The Crown of Thorns (Acanthaster planci) is a star fish belonging to the Echinodermata (“Spiny skin”) phylum. During its adult life stage, Crown of Thorns (COTs) are corallivores and predate on the live polyps of hard corals. Within a healthy and balanced reef ecosystem, COTs fulfil a number of important ecological roles. This includes promoting biodiversity of hard corals by preventing any one type of coral from dominating the reef (density dependent predation). COTs are also a key prey species for a multitude of organisms due to their complex, multistage life cycles, and are therefore important organisms living on the reef.

If the density of COTs increases within a particular area, the mortality of corals through COT predation may exceed the rate at which new corals are able to recruit in the same area. If such a high density of COTs is sustained over a long period of time, this may lead to a significant loss of hard corals within a reef ecosystem. During such events, the COT population is commonly referred to as an ‘outbreak population’. In the past, COTs outbreaks have led to mass mortality of corals, and have been responsible for the loss of entire reefs in certain areas of the world.

In the last 100 years, COTs outbreaks have increased both in severity and frequency throughout the world. COTs outbreaks are often associated with anthropogenic influences (human causes) such as agricultural/industrial waste leading to nutrient rich waters, often resulting in the increased survival of COT larvae, as well as overfishing of COT predators.

Research projects established by the Roctopus ecoTrust aim to monitor the population density of COTs, and identify areas where outbreak populations may form.  Research is also conducted on the habitat and dietary preferences of COTs in order to improve our understanding of the ecological effects of high COT densities around Koh Tao. If a COT outbreak is identified, the removal of COTs may be carried out in order to reduce the rate of coral mortality across an entire reef.