoctoral researcher at the GEOMAR Helmholtz Center for Ocean Research in Kiel, and participated in the research while working as a doctoral researcher on Axel Meyer’s research team. In their native Indo-Pacific native waters, the remaining species diversified into a single island environment, while another group broke through the Pacific Ocean across Africa, Europe, and America.
Traveling the world by raft
A particularly large amount of data collected for the study allowed the research team to create a particularly reliable seahorse tree that shows the link between species and global seahorse spreading pathways. Evolutionary biologist, dr. Schneider says, “If you compare the relationship between species and ocean currents, you’ll notice that seahorses were transported across the ocean.” If, for example, they were thrown out to sea during storms, they would grab their tails to catch anything they could find, such as a piece of algae or a tree trunk. These are places where animals could survive for a long time. Currents often flooded these “rafts” hundreds of miles across the ocean before they landed somewhere where seahorses could jump in and find a new home.
Since seahorses have existed for more than 25 million years, it was important to consider that ocean currents have changed over time as tectonic plates have changed. For example, about 15 million years ago, the Tethys Ocean was almost as large as today’s Mediterranean Sea. On the west side, where the Strait of Gibraltar is located today, it connects with the Atlantic Ocean. On the east side, where the Arabian Peninsula is today, it led to the Indian Ocean.
Tectonic shifts change ocean currents
The researchers, for example, could underline that seahorses may have colonized the Tethys Ocean across the Arabian Sea just before tectonic plates shifted and sealed the eastern connection. The resulting current, which flowed westward toward the Atlantic Ocean, brought seahorses to North America. A few million years later, this western connection also closed and the entire ocean of Tethys dried up. Ralf Schneider: “Until now, it was not clear whether the seahorses in the Atlantic were all of Arabian species that traveled south along the east coast of Africa, around the Cape of Good Hope and across the South Atlantic to reach the South America. We discovered that the other line of seahorses did just that, though later. “
Because the research team collected 20 animal samples from each habitat, it was also possible to measure genetic variations between individuals. And this was generally revealed: The larger the variations, the larger the population. “We can reconstruct the age of the variation based on its type. This allows us to calculate the size of the population at different times,” explains the evolutionary biologist. This calculation reveals that the population that crossed the Atlantic Ocean to North America was very small, supporting the hypothesis that it originated from only a few animals that ocean currents brought there holding on to a raft. The same data also showed that even today, seahorses from Africa cross the South Atlantic Ocean and bring their genetic material into the South American population.
Fast and flexible adaptation
Seahorses not only spread around the world traveling by sea currents, but were also surprisingly good at settling in new habitats. Seahorses have greatly modified their genomes and during their evolution have lost many genes, reappeared with new ones, or gained duplicates. This means: Seahorses change very quickly compared to other fish. This is probably the reason why different types of “bone spines” have developed quickly and independently of each other, which protect seahorses from predation in some habitats.
Some genes have been identified that show certain modifications for certain species, but are not the same for all species. Multiple rapid and independent selections have led to the development of thorns, and although the same genes play a role in this development, different mutations are responsible. This shows that the slower, sedentary seahorses were especially able to adapt quickly to their environment. This is one of the main reasons the research team cites because seahorses are so successful in colonizing new habitats.
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The genomic sequences of 21 seahorse species shed light on global propagation pathways and suggest convergent mechanisms of development of unusual bony spines. Nature Communications. DOI: 10.1038 / s41467-021-21379-x
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