What Makes Sea Dragons So Strange

Among the ocean's menagerie of bizarre creatures, sea dragons stand out. From a report: Relatives of sea horses and pipefish, sea dragons have long narrow snouts that they use like a straw to suck up meals of microscopic crustaceans. Instead of scales, the fish are covered in bony armor, and their backbones are kinked. Like their sea horse cousins, male sea dragons gestate a female's fertilized eggs in a pouch. They come in two groups of species, leafy and weedy. "Leafies" have elaborate branching appendages that make them virtually indistinguishable from the floating seaweed in their Southern Australian habitats. Weedy sea dragons are more streamlined but are also more colorful, with purple stripes and yellow polka dots. Bill Cresko at the University of Oregon studies sea dragon genetics to answer one fundamental question: He and his colleagues want to know "how the hell" these fish came to look the way they do. "We're just really fascinated by, 'How can you have an organism that looks like that? What has changed in the genome?'" he said. A study published in June in the Proceedings of the National Academy of Sciences tried to answer these questions. Researchers sequenced the genomes of leafy and weedy sea dragons and compared them with other fish. The strange appearance of sea dragons made the team think that there might have been something unusual happening with their fibroblast growth factor genes, "which are really important for development of things like teeth, which they don't have, or the shape of faces or appendage outgrowth, to name just a few," said Susie Bassham, a researcher in Dr. Cresko's lab and an author of the paper. But when they zeroed in on the animals' genomes, the researchers were surprised to see that sea dragons were missing several of these key developmental genes altogether. While the sea dragons were missing these growth genes, their genomes were packed with repetitive sections of code called transposons. This sort of repetitive code throughout the genome was once called "junk DNA," as scientists were not sure what it did. But transposons, or "jumping genes," are actually capable of cutting and pasting themselves from one spot within the genetic code to another, preventing other genes from shaping an organism's traits. The researchers cannot say for sure if the jumping genes are responsible for the absence of the growth factor genes. There is evidence of repeating genetic code near the spots where genes are missing, which might point to transposon activity, Dr. Small said. But scientists will need more genomes from across the fish family tree to confirm a cause-and-effect relationship.

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