Where does the sea dragon’s beautiful appearance come from? Decoding the secret of sea dragons

For scuba divers, spotting a lithe sea dragon, suspended among seaweed, adorned with shaggy feathers swaying in the ocean currents, is truly a sight to behold. Sea dragons have special features, lacking teeth, lacking ribs, and curved and folded spines.

Modern scientists have found genetic clues that may explain why sea dragons have such a distinctive appearance. Not only do their genomes contain many repetitive stretches of DNA that drive evolution, but they also lack a group of genes that produce teeth, nerves and many facial features in other animals.

Sea dragons belong to the same family as pipe fish and sea horses, order Syngnathidae, and are famous for having male pregnancies. However, they are oddballs in an already oddball group of fish. To find out why, the research team of Small, Bassham and colleagues sequenced the genomes of two species of seadragon: the leafy seadragon and the common seadragon, both found in cooler waters. off the southernmost coast of Australia.

Drifting with leaves provides excellent camouflage among kelp-covered reefs, and these slender animals can be difficult to spot. The third sea dragon species belongs to three rare species that have only just been discovered in the wild. All three sea dragon species are admired for their colorful, fantastical body shapes and long, blood-sucking crustacean snouts. But the ruby seadragon appears to have lost its leafy appendages like other species, while evolution eliminated its ornate outlines.

Scientists believe that sea dragons have evolved their physical characteristics quite rapidly over the past 50 million years, since they and sea horses branched off to form a new family. What’s unclear, however, is how they look so special. University of Oregon researchers collaborated with teams from the Birch Aquarium at Scripps Institution of Oceanography and the Tennessee Aquarium, to analyze samples from sea dragons in captivity.

The findings show that sea dragons, compared to their closest relatives, such as pipefish and seahorses, have in their genetic code a surprisingly large number of repetitive DNA sequences called transposons, or also known as ‘jumping genes’. So when mobile transposons jump around in the genome, they can make rapid genetic changes – which may explain why sea dragons evolve so quickly.

Compared to two distant relatives, the zebrafish and the stickleback fish, the genomes of the leafy seadragon and the grassy seadragon are missing part of a gene that plays an integral role in other vertebrates, including instructions on how to forming facial structures, teeth, limbs and even parts of the central nervous system. The loss of these genes, the researchers speculate, could explain how sea dragons develop their elongated facial features and beautiful frills. However, more investigations are needed to probe the evolutionary history of sea dragons and their relatives.

The research didn’t stop there. The team imaged a meter-long adult male sea dragon sample using high-resolution X-ray microscopic scanning, showing that decorative appendages likely evolved from thorn. The image shows that the support structure for the leaf paddles appears to be a complex of spikes, with fleshy appendages added at the tip. The team also found that these bony supports differ from the hard, solidified bone found in the fins of most bony fish, and instead appear to be stiffened by a grafted tissue core – which is an element that creates the unique body of male sea dragons.

The sea dragon has evolved to have a charming appearance, a glorious present. For all we know, there may still be a few more secrets in this species, hidden in its genome, that could be uncovered with further genetic comparisons.