A coral’s enemy: Crown of Thorns starfish (COTS)

What is the crown of thorns starfish?

The crown of thorns starfish (COTS) is the most voracious coral predator found on Indo-Pacific coral reefs. One starfish will eat a coral every day and can consume all of the corals within a 6-10 square meter area within a year.  They are responsible for destroying entire reefs during severe outbreaks. When COTS undergo population explosions, there may be ten COTS per square meter or more, with tens of thousands of animals invading a single reef. They tend to aggregate, piling on top of table corals and wrapping their bodies around delicate branching corals.  Their path of destruction often resembles a forest fire: they spread through a reef devouring the corals as they move and leaving only a few less preferred corals in their wake.


Outbreaks in the Maldives

Coral Reef CPR first began working in the Maldives in 2015 to tackle unnaturally high densities of COTS. This is the third reported outbreak in the Maldives. The first occurred in the 1970s, the second in the early 1990s was slightly larger and more widespread, while the current outbreak is the largest ever witnessed. It began in 2013 in North Malé Atoll; by October, 2015 large numbers of COTS were seen throughout Ari Atoll, two locations on Baa Atoll, one on Lhaviyani Atoll and four locations on South Malé Atoll. Localized but large densities were noted for the first time in 2016 on Shaviyani Atoll.


Why do outbreaks occur?

Excess nutrients: COTS larvae feed on plankton and normal survival rates are very low. When nutrients from sewage, fertilizer and run-off enter the usually nutrient-poor tropical waters, there plankton blooms providing these larvae with more food and their survival rates increase.


Overfishing of predators: Removing the few predators (Napoleon wrasse, pufferfish, triggerfish, trumpet triton) for the food and souvenir trades reduces predation pressure on the COTS, allowing their numbers to increase.

Life-history and biology: COTS are extremely resilient organisms, with a very rapid and successful life-history. They can regenerate their arms and oral discs within 5-6 months; they are highly fecund and one individual can produce up to 60 million eggs per breeding season (once a year); they can last 6-9 months with no food; they can live and move through very deep water (including between atolls!); their bodies are covered in poisonous spines making them less attractive to the few predators.

 What can be done?

Efforts have been undertaken in the Maldives to remove COTS from the reefs and also inject them with chemicals. This can be very successful, but it requires involvement by dive operators, recreational divers, scientists and resort staff and other volunteers. An effected reef requires an initial clean-up, along with follow up collection efforts to remove any missed starfish. Physical removal is very simple and low cost, our team uses a pvc pipe and mecotscollectingsh bag to collect starfish, sending full bags to the surface with an SMB to avoid injury. If injection methods are used, they must be conducted correctly otherwise the starfish can shed the injected limb, re-growing it rapidly. It is also vital that non-toxic substances are used when injecting so collateral damage to the reef is avoided. The only known effective and non-toxic substances are bile salts and vinegar.

Coral Reef CPR has partnered with Carpe Diem Maldives Pvt. Ltd. to detect outbreaks of COTS throughout the Maldives and remove them from affected reefs.

Dive teams of each of the 3 boats have been provided with the education, tools and mesh bags to remove them and will start removing them whenever possible during regular dives and register the information.

During the 10 night trip on the Vita in August 2016, Coral Reef CPR together with the dive team of the Carpe Vita  removed 242 COTs from different sites.


Check back frequently on the Carpe Diem website ( for the total numbers of COTS we have removed!