Covered with toxic spines, it has multiple arms and is capable of regeneration. With a voracious appetite, it expels its stomach out of its mouth to eat, leaving just skeletons in its wake.

But this is no sci-fi fantasy. The crown-of-thorns (COTS) starfish is a real life monster - and it’s invading our oceans.

Ranging in size from 25 to 80 centimeters wide, coral-eating COTS are the world’s second-biggest starfish - and increasingly plaguing coral reefs. An individual starfish produces 60 million eggs in a given season. They are rapidly multiplying as natural predators are dwindling due to overfishing and contaminated seas.

A starfish diet usually consists of branching corals, which grow faster than other species. But in the absence of these, any other coral, as well as sponges and algae, are fair game.

In healthy reef systems, starfish can help increase biodiversity by giving slower-growing corals a chance to flourish. But when COTS multiply, reef systems can be destroyed, with starfish populations able to strip them of 90% of their living coral tissue.

Fighting back

Current methods aimed at keeping COTS populations in check include taking the starfish ashore and burying them, injecting them with toxic chemicals or bile salts and building underwater fences to limit their movement.

However, these methods are expensive, labour intensive and can only realistically be justified for reefs that have a high socioeconomic or biological significance, such as tourist destinations.

Image: Barrierreef.org

So, scientists at the Queensland University of Technology, backed by funding from Google’s nonprofit arm, have come up with a new solution. Combining robotics and artificial intelligence (AI), they have created RangerBot, a low-cost, autonomous underwater vehicle capable of hunting down and destroying the starfish.

The technology is so accurate that it correctly identifies COTS in 99% of lab tests, even ignoring 3D printed fakes.

The robot follows a pre-programmed path, patrolling the reef with the help of thrusters. Sonar and cameras ensure it stays close to the reef but doesn’t bump into it and damage its delicate structure.

The robot’s cameras scan for starfish, trained to recognize the distinctive purple colouration, creeping motion and common positions they adopt. Once it detects a COTS, the robot lowers an arm to inject it with bile salts, digesting the starfish from the inside out.

Image: Barrierreef.org

The torpedo-shaped machine can stay underwater three times longer than a human diver, and operate during the night, when it may not be safe for divers but when the starfish are most active.

An urgent mission

Studies show that COTS outbreaks are second only to cyclones in terms of the level of destruction they cause to coral reefs.

Healthy reefs can recover from COTS outbreaks within 10 to 20 years. But combined with other negative impacts of climate change like coral bleaching - when high sea temperatures cause corals to drive out the algae living in their tissues and ultimately starve - the chances of recovery are lower.

Image: Barrierreef.org

Australia's Great Barrier Reef in particular has suffered from a number of severe COTS outbreaks in recent history, and scientists fear the reef may now be facing a perfect storm of damaging factors that would render it unable to recover. It has seen a 50% decline in coral cover over the last 30 years.