Sea Dragons Are Incredibly Strange Creatures

It is quite amazing to see a wispy sea dragon floating amid the seaweed, decorated with decorations that resemble leaves and move with the currents of the water.

However, sea dragons are more complex than the average diver realizes. Sea dragons may be bedazzling, but they also lack ribs, fangs, and have bent and kinked spines.

The genomes of sea dragons are rich with repeating DNA sequences that drive evolution, but they also lack a set of genes that give birth to teeth, nerves, and facial characteristics in other animals. These genetic hints may help scientists understand why sea dragons seem so remarkably unique.

The study's authors write in a recent report that studying sea dragon genomes has "lifted a veil on the evolution of sea dragon-specific traits" and "revealed intriguing evolutionary facets of this unusual vertebrate family, the Syngnathidae, as a whole".

Sea dragons are members of the Syngnathidae family, which includes pipefish and sea horses and is notable for having evolved male pregnancy.

"This group is just cool for a number of different reasons," explains University of Oregon evolutionary genomics expert Clayton Small, who co-led the work with colleague Susan Bassham.

"But sea dragons are oddballs in a group of already oddball fish." 

The leafy sea dragon and the weedy or common sea dragon, both of which are located in chilly seas off the southernmost beaches of Australia, were the two sea dragon species whose genomes were sequenced by Small, Bassham, and the team to find out why.

These skinny fish can be challenging to see as they drift across kelp-covered rocky reefs while concealing themselves with leaf-like fronds.

In fact, they are so secretive that the rare ruby sea dragon, one of just three kinds of sea dragons, was only recently spotted in the wild for the first time in 2017.

The three species of sea dragons are admired for their vivid, fantasy body shapes and their long, tubular snouts, which are used for sucking up crustaceans. However, the ruby sea dragon lacks the other species' leafy appendages, suggesting that evolution has stripped away the showy frills.

Since they and sea horses split off to establish a different family some 50 million years ago, scientists believe sea dragons rapidly evolved their ostentatious traits.

How they got to appear so unique, though, is less certain. In order to evaluate the samples they obtained from sea dragons bred in captivity, the researchers from the University of Oregon collaborated with specialists from the Tennessee Aquarium and the Birch Aquarium at Scripps Institution of Oceanography.

It turns out that sea dragons contain a very high amount of transposons, sometimes known as 'jumping genes', in their genetic code as compared to their nearest cousins, pipefish and seahorses.

Transposons may swiftly alter the genetic makeup of an organism by hopping throughout the genome, which explains why sea dragons developed so quickly.

The genomes of leafy and weedy sea dragons lack a significant portion of genes that play essential roles in other vertebrates, including instructions on how to form facial structures, teeth, limbs, and even portions of the central nervous system. This is in contrast to two distant relatives, zebrafish and sticklebacks.

More study will be required to fully understand the evolutionary history of sea dragons and their cousins, despite the researchers' inclination to hypothesize that the loss of these genes may have contributed to the development of sea dragons' elongated facial features and extravagant frills.

The decorative appendages most likely originated from spines, according to high-resolution X-ray microscope scans of a foot-long adult male weedy sea dragon specimen that the researchers also conducted.

"We could see that the support structures for the leafy paddles appeared to be elaborations of spines [with fleshy appendages] added to the ends," according to Bassham

The scientists also observed that these bony supports were not like the ossified, hardened bones seen in the fins of other bony fish, but rather appeared to be strengthened by a core of collagenous tissue, contributing to the explanation for how the sea dragon's distinctive body architecture came to be.

The end product, whatever sea dragons originated, is magnificent, even gorgeous. It's possible that sea dragons still have some hidden genetic mysteries that can be discovered through more genetic comparisons.

The study was published in PNAS.