A masked marauder has emerged unexpectedly from the ocean to rescue a dying coral reef from destruction in the nick of time.
With the dramatic flair of comic-book superhero Batman, a batfish has saved a coral reef that was being choked to death by seaweed – although the fish was never previously known as a weed-eater.
Scientists at the ARC Centre of Excellence for Coral Reef Studies (CoECRS) who were studying how coral reefs are lost to weed were astonished when, after removing a cage from a particularly weedy bit of reef, the rare batfishes emerged out of the blue and cleaned up most of the weed.
“Worldwide, coral reefs are in decline,” says Professor Dave Bellwood of CoECRS and James Cook University. “Commonly this takes the form of the coral being smothered by weedy growth, a transition known as a phase-shift which is very hard, if not impossible, to reverse.”
“Research internationally has found that a major factor in this shift is the over-fishing of weed-eaters like parrot and surgeon fish – which normally keep the coral clean of weedy growth.”
Prof. Bellwood and colleagues Prof. Terry Hughes and Andrew Hoey were testing a weed-infested patch of coral near Orpheus Island on Australia’s Great Barrier Reef to see whether local herbivorous fish could restore it to a normal state.
The ensuing action was captured on underwater TV cameras. When the cage was removed from a particularly weedy patch, local herbivores pecked at it but made little impression on the dense growth of sargassum weed.
“Then these batfish showed up and got stuck into it. In five days they had halved the amount of weed. In eight weeks it was completely gone and the coral was free to grow unhindered,” Prof. Bellwood explains.
The turnaround was due mainly to one species of batfish, Platax pinnatus, which is comparatively rare on the GBR and was thought to feed only on invertebrates.
The event surprised the scientists in two ways, he says. First, it showed that the species one would normally expect to “mow the weeds” may make little impression on a heavily-overgrown reef, dashing researchers’ hopes that herbivores may be a way to restore heavily weed-infested reefs.
And second, it shows that in nature, help can come from a totally unexpected quarter – from a fish that itself may be at some risk.
“Batfish represent a ‘sleeping functional group’, meaning they are capable of performing a vital role in the life of the reef, but do so only under exceptional conditions,” Prof. Bellwood says.
“Platax are relatively rare on the Great Barrier Reef and currently have no specific legal protection. They are vulnerable because their large size makes them attractive to spear-fishers, while they depend as young fishes on coastal mangroves which are in decline in many areas.
“Indeed, the resilience of inshore GBR reefs may be closely tied to the fate of mangroves and their suitability for batfish recruitment.”
Batfishes may be one of the last intact herbivore populations capable of reversing serious weed overgrowth of inshore coral reefs, he adds. The reef has already all-but lost one major group of weed-mowers, the dugongs, while another – green turtles – is seriously endangered.
“If Platax is the last grazer of dense weedy stands on inshore coral reefs and it goes into decline, the capacity of these reefs to recover from phase-shifts could be lost.”
For scientists and reef managers, the batfish has thrown up a new challenge – how to identify other ‘sleeping functional groups’ that may prove lifesavers for the reef, but whose habits and abilities we as yet know nothing about.