The sight of a wispy sea dragon, dangling among the seaweed, adorned with leaf-like decorations ruffling in swaying ocean currents, is truly spectacular.
Sea dragons have more to offer than meets the eye. As beauteous as they may be, sea dragons also have teeth, lack of ribs, and their spines are curvatured and kinked.
Scientists have identified genes that might explain why sea dragons are so distinctive. Not only are their genomes packed with repeating DNA fragments that drive evolution, but they are missing a group of genes that give rise to teeth, nerves, and facial features in other animals.
In a new paper, researchers examine the genomes of sea dragon genomes and reveal interesting evolutionary aspects of the unusual vertebrate family, the Syngnathidae.
The Syngnathidae, which is well-known for having evolved male pregnancy, belongs to the same family as pipefish and sea horses.
"This group is really cool for a number of reasons," says Oregon evolutionary genomics researcher Clayton Small, who co-led the research with fellow researcher Susan Bassham.
"But sea dragons are oddballs in a group of already oddball fish."
Small, Bassham, and their colleagues sequenced the genomes of two sea dragon species: the leafy sea dragon and the weedy or common sea dragon, both of which are found off Australia's southernmost coasts.
These slender fish can be difficult to spot because of their leaf-like fronds that help conceal them in kelp-covered rocky reefs.
The third (of only three) sea dragon species, the rare ruby sea dragon (which was not sequenced in this study), was only seen in the wild for the first time in 2017.
Theruby sea dragon appears to have lost the leafy appendages seen by the others, due to evolution eliminating the extravagant features.
Sea dragons and sea horses have evolved their elaborate features fairly swiftly, in the previous 50 million years or so, since they joined forces to form a new family.
Exactly how did they become so distinctive? For this research, the University of Oregon researchers teamed up with scientists from the Birch Aquarium at the Scripps Institution of Oceanography and the Tennessee Aquarium to examine what samples they had from sea dragons raised in captivity.
Sea dragons have, when compared to their closest relatives, pipefish and seahorses, a surprising number of repetitive DNA sequences called transposons, otherwise known as 'jumping genes.'
Transposons are so mobile, moving around in the genome, that they can make quick genetic changes, which might explain why sea dragons evolved so quickly.
The genomes of leafy and weedy sea dragons lack a significant portion of genes that play critical roles in other vertebrates, such as instructions on how to make facial structures, teeth, and even parts of the central nervous system.
While the researchers were tempted to speculate that the loss of these genes might explain how sea dragons developed elongated facial features and fabulous frills, more research will be required to investigate sea dragons and their relatives' evolution.
The researchers were not done yet, and they also examined a foot-long adult male weedy sea dragon specimen using high-resolution X-ray microscope scans, which revealed that the ornamental appendages originated from spines.
"We could see that the leafy paddle support structures appeared to be elaborations of spines [with fleshy appendages] at the ends," says Bassham.
The researchers discovered that these bony supports differed from the hardened, ossified bones found in the fins of most bony fish, and instead appeared to be stiffened by a core of collagenous tissue, further illustrating the origins of the sea dragon's unique body structures.
The results, however sea dragons evolved, are spectacular and stunning. For all we know, sea dragons may still have a few more secrets hidden in their genome, which may be revealed when further genetic analyses.
The study was published in the Proceedings of the National Academy of Sciences.