On a coral reef, the corals themselves are keystone species.  The absence of coral means a reduction of reef-dependent fishes, which in turn disrupts ecosystem functions further up the chain.  Perhaps the deadliest predator to corals is an unlikely one.  The crown-of-thorns starfish (Acanthaster planci), or “COTS”, is responsible for up to 90% of coral cover reduction in some parts of Indo-Pacific reefs.  This has had devastating and widespread effects, and many research efforts are seeking a solution.

This damage to this coral is quite apparent, as three COTS prey upon it. Image copyright © Borut Furlan.

The damage to this coral is quite apparent, as several COTS prey upon it. Image copyright © Borut Furlan.

The complex existence of the crown-of-thorns starfish

In normal population densities, COTS are actually very important members of the reef ecosystem.  They prey upon fast-growing corals, which allows slower-growing corals a chance to establish and survive competition from the faster corals.  This allows a diversity of coral species that attracts healthy fish and invertebrate populations.  But this beneficial “service” isn’t why COTS is so famous; the truth is a little creepier.

Every so often, thousands of COTS will emerge from the depths onto shallow reefs in the Indo-Pacific and wipe them clean.  Their method of feeding is just as strange.  They expel their digestive system outside of their bodies to cover their prey, then digest them from the outside by excreting a mixture of enzymes.  After devastating the reef’s coral, other marine invertebrates, and even each other, they disappear as mysteriously as they appeared.

The reason for the outbreaks is still widely misunderstood, but it is commonly associated with elevated nutrients and over-fishing in waters of heavily-populated areas.  However, recent COTS outbreaks have been discovered in a group of atolls in the central Indian Ocean, where the islands are completely uninhabited by humans and the waters are rarely, if ever, fished.

The search for defense of COTS invasions

COTS can have up to 21 arms, and can be brilliantly colored. Poisonous spines protect these animals from most would-be natural predators. Photo by Suzanne R. Livingstone.

While scientists continue to look for population-level control methods, current techniques focus on killing as many individuals as possible.  In the past, individuals were removed or even chopped into pieces.  But these methods were slow and ineffective, and were too dangerous for the people involved due to contact with the poisonous spines.  Now the most common method is a lethal injection of a mix of chemicals, which a diver can administer at a safer distance.  The problem with this method, though, is that the chemicals used are toxic to many other marine animals, as well as the SCUBA divers themselves.

Other lethal mixtures that have been tested, such as bile salts, have been effective at killing COTS, but require permits to use and may encounter quarantines when being shipped across international borders.  All in all, the past and current control methods are either too dangerous to perform, or it is too difficult to acquire the substances needed.  In the search to find a safer and more easily-obtainable solution for injection, scientists have stumbled upon an unlikely answer.

A possible simple solution to massive coral predation

Lethal injection is a safer method than removal.

Lethal injection is a safer method than removal.  Photo by Dive Queensland.

Vinegar.  The cheap and common household staple was tested as an injection substance for COTS eradication in a study conducted on the Great Barrier Reef and in Papua New Guinea.  The results showed 100% mortality within 48 hours of injection.  Echinoderms (the Phylum that includes starfish, sea cucumbers, and sea urchins) are poor regulators of internal pH.  Since vinegar is acetic acid, this rapid change in pH causes tissues to rapidly die off and render the COTS immobile, eventually leading to death.

Over the past 30 years, the Great Barrier Reef has seen a 50% reduction in coral cover, and COTS is responsible for much of that decline.  The euthanization of individual animals is not a long-term effective solution to the problem; scientists would never be able to keep up with the outbreaks.  However, it could be enough to save individual reefs while the search for an effective population-level control method continues.  The identification of an accessible and cheap injection solution will help make that possible.  More research will be needed before the method is widely employed on reefs in order to ensure other marine wildlife would not be harmed in the process. Perhaps this is even a job for the robotic COTS hunter that is being deployed here.

Currently, only scientists holding permits may inject and/or remove COTS, but if safe and easy solutions are identified, it’s possible a citizen science project could allow recreational divers to participate in eradication efforts, much like with invasive lion fish in the Atlantic.

Sources:

Barrier Reef: Vinegar could curtail coral-eating starfish. BBC World News – Australia. Web. 23 September 2015. <http://www.bbc.com/news/world-australia-34337394>

Einarsson, Lisa Bostrom and Jairo Rivera-Posada.  “Controlling outbreaks of the coral-eating crown-of-thorns starfish using a single injection of common household vinegar. “ In Coral Reefs: Journal of the International Society for Reef Studies.  Berlin: Springer, 2015.

Roche, R.C., et al.  August 2015.  Localized outbreaks of Acanthaster plank at an isolated and unpopulated reef atoll in the Chagos Archipelago.  Marine Biology 162(8): 1695-1704.

Sheppard, Charles R.C., Simon K. Davy, and Graham M. Pilling.  The Biology of Coral Reefs.  Oxford University Press, 2012.  Print.