By Jessica Z. Crawford
Like their terrestrial namesakes, seadragons seem to belong to a mythological world. Their hypnotic, leaf-like limbs suggest they are related somehow to both fish and seaweed.
Scripps Institution of Oceanography at UC San Diego marine biologists Greg Rouse and Nerida Wilson didn’t expect to feel such a strong fascination for these animals. They don’t usually study the celebrities of the sea. They spend most days elbow deep in muddy worms, slimy slugs, and other unglamorous ocean critters that lack backbones.
But in 2004, their research interests drifted from the grimy to the glitzy. At that time, Rouse and Wilson were working in southern Australia, in the heart of seadragon country, where the animals are so popular that festivals are held in their honor.
Seadragons belong to the fish family Syngnathidae –– which also includes pipefish and seahorses (yes, male seadragons carry babies like their cousins) –– and they are generally grouped into three species: leafy, weedy, and ribboned.
Rouse and Wilson research the genetic ties of the invertebrates they typically study, so they naturally began asking the same questions about seadragons – Which species are related? How close are they connected? Are they all really seadragons?
They were astounded to discover a lack of biological information about such a celebrated creature, and a new research project was born.
Today, Rouse and Wilson are debunking some long-held assumptions about the animals. Using DNA analysis, the researchers have unlocked their evolutionary history, discovering that one species should probably be designated as two species, and that another isn’t a seadragon at all.
Their findings could spur major changes in the way seadragons are protected throughout their habitat.
Research Down Under
Leafy and weedy seadragons hug the coast of southern Australia in shallow waters ranging as far east as Sydney and as far west as Perth. As the official emblems of two states, they are among the most renowned fishes on the continent. The ribboned seadragon is found in northwestern and northern Australia.
However, DNA testing had never been used to confirm whether these species were related, leading Rouse and Wilson to wonder how the species were genetically linked.
“We knew seadragons were of great interest to people, but no one really knew anything about their genetics,” said Wilson, who worked with Rouse at the South Australian Museum in Adelaide at the time. “It was amazing that no one else was working on this.”
Aided by the National Geographic Society, the researchers and colleagues from the museum began to collect tissue samples from leafy and weedy seadragons across southern Australia. Since seadragons live close to shore, the scientists relied on tips from local scuba instructors for the best spots to find the animals.
That knowledge led them to some of the most spectacular diving spots on the continent. Once they spotted one of the famously camouflaged creatures, the pair performed a series of delicate steps to obtain their samples.
First they photographed the seadragon from several angles to measure different parts of the body, which would help determine whether the anatomy of the animals differ across their range.
Then, Rouse and Wilson gently held the seadragon in place while they photographed its head against a measuring grid and took a small snip of tissue from one of its appendages. The sample was then stored in a sealable bag and towed to shore for preservation.
The research continued when Rouse and Wilson came to Scripps Oceanography in 2006, where the UC San Diego Academic Senate further supported their research.
In the end, the pair collected tissue samples from more than 150 seadragons, which, in combination with other data, confirmed for the first time that leafy and weedy seadragons are each other’s closest relatives.
Help from Friends
The ribboned seadragon, however, remained a mystery. After five years of searching, the team still lacked tissue samples from the animal and couldn’t determine its genetic connection to the other seadragon species.
Rouse and Wilson considered abandoning their quest to test all three species until they received an unexpected call last year from very close to home.
A newly acquired ribboned seadragon at Birch Aquarium at Scripps had just died, and aquarium curators hoped that Rouse would find scientific interest in the specimen.
“We were thrilled that they were here,” Rouse said. “It was really ironic that we had spent all this time trying to get them, and our own aquarium ended up having some.”
From that specimen, the researchers were able to decode enough of the genes to show that ribboned seadragons are not related at all to leafy and weedy seadragons. They are instead a species of pipefish.
Weeding Out the Species
Rouse and Wilson say their findings about weedy seadragons will likely have the most impact.
The geographic range of this species extends more than 3,200 kilometers (2,000 miles) around the entire base of Australia, and populations are found on both sides of the Australian Bight, a vast stretch of ocean across the southern arc of the continent.
Scientists have generally regarded weedy seadragons as one species, but genetic information and slight differences in body structure between the eastern and western populations suggest that weedy seadragons should probably be divided into two species, according to the scientists.
“There’s a lot of debate about what defines a species and where you should break a species into two,” Rouse said. “We’ll make a decision in due course about whether they should be considered two. It’s quite an important decision to make.”
“It would not involve us making a new species name,” Rouse added. “Rather, we would be resurrecting a preexisting species name for the western weedy seadragons.”
The verdict would likely change the way weedy seadragons are protected. At the moment, the animals are considered one species and protected under the various conservation practices of each Australian state where they are found. They’re also listed as one species under the International Union for the Conservation of Nature (IUCN), the world's main authority on the conservation status of plants and animals.
If they were broken into eastern and western species, the Australian states would have to reassess their legislation for safeguarding the animals and the IUCN would need to determine how to list the species, the scientists said.
“The states would need to consider the population closest to them, not weedy seadragons across their entire range,” Wilson said. “It would basically change the spatial structure of the management.”
Weedy seadragons are currently listed as “near-threatened” under the IUCN because of human impacts to their near-shore habitats. Storm water and sewage discharge are degrading their already sensitive rocky-reef homes.
Listing “weedys” as two species would enable the states to tailor their conservation laws to better protect both populations, the scientists said.
“Like all coastal animals, they basically suffer from us – our development around the coast, the amount of waste water coming out, and things like that,” Wilson said. “They’re really at that interface of the water and the land. Taking care of the environment is the best thing we can do for them.”
Rouse and Wilson’s research about the evolutionary relationships of seadragon species will be featured in Birch Aquarium at Scripps’ newest exhibit, There’s Something About Seahorses, opening Nov. 14. To learn more about the exhibit, visit aquarium.ucsd.edu.