You may be forgiven for having a cuddly mental image of starfish, the apparently docile creatures that seem to exist purely for the purpose of adorning the seafloor for the pleasure of all who gaze upon them. But the reality is very different, at least in the case of one particular species, Acanthaster planci, a large, multi-armed variety that is silently, insidiously destroying the corals of Asia and Oceania.

This sea star’s popular name, crown-of-thorns starfish (COTS) – derived from the biblical headgear of the same name – aptly describes this carnivorous predator. The COTS, which can grow as large as a dinner plate (35 centimetres) and boast up to 21 arms, features long, thorn-like spines on its arms and upper surface which are stiff and sharp, and should they perforate the soft tissue of an unsuspecting predator, a detergent-like toxin is released into the wound. In humans, this causes an unpleasant stinging pain that persists for several hours and the swelling may take a week to come down.

But it is not the potential discomfort to humans that concerns scientists studying A. planci. In the subtropical and tropical areas where it is found – the Red Sea, the east African coast, the Indian Ocean, and right across the Pacific Ocean to the west coast of Central America – COTS is doing serious damage to both hard and soft coral communities. Australia’s Great Barrier Reef, already seriously threatened by the coral bleaching associated with warmer seas and water pollution, has seen COTS numbers increase alarmingly in recent years, and divers have been forced to conduct “search and destroy” missions to try and eradicate the scourge.

An unusually brightly coloured crown-of-thorns starfish, Thailand © Jon Hanson/flickr/Wikimedia Commons

An unusually brightly coloured crown-of-thorns starfish, Thailand © Jon Hanson/flickr/Wikimedia Commons

Appetite for destruction

Studies have shown that an individual starfish can consume up to six square metres of coral reef per year. That may not sound like much, but when you consider the staggering numbers – there are an estimated 11 million along Queensland’s east coast alone – collectively COTS represent a serious threat. Left unchecked, the World Wildlife Fund has suggested that populations in the Great Barrier Reef region could explode to as many as 60 million over the next five years.

Some scientists believe that the fertilizer used by cane sugar farmers is partly to blame. The starfish’s larvae feed on phytoplankton, which in turn feed on nutrients in the water, and with agricultural runoff increasing nutrient concentrations, there are bigger populations of phytoplankton and therefore bigger populations of COTS larvae surviving into adulthood. This runoff of nutrients and contaminants from the land is already lowering marine ecosystem health and coastal water quality across Australia, creating more problems for reef systems already stressed by increases in water temperature due to the current El Niño event and the ongoing effects of human-induced global warming.

Adding to the problem is the fact that a starfish’s metabolic rate is directly affected by the temperature of the water surrounding it. Thus, COTS do significantly more damage, and over a wider area, in summer than in winter, and likewise higher water temperatures means increased locomotion and feeding rates.

Crown-of-thorns starfish overcome corals in French Polynesia © Rore bzh/Wikimedia Commons

Taking on the COTS

In Australia, the Association of Marine Park Tourism Operators (AMPTO) in far north Queensland has been overseeing a programme to try and tackle the huge numbers of A. planci. The programme trains disadvantaged youth to become divers and destroy the COTS – one starfish at a time. While some 400,000 of the animals have been removed since 2010, with millions more left to eradicate, it’s proven an almost impossible undertaking, especially as funding is limited.

An even more effective method of population control is killing the starfish by injecting them with sodium bisulphate, which is deadly to the COTS but is not harmful to the surrounding ecosystem. A 2008 study by the Great Barrier Reef Marine Park Authority showed that divers had kill rates of up to 120 starfish per hour per diver. More recently, in 2015, research by Queensland’s James Cook University showed that normal household vinegar was also capable of destroying COTS, with no residual harm to the marine environment.

However, the fight against the COTS infestation along the Great Barrier Reef has really gone high-tech lately, with Queensland University of Technology (QUT) researchers trialling a new autonomous underwater robot to do the dirty work. As the video below shows, the COTSbot uses a computer vision and machine learning system to locate starfish and then administer a lethal injection of thiosulfate-citrate-bile salts-sucrose agar (TCBS) to take them out. A single injection of TCBS results in the starfish’s death with 24 hours, but again everything else on the reef is left unharmed. Thousands of still images and videos of the reef were used by the QUT team to train the torpedo-shaped vehicle to distinguish COTS from a wide range of coral species, achieving a detection performance of over 99 percent.

COTSbot: Crown-of-thorns starfish detection system (Dr Feras Dayoub/Queensland University of Technology/YouTube)

There is evidence that the much-maligned crown-of-thorns starfish doesn’t quite deserve its forbidding reputation. Some ecologists assert that as a component part of reef systems, and in sufficiently low densities, A. planci plays an important role in preventing slower-growing coral from being overpowered by faster-growing varieties, which maintains coral biodiversity. So the real issue is not so much COTS but their overabundance – which is very likely caused, at least in part, by the damaging effects human activity is having on our oceans.