A team of marine biologists from Scripps Institution of Oceanography and the Western Australia Museum has captured on video the first-ever field sighting of the recently-discovered species of seadragon — the ruby seadragon (Phyllopteryx dewysea).
“Until recently, only two species of seadragon were known, the leafy seadragon (Phycodurus eques) and the common seadragon (Phyllopteryx taeniolatus), both from Australia. In 2015, we described a new species of seadragon, Phyllopteryx dewysea,” said Scripps Oceanography Professor Greg Rouse and colleagues.
“Although the leafy and common seadragons are well known and commonly seen in aquarium exhibits world-wide, the ruby seadragon was known only from four preserved specimens, leaving many aspects of its biology unknown.”
“The only recent live-collected specimen was trawled from the Recherche Archipelago, a cluster of over 100 islands in Western Australia. We took a small remotely operated vehicle (miniROV) to this locality to obtain the first images of live ruby seadragons,” they explained.
The video footage of the ruby seadragon shows off its intense red color and reveals that its habitat is very different from the algal reefs occupied by its relatives.
The footage also shows that the species lives at a depth of more than 50 m — beyond recreational SCUBA diving limits.
The biologists suggest that this may be why the fish went undiscovered for so long. Its closest relatives inhabit much shallower depths of 3-25 m.
“The documentation of living ruby seadragons at more than 50 m depth confirms that these fish live at deeper depths than leafy and common seadragons and in a very different habitat,” they said.
“Leafy seadragons occur along the coast of Western and South Australia from south of Perth to east of Adelaide and are generally found in depths from 3 to 25 m near brown algae, seagrass, and sand.”
“Common seadragons have wider geographic range, extending east to the central New South Wales coast and also around Tasmania. They show a similar depth preference to leafy seadragons and are also known for living associated with rocky reefs, sand patches, kelp and seagrass.”
The footage also reveals some striking differences between the three species.
One unique characteristic of the ruby seadragon is that it lacks leaf-like appendages — a distinctive feature of the bodies of common and leafy seadragons, which camouflages the fish among the sea grass and kelp meadows in which they live.
“The video results for the ruby seadragon clearly show that these fish lack dermal appendages as opposed to the prominent appendages of leafy and common seadragons,” the researchers said.
“In the sparse habitat they occupy, appendages would serve little purpose as camouflaging agents and could add significant costs in drag or fluid resistance, particularly in strong surge.”
“It appears that at these low-light depths, an efficient camouflage strategy for ruby seadragons is to rely on cryptic red coloration.”
The team was also surprised to find that the ruby seadragon has a prehensile tail which it may use to hold on to objects to avoid being swept away by strong sea surge.
“Surprisingly, we also saw that the ruby seadragon has what appears to be a prehensile tail. Although neither fish was observed to use it in a direct hold, the conditions on the day were relatively calm. The fish would often be exposed to much stronger surge, and then may well use their tails to stop from being swept off the very limited reefal habitats,” the authors said.
“The discovery that the ruby seadragon has a prehensile tail complicates the scenario of the evolution of prehensile tails in this group, as it is the closest relative to the common seadragon, which cannot bend its tail.”
Further study is needed to clarify whether the ruby seadragon has re-acquired a prehensile tail that was lost in a common ancestor, or if the absence of a prehensile tail in the common and leafy seadragons has independently evolved in each of the species.
The team’s findings were published online January 13 in the journal Marine Biodiversity Records.
Greg W. Rouse et al. 2017. First live records of the ruby seadragon (Phyllopteryx dewysea, Syngnathidae). Marine Biodiversity Records 10: 2; doi: 10.1186/s41200-016-0102-x