Among the collection of bizarre ocean creatures, sea dragons stand out. Relatives of seahorses and pipefish, sea dragons have long, narrow snouts that they use as a straw to suck up meals from microscopic crustaceans. Instead of scales, these fish are covered in bony armor, and their backbones are twisted. Like their seahorse cousins, male sea dragons gestate the female’s fertilized eggs in pouches on their tails.
They belong to two species groups – leafy and common. The “leaks” have elaborate branched appendages that make them almost indistinguishable from the floating algae in their habitat in South Australia. “Ordinary” sea dragons are more streamlined, but also more colorful, with purple stripes and yellow polka dots.
Bill Cresko of the University of Oregon studies sea dragon genetics to answer a fundamental question: he and his colleagues want to know how these fish came to be the way they are.
“We’re really fascinated by, ‘How is it possible for an organism like this to exist? What has changed in the genome?'” Cresko said.
A study published in June in the Proceedings of the National Academy of Sciences attempted to answer these questions. The researchers sequenced the genomes of leafy and common sea dragons and compared them with those of other fish.
The sea dragons’ strange appearance made the team think that something unusual might have happened to their fibroblast growth factor genes, “which are really important for the development of things like teeth, which they don’t have, or the shape of faces.” or the growth of appendages, to name just a few,” said Susie Bassham, a researcher in Cresko’s lab and one of the authors of the paper.
But when they focused on the animals’ genomes, they were surprised to see that sea dragons lacked several of these key developmental genes.
“I really didn’t believe it at first,” said Clay Small, another author on the paper, also at the University of Oregon.
Although sea dragons lacked these growth genes, their genomes were filled with repeating sections of code called transposons. This type of genome-wide repetitive code was once called “junk DNA” as scientists weren’t sure what it did. But transposons, or “jumping genes,” are actually capable of cutting themselves from one point in the genetic code and pasting themselves into another, preventing other genes from shaping an organism’s characteristics.
The researchers cannot say for sure whether jumping genes are responsible for the absence of growth factor genes. There is evidence of repetition of the genetic code near the points where genes are missing, which could indicate transposon activity, Small said. But scientists will need more genomes from the entire fish family tree to confirm a cause-and-effect relationship.
In addition to studying the sea dragon’s DNA, the team took CT scans of a sea dragon at the highest resolution they were aware of. The X-ray images gave them information about the fish’s appendages, which researchers now suspect are modified spiny growths.
Axel Meyer, a sea dragon researcher at the University of Konstanz in Germany, said the study increases understanding of extreme body plans. “This is a creature that is symbolic of the exuberance of evolution. It’s as if evolution has gone mad,” said Meyer, who was not involved in the study. “The fun of being an evolutionary biologist is that you can study these crazy animals and try to understand them genetically.”
Sea dragon DNA can offer insights into how to conserve them, says Cresko, which is critical because they are rare in the wild and difficult to keep in captivity. A single fish can cost upwards of $10,000, and they proved nearly impossible to breed.
“Believe me, there are a lot of people I work with who don’t want to take care of them because it’s too much work,” said Leslee Matsushige, who has treated sea dragons at the Birch Aquarium at the Scripps Institute of Oceanography in San Diego for more than 20 years. Fish are sensitive to fluctuations in temperature and light, requiring gradual changes to mimic sunrise and sunset. Matsushige even tried to match the tank’s night light to the phases of the moon.
Despite the difficulties, Matsushige said it’s important that aquariums like Birch and the one in Tennessee — which provided sea dragon tissue for the DNA study — find ways to help the fish reproduce. As climate change threatens their habitats, sea dragons’ survival may depend on breeding programs in zoos and aquariums, so it’s important to “know as much as we can”.
Translated by Luiz Roberto M. Gonçalves
