A THORNY SITUATION
Marine scientists are trying to battle plagues of coral-eating starfish on the Great Barrier Reef
Upon first glance, the crown-ofthorns starfish looks a lot like an enemy creature you’d find in a nature-based video game. Long spikes cover its body, which can reach 2½ feet in diameter. It’s somewhat reminiscent of a landmine, if a landmine had 14 to 21 movable arms.
But its exterior isn’t nearly as intimidating as its appetite. By the time a crown-of-thorns matures at four months old, it eats live coral voraciously — up to its own body weight in one evening. Just one can consume 20 to 32 feet of living coral a year. Thirty of them per 2½ acres can kill an entire reef ’s coral.
These starfish are the second biggest threat to the already endangered Great Barrier Reef — the world’s largest coral reef system, right behind tropical cyclones. Over the past decades, they have attacked the reefs in waves of outbreaks, one of which spiked their population to as many as 1,000 starfish per 2½ acres.
The Great Barrier Reef covers nearly 865 million acres off the coast of Australia, which means about 350 billion starfish live there. The starfish, the most fertile invertebrate in the world, stripped 150 reefs of coral within the Great Barrier system and damaged 500 more in just a few years.
These enormous outbreaks initially confounded scientists, but they have begun to make progress in understanding them — discovering a surprising biological trait — and in fighting them with the development of an underwater robot to attack them.
The recent discovery that crownof-thorns starfish, called COTS for short, can delay their transition to adulthood for at least six years means they can lie in wait without needing to feed on coral while a damaged reef heals itself, then feast on it as soon as its healthy coral has grown back.
“Because COTS juveniles have the ability to stay in an algae feeding form for up to six years, there could be an accumulation of multiple generations of juveniles that are happily feeding on algae until there is a specific cue that catalyzes their transition to feeding on corals,” says Paul Barber, professor in the Department of Ecology and Evolutionary Biology at the University of California at Los Angeles.
Imagine hundreds of thousands of crown-of-thorns living that way around a single reef system — its living coral couldn’t withstand the assault. This starfish’s development suspension ability isn’t the only driver of outbreaks. Human interference is responsible, as well.
“The science tells us that elevated fertilizer and other pollutants from primary production run-off is causing an increase in phytoplankton, which is the main food source for COTS larvae,” says Anna Marsden, managing director of the Great Barrier Reef Foundation.
Overfishing and the removal of the starfish’s natural predators, such as the giant triton snail, also have contributed to outbreaks.
Under normal circumstances, crown-of-thorns starfish play a useful role in reef vitality and diversity because they prefer to eat the faster-growing coral, which helps slow-growing coral varieties gain a foothold. However, when their population isn’t kept in check, they can easily become a devastating invasive species.
The Great Barrier Reef Foundation has been working with local farmers to help reduce the amount of pollutant run-off into the reef ’s waters, which hopefully will lessen the number of outbreaks. It recently launched a $37.5 million crown-of-thorns starfish control program that is exploring innovative surveillance and control methods.
Scientists have been exploring methods to mitigate crown-ofthorns outbreaks since the early 1960s, when the escalation in outbreaks was first noted. In 2015, researchers at James Cook University
discovered that a 20-millilitre dose of vinegar would kill a starfish in 48 hours. Before that, the only known injectable poison was bile salts, which are expensive and harder to get in large amounts.
But coming up with a good culling tool was only half the battle. To administer either of these injections, diving crews had to hunt down the starfish one by one. As a result, it’s been difficult to make much of a dent in their population.
For example, in 2015, the Great Barrier Reef was home to anywhere between four and 12 million crown-of-thorns starfish. In one year, two crews working fulltime were able to eradicate only 350,000. Each female lays about 65 million eggs a year.
Clearly, crown-of-thorns starfish control via human divers alone is an uphill battle.
Enter the Rangerbot. The Rangerbot is an autonomous underwater vehicle (AUV) and the first vision-based robot designed to protect coral reef systems. It was developed by a team of scientists led by Matthew Dunbabin, professor of science and engineering at Queensland University of Technology over the past six years.
“Given the extreme size of the Great Barrier Reef and many threats to humans, we wanted a ‘tool’ that could allow the authorities to scale back the manual eradication program,” Dunbabin says.
The idea for the bot was born 14 years ago.
“In 2005, I developed a robot which proved that vision can be used to allow a robot to estimate its position in the reef and avoid obstacles,” Dunbabin says.
However, computer technology was not yet advanced enough to allow for real-time detection of crown-of-thorns. That changed in 2014 with advancements in Deep Learning technology, which allowed Dunbabin’s team to construct a robot that could be programmed to detect and inject the starfish with bile salts all while operating autonomously. Over the next few years, they refined the prototype to be smaller, lighter and less expensive so that a bot army could eventually be commissioned.
In 2016, the Rangerbot won the Google Impact Challenge Australia and joined forces with the Great Barrier Reef Foundation in an effort to accelerate the development process. By late 2018, the Rangerbot was fully operational in the Great Barrier Reef. In 2019, the team was able to increase its capabilities so that it can also help with coral larvae reseeding along the Great Barrier Reef and reefs in the Philippines.
“(We) have been integrating them with robotic boats to further increase their ability to deliver large amounts of coral larvae to damaged reefs,” Dunbabin says.
The Great Barrier Reef is an irreplaceable wonder of the natural world. Its 3,000 reef systems spanning over 214,000 miles — roughly the size of New Zealand — are home to more than 1,500 species of fish, 400 types of hard coral and one-third of the world’s soft coral. In just the past 30 years, half of that coral has been lost, largely because of bleaching events and crown-of-thorns outbreaks.