The Crown-Of-Thorns Starfish

Last week I discussed the basic biological history of sea stars including why they are an important species in most marine habitats. The species of focus this week is a sea star who preys on a very vulnerable animal: corals. Acanthaster planci, the Crown-of-Thorns sea star (COTS) has had major negative impacts on coral reefs since 1962.

About

The COTS can reach up to a half meter in diameter making it the largest tropical seastar. The COTS has evolved to grow many venomous thorns on their aboral side to protect them from predators while they eat. It can take up to six hours for them to feed! The venom can cause varying levels of reactions in humans if they puncture your skin. COTS occur naturally throughout reefs in the Indo-Pacific region. They are not found in the Atlantic Ocean.

Their densities vary in cyclic oscillations from mainly very low to brief levels of un-sustainably high densities. If the conditions are just right, then an outbreak occurs and devastates hard coral communities. In the last 40 years COTS have been one of the major causes of coral decline in the Great Barrier Reef system. The cause of their outbreaks is hotly debated among marine scientists today. Some claim that outbreaks are methodic, slow-paced and diffusive.

Outbreaks

Populations of the COTS can destroy up to 6 km² of coral tissue in just one year. To put that into perspective, that’s over 1,121 football fields worth of corals. Whoa. In localized areas, 90% of coral mortality has resulted from outbreaks of the COTS on the Great Barrier Reef, Guam, American Samoa, and Japan. Some potential triggers for outbreaks include hurricanes, El Niño, rising sea temperatures, predator removal and nutrient discharge. The main and most accepted diver for outbreaks is an increase in phytoplankton food biomass as a result of environmental and anthropogenic events. COTS larvae feed on phytoplankton. What this means is if there is more food available for the larvae, they are more likely to survive and settle onto coral reefs. 

Impact

Since COTS can consume a threatening amount of reef building corals, (a special kind of coral that creates the structural integrity of reefs) this results in major alteration of reef systems and long term consequences. I would guess if corals did not have the added stresses of ocean acidification and rising temperature, they could recover from these outbreaks. Some long term consequences include more benthic algae, collapse of the reef’s structural complexity, declined productivity, and the deterioration of biodiversity. Once one area has been decimated, the starfish will migrate to nearby reefs and begin feeding there. 

What Can Be Done?

Since there is still a lot unknown about the COTS outbreaks, managing requires preventative and tactical actions. Local populations and management authorities are taking measures to eradicate the COTS from vulnerable reefs. Since the 1980s, the Great Barrier Reef Marine Park Authority has partnered with several research institutions and government agencies to better understand what can cause severe outbreaks and what management responses will help the coral reef ecosystems. The management framework they have includes:

• Preventative actions to reduce causes of outbreak and spread
• Tactical action to reduce the damage when an outbreak occurs
• Monitoring reefs to observe any warning signs of a developing outbreak
• Prioritizing research to improve COTS management

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References

Kayal M, Vercelloni J, Lison de Loma T, Bosserelle P, Chancerelle Y, Geoffroy S, et al. (2012) Predator Crown-of-Thorns Starfish (Acanthaster planci) Outbreak, Mass Mortality of Corals, and Cascading Effects on Reef Fish and Benthic Communities. PLoS ONE 7(10): e47363. https://doi.org/10.1371/journal.pone.0047363

Australian Institute of Marine Science (Director). (2011, June 16). COTS Closeup [Video file]. Retrieved September 6, 2020, from https://youtu.be/RBkv_SSvm5U

Pratchett, M.S.; Caballes, C.F.; Wilmes, J.C.; Matthews, S.; Mellin, C.; Sweatman, H.P.A.; Nadler, L.E.; Brodie, J.; Thompson, C.A.; Hoey, J.; Bos, A.R.; Byrne, M.; Messmer, V.; Fortunato, S.A.V.; Chen, C.C.M.; Buck, A.C.E.; Babcock, R.C.; Uthicke, S. Thirty Years of Research on Crown-of-Thorns Starfish (1986–2016): Scientific Advances and Emerging Opportunities. Diversity 2017, 9, 41.