Similar to seahorses, sea dragons are somewhat strange looking. These fish have a body covered with leaf-like decorations that shake when the current hits them, a tubular snout, they lack ribs and teeth, and their spines are curved and twisted. Scientists may have found the reason for their odd appearance: They are missing a group of genes that give rise to teeth, nerves, and facial features in other animals. Furthermore, sea dragon genomes are packed with repetitive bits of DNA that drive evolution.
Analysis of sea dragon genomes has “lifted the veil on the evolution of sea dragon-specific traits” and “revealed intriguing evolutionary facets of this unusual family of vertebrates, the Syngnathidae , as a whole,” write the team responsible for the study in a new article that has been published in PNAS .
Sea dragons belong to the Syngnathidae family, which also includes pipefish and seahorses. All of them famous because it is the male and not the female who gets pregnant . “This group is great for a number of reasons,” says evolutionary genomics researcher Clayton Small of the University of Oregon, who co-led the study with colleague Susan Bassham. “But sea dragons are rare bugs in an already rare group of fish.”
In their study, the researchers sequenced the genomes of two species of seadragons: the leafy seadragon and the common or weedy seadragon , which live in cold waters off the southernmost coasts of Australia.
There is a third species of sea dragon: the ruby sea dragon, which was first seen in the wild in 2017. These slender, rare fish drift in their leaf-like ornaments and hide in rocky reefs, among the algae. It is not easy to detect them.
The three species of sea dragon do not go unnoticed and are often praised for their slender and colorful bodies, for their tubular snouts with which they eat crustaceans and for the leaf-like appendages that cover them. The ruby dragon, however, does not sport these “adornments”. Evolution has eliminated them.
Sea dragons are believed to have evolved into what they are today in the last 50 million years, very quickly by evolutionary standards. It was on that date that they separated from the seahorses. What is unknown is how they have come to have such a curious appearance.
To explain why sea dragons look curious, researchers from the University of Oregon teamed up with scientists from Scripps Institution of Oceanography’s Birch Aquarium and the Tennessee Aquarium to analyze samples they had of sea dragons bred in captivity.
What the researchers saw was that, compared to their close relatives pipefish and seahorses, seadragons possessed a very large number of repetitive DNA sequences called transposons, also known as “jumping genes ,” in their genome. Transposons are “jumping genes” because they can move around the genome causing rapid genetic changes. This could explain why these animals evolved in such a short time.
Regarding the appearance of these animals, the scientists compared the two species of sea dragons, the leaf and the weedy, with two distant relatives, the zebrafish and the stickleback. It turns out that sea dragons do not have a part of the genes that contain the instructions for forming facial structures, teeth, limbs and even parts of the brain’s nervous system . Despite the findings, the researchers are cautious and do not say outright that gene loss is the reason for the sea dragon’s odd appearance. More research is needed, they say.
The team has also taken a high-resolution scan of an adult sea dragon and concluded that the ornamental appendages likely evolved from spines. “We could see that the supporting structures of the bladed blades appeared to be elaborations of spines [with fleshy appendages] added to the ends,” says Bassham. Furthermore, the bony supports are not like the hardened fin bones of most bony fish but appear to be hardened by a core of collagenous tissue. Yet another clue as to how the sea dragon’s unique body structures came to be.
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Referencia: Small, C. , Bassham. S. et. al. 2022. Leafy and weedy seadragon genomes connect genic and repetitive DNA features to the extravagant biology of syngnathid fishes. PNAS. DOI: https://doi.org/10.1073/pnas.2119602119
