Crown of Thorns Starfish: A Primer


I have tried looking up information about the control of the Crown of Thorns Starfish (or what we commonly refer to as COT). And here are some of the facts:

1. WHAT IT LOOKS LIKE: The crown-of-thorns starfish (Acanthaster planci) is a red-colored starfish with thorn-like spines sprouting all over its body for protection. These echinoderms grow to a diameter of up to 40 cm across and have 12 to 19 arms extending from their center. The crown of thorns can grow from the size of a grain of sand to the size of a dinner plate. An exceptionally large crown of thorns can grow to be the size of a car tire.

2. WHAT IT CAN DO TO PEOPLE: If accidentally touched or stepped on by humans, the starfish’s long spines (which release a neurotoxin) are capable of pricking and stinging, inflicting great pain that can last for hours, as well as nausea and vomiting. Frequently, the area around the puncture turns a dark blue (erythema) and begins to swell (edema). The swelling may persist for a number of days or weeks. If you are pricked by a crown of thorns, it is important to ensure any remnants of the spine are completely removed.

First aid treatment is the same as that given for all venomous fish stings:

Remove any very loose spines. Spines that are embedded should be left until medical help is available. Avoid excessive tugging at spines as the tips may break off and be difficult to locate.
Place the injured part into hot, but not boiling water. The temperature of the water should be tested by someone other than the patient before use.
Apply local anaesthetic around the wound to provide pain relief.
Clean the wound, washing out with sterile hot saline solution.
Dress and loosely bandage the wound.
Arrange bed rest and antibiotic treatment as appropriate.
Supervise any further pain relief.

Note: Do not apply an arterial tourniquet, nor a pressure or immobilisation bandage.

Although there is no physiological explanation, a widespread traditional remedy for spinings is to hold the starfish’s lower surface (with the suction feet) against the wound. This allegedly relieves pain and disability.

3. WHERE THEY LIVE: Crown-of-thorns starfish are found on coral reefs in the tropics ranging from the Red Sea, throughout the Indian and Pacific Oceans, and all the way to the Pacific coast of Panama.

4. WHAT THEY EAT: The starfish is a coral reef predator (a corallivore) and preys on the coral polyps by climbing onto them, extruding its stomach over them, and releasing digestive enzymes to then absorb the liquified tissue. They feed alone at night, maintaining a constant distance between themselves and other crown-of-thorns starfish. During times of food shortage, these creatures can live on their energy reserves for over six months.

5. PURPOSE: Before overpopulation, crown of thorns kept the fast growing coral from overpowering the slower growing coral. In an outbreak it destroys coral mercilessly.

6. HOW TO GET RID OF THEM IN AN OUTBREAK: Collection by hand has been a successful control method but only in small areas and only when performed periodically until the COT population has gone down to a manageable number, meaning, when the balance of more coral and few COT is restored.

Poison injection is another way. Sodium bisulphate, or ‘dry acid’, is the best choice. Add about 1/3 cup (140 grams) of the chemical to each litre of seawater. One litre of the solution is enough to kill about 40 adults starfish. Because the solution is colourless and difficult to see underwater, add a food colouring as well. The mixture can be injected using a standard agricultural injection gun. In Australia, the DuPont Veldspar Spot Gun fitted with a longer 50 cm needle and 5-litre plastic bladder is recommended. To inject the starfish, set the dose meter on the gun to 2 ml. Push the needle under the skin of the central disk of the starfish and pull the trigger. Inject the starfish three times this way.

7. WHAT CAUSES AN OUTBREAK: Outbreaks of huge numbers of these starfish are believed to be caused by agricultural runoff which causes algal (phytoplankton) blooms which provide food for the larvae and enhance their survivorship. The connection lies probably in the increased amount of algae supplying coral polyps (which eat the eggs of COT) with large amounts of food, thus reducing predation on the starfish’s eggs. Since the eggs drift considerable distances in the plankton, the runoff would have to occur on the settling, not the spawning, grounds. This also explains the phenomenon of massive outbreaks seemingly appearing out of nowhere, with no previous indication of an increasing population at the affected site.

The other suggests that outbreaks are a result of man’s interference since he has (inadvertently?) removed the main predators of the starfish.

8. THEIR NATURAL PREDATORS: Notable predators of COT (Ancanthaster planci) include the Giant Triton (Charonia tritonis), a species of shrimp, a species of worm, and various reef fish (especially the Humphead wrasse) which feed on larvae or small adults.

9. HOW LONG OUTBREAKS LAST: The duration of outbreaks on individual reefs varies widely: some may last for 4-5 years whilst others may have run their course within 1-2 years. The duration of an outbreak probably depends on the size and shape of the reef, the number of starfish in the population, the amount of corals available and certain physical factors (eg. weather conditions).

In general, reefs pass through a series of 5 distinct stages during an outbreak of crown-of-thorns starfish. This is termed an outbreak cycle. These stages represent the main changes in abundance of the crown-of-thorns starfish and its coral prey:

Stage 1. Before the outbreak (high live coral cover and very few crown-of thorns starfish)
Stage 2. Start of the outbreak (increasing numbers of crown-of-thorns starfish and declining amounts of coral).
Stage 3. Height of the outbreak (very high numbers of crown-of-thorns starfish and large amounts of recently dead coral).
Stage 4. Decline of the outbreak (rapidly declining numbers of crown-of-thorns starfish and low live coral cover).
Stage 5. Recovery from the outbreak (few crown-of-thorns starfish and increasing amounts of live coral).

10. HOW THEY REPRODUCE: The COT reproduces sexually. Females shed eggs into the water which are fertilised by sperm released from nearby males. Starfish need to be examined internally to determine their sex. As yet there are no data to indicate the relationship between rates of fertilisation and the density of spawning starfish. The rate of fertilisation of eggs would be expected to increase with greater numbers of spawning starfish and/ora decrease in the distance between them.

The bell-shaped posture normally is adopted by both male and female starfish during spawning. They release what looks like a cloud in the water, much like when we disturb the silt. The female releases a pheromone to invite the nearby male to release sperm. Recent studies have shown that a large female starfish (about 40 cm in diameter) may produce about 60 million eggs during one spawning season.

Crown of Thorns Starfish
Controlling Crown of Thorns Starfish
Crown of Thorns Starfish: Questions and Answers


It seems that what must be done to save our playground is to keep collecting the COTs by hand. It must be done periodically which means, that if you dream of diving in the rich coral gardens, you must make certain sacrifices every now and then. Perhaps you could convince your group to spend one dive on a heavily infested dive site. The COT population is alarming. And it will not take long before all the dive sites in Anilao are covered because Anilao is a small area and the currents will take them whither and thither.

I am not a COT expert. I found this information on the internet. I hope that by writing about it and showing you what the COT can do, maybe I can convince you to collect COT. We need all the help we can get.

If there are COT clean-up volunteers, please contact Dave E. Santos so we can organize another clean-up.



  1. May I suggest that we ask resort owners or DM’s/instructors report outbreaks so we as divers know where to collect. This way, we can “donate” one dive per trip to collecting COT’s. If most do their part, the outbreak can be controlled.

  2. okaye info. but could be better.

  3. pretty cool and good information. but i needed more because i keep getting almost the same information everytime for my project. But thanks you had a little of something that the other didn’t have.

  4. Hi I live in Cooktown and I’m a keen diver I would love 2 join the team 2help clean up this pest I think about this all the time and make me sad could imagine life with out the great barrier reef please call me on 0400006588 I will help sorry I am not good at reading and writing so please call me thank you dom

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