Sea dragons are incredibly strange creatures, and we may finally know why

(ORDO NEWS) — Seeing a sea dragon floating among the seaweed, adorned with leaf-like ornaments that sway in the swaying ocean currents, is a truly unforgettable sight.

But sea dragons are more than meets the eye. As dazzling as they are, sea dragons also lack teeth, no ribs, and their spines are bent and twisted.

Now scientists have found genetic clues that could explain why sea dragons look so strikingly distinctive – not only are their genomes filled with repetitive DNA fragments that determine evolution, but they also lack a group of genes that give rise to teeth, nerves and facial features. other animals.

Studying the genomes of sea dragons “has lifted the veil on the evolution of sea dragon-specific traits” and “revealed intriguing evolutionary aspects of this unusual vertebrate family, Syngnathidae, as a whole,” write the study’s team of authors in a new paper.

Sea dragons belong to the same family as trumpeter fish and seahorses, the Syngnathidae, which are known for developing male pregnancies.

“This group is amazing for a number of reasons,” says evolutionary genomics researcher Clayton Small of the University of Oregon, who led the study with colleague Susan Bassham.

“But sea dragons are the oddballs in a group of already oddball fish.”

To find out why, Small, Bussham and their team sequenced the genomes of two species of sea dragon, the leaf sea dragon and the weed or common sea dragon, that live in the cool waters off Australia’s southernmost coast.

These slender fish are hard to spot as they drift through the kelp-covered rocky reefs, their leaves helping to camouflage them.

In fact, it’s so elusive that the third (of just three) sea dragon species, the rare ruby ​​sea dragon (which was not sequenced in this study), was only first seen in the wild in 2017.

All three species of sea dragon are revered for their colorful, bizarre body shapes and long tube-like snouts that suck up crustaceans, but the ruby ​​sea dragon seems to have lost the leaf-like appendages of other species, and evolved without extravagant frills.

Scientists believe that sea dragons have acquired their extravagant features quite quickly, in the last 50 million years or so, since they and sea horses separated to form a new family.

However, it is not entirely clear how they began to look so peculiar. So for this study, scientists at the University of Oregon teamed up with scientists from the Birch Aquarium at the Scripps Oceanographic Institution and the Tennessee Aquarium to analyze their captive-bred sea dragon specimens.

It turned out that sea dragons, compared to their closest relatives – trumpeter fish and seahorses, have in their genetic code a surprisingly large number of repetitive DNA sequences called transposons, otherwise known as “jumping genes”.

Transposons are so mobile that by jumping around the genome, they can create rapid genetic changes – this may explain why sea dragons have evolved so quickly.

Compared with two distant relatives, zebrafish and sticklebacks, the genomes of leafy and weedy sea dragons lack a number of genes that play an indispensable role in other vertebrates, containing instructions for the formation of facial structures, teeth, limbs, and even parts of the central nervous system.

While researchers have been tempted to speculate that the loss of these genes could explain how sea dragons developed their elongated facial features and fabulous frills, more research will be needed to understand the evolutionary history of sea dragons and their relatives.

But the researchers didn’t stop there: they also took high-resolution X-ray microscope images of an adult male weedy sea dragon, which showed that the ornamental appendages most likely originated from the spines.

“We saw that the support structures for leaf-shaped lobes appear to be a development of spines [with fleshy appendages] added to the tips,” Bassham says.

The team also noticed that these bony supports are different from the hardened, ossified bones found in the fins of most bony fish, and instead stiffness is provided by a core of collagen tissue, adding to the story of how the sea dragon’s unique body structures came about.

No matter how the sea dragons have evolved, the results are magnificent, even glamorous. As far as we know, sea dragons may have a few more secrets hidden in their genome that could be revealed through further genetic comparisons.

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