There is a silent killer in our reefs, a slow, underwater assassin: the Crown-of-Thorns starfish.
At low density, the corallivorous starfish, Crown-of-Thorns (Acanthaster planci), naturally occurs in coral reefs. Yet sudden increases in population, known as ‘outbreaks’, represent one of the most important disruptions to the coral reef. Acanthaster planci (Linnaeus, 1758) is an incredible machine, built to feed and reproduce – a Darwinian nightmare. This starfish is found practically everywhere in the Indo-Pacific Ocean, from the Red Sea to Japan and from the coasts of South Africa to California.
These little crawlers can grow up to 80 cm, weigh up to 4 kg in adulthood and have bright shades of blue, purple, yellow, orange and red. If you think you have seen one before with five arms, it’s probably not it. Crown-of-Thorns (COT) have between seven and 22 arms, all covered with thorns loaded with saponin, a kind of steroid that can lead to reactions such as an anaphylactic shock.
As for most echinoderms, its reproduction is external, taking place during the hottest months each year. Larger females can expel a quantity of approximately 60 million gametes.
Many communities in the Pacific Islands depend on their reefs for survival, seeing them as a source of food and income as well as protection against coastal erosion and disaster events such as tsunamis and hurricanes. For all these communities, the ravages of this starfish are a threat to their lifestyle and their safety. Fighting against COT becomes a necessity.
Everywhere it goes, the coral is devoured, with such efficiency that it is often compared to a cyclone. Only the skeleton remains, white, immaculate, lifeless. Not only does it destroy the coral, but it also causes a cascade of malignant effects on the coastal community, drastically reducing the surrounding biodiversity.
The first cycles observed in the 1920s had a recurrence of about fifteen years, hence the coral was then able to recover. The fast-growing coral, such as Acropora, is preferentially consumed, potentially allowing slower species the time and space to develop. There was a balance. Nowadays however, these cycles have shortened. From one year to the next, scientists can observe different outbreaks of Acanthaster on the same island and on the same reef, leaving the coral no chance to recover.
Not only has the frequency of outbreaks increased in recent decades, but the number of individuals on the reef has increased as well, reaching peaks such as the one observed by Kayal in 2011 with 53,750 starfish per square kilometer in Moorea, French Polynesia.
The reasons are multiple but linked by their origin; climate change counts among its causes. Global ocean warming allows these starfish to have a longer period of reproduction. The larvae born during these mating periods feed on phytoplankton, which itself is prolific, probably as a result of episodes of strong terrestrial run-off. The larvae grow better and almost without predators as the reef becomes depleted of its fish, often harvested by human populations.
Ultimately, what goes around comes around; the United Nations Environment Program estimates the cost of coral loss to be around 172 billion USD per year. This includes, among other things, the loss of food, raw materials, natural protection from weather events, water purification, and tourism. The disappearance of this incredible biodiversity is however, priceless.
Right now, these outbreaks can only be contained by human intervention and these interventions are limited by financial and human resources, as well as by the size of the area concerned.
Over time, different strategies have been developed to thwart these invasions. Several groups of scientists tried to set up fences, underestimating the agility of the Acanthaster; they also tried to destroy them by cutting them in two before tossing them back to sea, underestimating this time, their incredible capacities of regeneration. Biological control also resembles a stalemate: its main predator, the triton shell (Charonia tritonis), is intensely poached for its shell and difficult to breed in captivity; plus, it counts many different species of starfish in its diet.
One technique, still used by the communities of the Pacific, has proven its worth over the years: it is simply called ‘Hand-removal’ followed by a burial in the sand that the tide will not reach. On a small scale, this technique is undeniably efficient, but it finds its limitations with greater outbreaks.
Although it is a promising alternative, its cost remains extremely high for most Pacific island countries.
Another solution, however, has come from Vanuatu.
It became essential to develop a technique that was both efficient and cost effective and in 2014, a team composed by the IRD, the CRIOBE and the Vanuatu Fisheries Department developed a new injectable solution based on natural acid: lime juice.
Extracted from the fruit of Citrus aurantifolia, the juice contains a high percentage of citric acid and has never been tested on the Acanthaster before. This tree has a huge advantage as it is present throughout the Pacific. Harvesting its fruit will always be easy and cheap. At the same time, vinegar was also tested, all these experiments taking place in Vanuatu. Various parameters were measured such as quantities, number of injections and solutions used. For each treatment, 32 individuals were collected and injected; a total of 256 starfish. The best results were achieved with 20ml of lime juice injected twice into the starfish. The team observed 100% mortality within 21 hours. The same numbers were observed with vinegar, with a 100% efficiency and mortality under 23 hours. The potential of this method has been confirmed by both an in situ and a laboratory comparative test, and similar results were obtained regardless of whether the environment was artificial or not.
The team didn’t stop there and their experiments were coupled with a thorough analysis of the starfish’s immune system. Due to its uncontrollable nature, triggering a disease in this species would be undesirable. The analysis of the immune functions showed that the treatment does not cause a disease but a disruption of their internal pH.
Although this technique of regulation is only a short-term answer for a phenomenon for which causes are yet to be clarified, its efficiency is gaining popularity. The lime juice injections are becoming a small revolution that could give communities the chance to eliminate a thousand Acanthasters with only 10 to 20 litres of lime juice or vinegar, all of that without harming the ecosystem. The only real cost is that of the injection guns: the lime itself only needs to be harvested, squeezed and filtered. Citric acid or acetic acid in powder, which is easier to transport, could also be considered.
The Vanuatu Department of Fisheries is currently working on a way to intervene at any location, training groups of ‘vigilantes’, locals on the lookout for any outbreak, and using this new method to protect our reefs.
In Vanuatu, COT have already started going on a rampage. They have been spotted in high numbers on Aore and off the coast of Santo, in Crab Bay, Malekula, Emae and Emao and around Efate. The outbreaks may have been stopped in time but the reefs still suffered some loss.
An undetected outbreak could bloom anywhere, and have devastating consequences if it remains unseen.
It doesn’t take a background in Marine Biology to help, more like a pair of fins and a snorkel. Well aware of the challenge presented by the wide-spread islands of Vanuatu, the IRD and the Fisheries Department have come up with a solution to map the outbreaks: an interactive map that is called Oreanet. Available on the Fisheries’ website and as a mobile app, anyone can pinpoint where they saw these starfish on a map. You will need to count the starfish found in 1sqm, and record how far you swam. It is a very simple method that could effectively save a reef and a whole community. In the end, the goal is not to find as many starfish as possible – on the contrary, zero is of course, the best outcome. If you find any, go to https://fisheries.gov.vu/ and/or contact the Fisheries Department.
Story and photography by Gregoire Moutardier with the collaboration of Jayven Ham and Pascal Dumas.
Grégoire Moutardier arrived in Vanuatu in 2014 with the IRD. At the Fisheries Department, his master thesis on Crown-of-thorns starfish lead to the Franco-Vanuatu discovery of lime juice injection. Now he is the manager of Okeanos Vanuatu, a traditional sailing boat (vaka) using only renewable energy. The vaka is used for disaster relief, eco tourism and occasional COT surveys.
Jayven Ham, from Malekula, has been a fisheries biologist at the Fisheries Department of Vanuatu for the last 10 years. He is an expert in marine protected areas, trochus, sea cucumbers, giant clams and crown-of-thorns starfish. He was part of the team that studied the efficiency of hand-removal of COTs and the lime injection experiments.
As a research scientist for IRD (French National Research Institute for Sustainable Development), Dr. Pascal Dumas has conducted research on tropical coastal ecosystems for the past 15 years. He is the coordinator of the COT monitoring initiative OREANET (Oceania Regional Acanthaster Network) and coordinates research on community-based management of coastal resources with the Vanuatu Fisheries Department.