Last Update: 09 September 2012

Crown of Thorns

The Crown of Thorns (COT), known scientifically as Acanthaster planci, is a very large starfish that commonly occurs in the Indo-Pacific waters. They can reach up to 1 meter in diameter and have 21 arms. In large numbers, COTs can provoke extensive coral bleaching. For this reason, the number of COTs needs to be monitored regularly.

  • COT starfish
  • Mouth of COT starfish
  • Bleached coral caused by COT feeding
  • COT starfish
  • COT starfish
  • Manual collection of COT
  • Bleached coral caused by COT feeding
  • Manual collection of COT

Feeding behavior

COTs are carnivorous feeding actively on corals by extruding their stomach out through their mouth and turning it inside-out in a process called eversion. Digestive enzymes are then secreted directly onto the coral tissue that is thus digested and reabsorbed. The feeding process can take from 4 to 6 hours and COTs can feed twice per day. In one day a single starfish can cover a distance of up to 20 meters. Juvenile COTs feed mostly on coralline algae but around six months of age, they start feeding on corals. COTs showed a preference for branched corals, in particular the Acropora genera but in case of competition for food due to high numbers of starfish, they will eat most types of corals. COTs are usually more active at night and spend the day hidden in sheltered places. However during outbreaks they are seen equally during the day and at night.



When they reach the age of 2-3 years, COTs are sexually mature and can start breeding. They will be actively breeding for the next 5 to 7 years. During the spawning season, starfish release eggs and sperm in the water. When the eggs are fertilized, they will develop into larvae that will settle into a reef after having drifted for 2 to 4 weeks. In order to maximize the chances of eggs fertilization, starfish tend to gather during the spawning season. One starfish can produce 10 to 12 million eggs per individual per spawning season. This strategy (producing a large number of eggs) is quite common among marine invertebrates as usually the survival rate of eggs and larvae is very small.


What is an outbreak?

An outbreak is simply defined as the unusual occurrence of high number of individuals in a limited space. COTS outbreaks are classified either primary or secondary. Primary outbreaks that are due to a combination of biological and physical factors permit the animals to spawn successfully and survive through their first year. Secondary outbreaks are caused by infestation of new larvae/recruits from the spawning of COTS at nearby reefs mainly from primary outbreaks. Outbreaks are classified as incipient outbreaks (high number of juveniles likely to reach maturity in 1-2 years), spot outbreaks (high densities of COTs in few areas of the site, but not elsewhere), and active outbreaks (more than 30 COTs per hectare over large areas of the reef)


COTs outbreaks in the Egyptian coast of the Red Sea:

During 1994-8, 26 Indo-Pacific nations have reported COTs outbreaks of different magnitude, including the Egyptian coasts of the Red Sea. It is believed that a primary outbreak occurred at the Jackfish Alley of the Ras Mohammed National Park in 1994 followed in 1998 by an active secondary outbreak that infested numerous reefs along the Egyptian coasts of the Red Sea starting from Dahab on the Gulf of Aqaba to the Sudanese border. This was the first time that such an event was observed in the Egyptian Red Sea. In order to control the problem, the Egyptian government authorized the removal of a quota of starfish from reefs with high economic value. More than 100,000 COTs were removed from these sites, thus saving hundreds of hectares of living corals that kept their economical value and attractiveness for tourists.


Causes of COTs Outbreaks:

Up to date, the possible causes of COTs outbreaks are still unknown. There are two main theories concerning the outbreaks:

1) Outbreaks are natural phenomena and happen periodically;

2) Outbreaks are the consequence of human impact.

Historical accounts of COTs outbreaks make scientists believe that these events are natural; however there is no doubt that human factors like overfishing of COTs predators, runoff nutrients, pollution and climate change have increased the frequency and intensity of the outbreaks, resulting in severe damages to the living corals.


Manage COTs Outbreaks:

Before presenting the most common management strategies to control COTs outbreaks, it is important to keep in mind that:

1) COTs are not an invasive species, they are part of the coral reef ecosystem;

2) because COTs are an important element of coral reefs, the removal of all animals is not desirable;

3) the reasons behind outbreaks are still unknown and they could simply be a natural process with which we should not interfere.

For these reasons, it is not recommended to eliminate or remove COTs on large scale. However starfish can be collected from small areas of special value for tourism or science in particular.


At present, two strategies exist to control COTs outbreaks:

1) Clean ups or removal of the COTs: this solution is the easiest and probably the cheapest when the area to be covered is not larger than a few hundred square meters. COTs should be collected and buried ashore.

2) Injection of poison to kill individuals: the most effective poison that has been used up to now is the sodium bisulphate that is relatively inexpensive and, more important, doesn’t damage other organisms. This method requires trained people to use a large syringe to inject the poison on each individual. It has proven to be useful but could be expensive and time demanding.

For these management practices to be effective, the help and support of all coral reef users is strongly needed.


Need for regular monitoring/reporting programs:

In the past weeks, HEPCA has received some notifications on the presence of unusual densities of the COTS on some reefs. The most recent notification concerned Carless reef. These reports are now under assessment by HEPCA scientific team. Based on this, HEPCA decided to establish and implement a monitoring and warning program to follow up the distribution and abundance of the COTS on our reefs based on reports from the diving community of the Red Sea.


How can you help:

a) Report to HEPCA when you see unusual densities of the COTS sending us the following data (use the form provided below):

  • Date of dive/survey
  • Location: latitude/longitude data or the name of the reef
  • Estimation of the area covered
  • Depth at which you have seen the COTs
  • Number of COTs observed

b) Take part in the clean ups: when reports of primary or secondary outbreaks will be confirmed by HEPCA scientific tem, we will organize clean-up events in order to collect as many animals as possible from specific reefs.


It is well known now that if there is evidence for a primary outbreak, a secondary outbreak will take place a few months or years later and could affect a larger area as adult individuals will move from deep to shallow water when water temperature will rise.


How to collect COTs:

Please, do not start cleanups by yourself without previous instructions! Collecting COTs needs to be carried out using very specific techniques in order not to cause more harm than good. 

In order to safely collect COTs, you will need gloves, a hook or a spear and a bag to put them. Mesh bags resulted very useful and practice in previous clean ups.

Do not attempt to collect COTs with your hands, remember that COTs spines are very venomous and produce a neurotoxin that can cause sharp stinging pain that can last for hours, as well as nausea and vomiting.



Report unusual densities of COTs to us:



Crown of thorns starfish - identification

Learn more about COTs and how to identify them

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COT survey form

Download form to participate in our crown of thorns survey

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