The Secret Lives of Koh Tao’s Crown of Thorns

The Crown of Thorns Starfish, Acanthaster Sp., is a large sea star whose diet consists almost entirely of hard corals. This sea star has been known to cause devastating losses to coral reefs in areas where their populations rise, or where corals are particularly stressed by other threats such as development, over-fishing, physical damage, or bleaching. For years, we at the NHRCP have been monitoring Acanthaster populations around our island, and performing removals in areas where populations are relatively high, or coral levels are low. In 2014 we removed 138 individuals from reefs around our island, and in 2015 we removed over 277. After so many removals we started to wonder where the new sea stars are coming from, what depths are they living at, what are the population dynamics, and what corals do they prefer to eat. All of these points where investigated in detail by one of our interns, Leon Haines.

Leon did a study on the population of Crown of Thorns star fish from Koh tao, using data collected both during removals and through focused surveys he performed. The project was done for his degree of integrated Coastal Zone Management at the Van Hall Larenstein University of Applied Science. Recently, he finished his report on the project, titled “The dietary preferences, depth range and size of the Crown of Thorns Starfish (Acanthaster spp.) on the coral reefs of Koh Tao, Thailand”. Many of his findings have been quite interesting in the context of previously published literature, and it would appear that the COTs populations around Koh Tao differ from many others around the Indo-Pacific Region.

Most studies of Crown of Thorns starfish have concluded that their primary food sources are corals of the genre Acropora and Pocillopora. Leon’s study looked at 248 individual starfish, collected at 14 sites around Koh Tao from January 2014 to March 2015. He found that the preferred food source of the sea stars was mushroom corals (Fungia), accounting for 17% of observed feeding. This was followed by Pavona (15%), Porites (14%), then Acropora and Favia (both at 9%).

Although according to much of the available literature the sea stars feed mostly at night, in his study, 78% of the sea stars were found feeding during the daytime. The sea stars were found mostly between 6-9 meters depth, followed by 9-12 meters depth. Very few of the sea stars were found shallower than 3 meters or deeper than 18 meters. Interestingly, the sea stars were found at slightly shallower depths around February and October, and at deeper depths around April. These movements are most likely in response to spawning timing, as the animals move to shallower depths to create more concentrated populations.

When Leon ranked sites by number of Crown of Thorns collected, he found that King Kong Rock, Laem Tien, and Lighthouse had the highest numbers. The average size of the sea stars was 35 cm in diameter and the largest was 52 cm. No sea stars smaller than 12 cm were found at any of the sites, leaving the question as to where the larvae recruit and live out their first few months around the island.

Leon’s study has helped to contribute to the growing knowledge on this outbreak-prone coral predator around the island of Koh Tao. Crown of Thorns have already been documented to contribute greatly to coral reef decline in many reefs around the globe, most notably the Great Barrier Reef and Coral Triangle Region. Having more knowledge on their local population dynamics, behavior, and feeding preferences helps our program take a more proactive approach to Crown of Thorns Management around our island.

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