Crown of Thorns Starfish (CoTS) – The complicated story of a natural predator on coral reefs

Written by: Manu madhavan

Edited by: Sofia Perez and Bobbie Renfro

Crown-of-thrown starfish (CoTS) Acanthaster planci, is a large species of starfish (Echinoderm) that is covered in crown-like spikes and feeds on living coral polyps, causing widespread damage to the coral reef’s ecosystem in the Pacific when this native species becomes highly abundant. 

While the starfish is native to the Indo-Pacific region, it can occasionally act like an invasive species due to its dense populations and the devastation it is capable of wreaking on the Great Barrier Reef’s corals (Gruber, K. 2015). Outbreaks of CoTS can result in the loss of large areas of coral reef cover, which can significantly impact the overall health of the coral reef. While, the coral cover can be restored, the process is slow. This predatory starfish prefers fast-growing coral, but will also eat boulder coral, which are massive coral formations that take hundreds of years to grow. As the starfish coils around its prey, it ejects its stomach onto the coral and the coral is digested outside of the starfish’s body (Hadhazy 2008).  The decline in coral cover can reduce biodiversity and cause habitat loss for other reef organisms, such as fish and mobile invertebrates. This can have a cascading effects on the entire reef ecosystem. 

The adult crown-of-thorns starfish contains many long spines coated with saponin toxins known as Asterosaponins that causes irritation to puncture wounds  (Howden et al., 1975; Shiomi et al., 1988). 

CoTS contain a mixture of these saponins, and at least 15 chemical studies have been conducted to characterize them.  They have detergent-like properties. If you keep these starfish in small amounts of water with aeration these compounds actually produce foam on the surface of the water, like soap! A. planci has no mechanism for injecting the toxin, but as the spines perforate the tissue of a predator or unwary person, saponin-containing starfish tissue is lost into the wound. In humans, this immediately causes a sharp, stinging pain that can last for several hours, persistent bleeding due to the haemolytic effect of saponins, and nausea and tissue swelling that can last for a week or more. Brittle spines may also break off and become embedded in tissue, requiring surgical removal. These defenses were of course not meant for humans, they are meant to protect the starfish against predators, but they are a hazard to us as well.

Reef recovering after Crown of Thorns Starfish outbreak.

In addition to the ecological impacts, CoTS outbreaks can also have economic consequences. Coral reefs are essential for the fishing and tourism industries, and the loss of coral cover can reduce the productivity of these industries.

The story of Crown of Thorns Starfish is a complicated one. They are native species but they cause significant coral loss when they exhibit outbreak population booms and these days corals can’t handle yet another stressor. To mitigate the impact of CoTS on coral reefs, various management strategies have been developed. These include the physical removal of CoTS and the introduction of natural predators such as hump head wrasse, titan triggerfish, and giant triton snails.  However, the effectiveness of these strategies can vary depending on the outbreak’s severity and the particular circumstances of each coral reef system. The impact of CoTS on coral reefs is significant and wide-ranging. It can lead to declines in coral cover, reductions in biodiversity, and adverse effects on local fishing and tourism industries. Effective management strategies are needed to mitigate their impact but these should be used only in areas of outbreaks when necessary to reduce but not eliminate this native species to protect the health of coral reefs for future generations.


Howden, M.E.H.; Lucas, J.S.; McDuff, M.; Salathe, R. Chemical defenses of Acanthaster planci. In Crown-of-Thorns Starfish Seminar Proceedings; Australian Government Publishing Service: Canberra, Australia,1975; pp.67–79 

Shiomi, K.; Yamamoto, S.; Yamanaka, H.; Kikuchi, T. Purification and characterization of a lethal factor in venom from the crown-of-thorns starfish (Acanthaster planci). Toxicon 1988, 26, 1077–1083 

Hadhazy, A. (2008, July 21). Fishing bans may save corals from killer starfish. Scientific American. Retrieved from

Gruber, K. (2015, March 23). The Great Barrier Reef is under siege. The Washington Post. Retrieved from

Platt, J.R., (2016, January 1). A Starfish-Killing, Artificially Intelligent Robot Is Set to Patrol the Great Barrier Reef. Scientific American. Retrieved from

Rivera-Posada, J., et al., (2014). Bile salts and the single-shot lethal injection method for killing crown-of-thorns sea stars (Acanthaster planci). Ocean & Coastal Management, 102 Part A:383-390.

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