Currently, the crown of thorns sea star (COTS) is wreaking havoc on coral reefs across the Indo-Pacific region, including the Great Barrier Reef.
Although not an invasive species, outbreaks of the crown of thorns sea star have resulted in up to 90% coral mortality in infected reefs. These sea stars have a truly insatiable appetite for corals!
What Causes An Outbreak?
In controlled populations, the crown of thorns sea star is not a threat to coral reefs. The sea stars are part of a cyclic pattern and experience an outbreak once every 17 years. However, local outbreaks can occur unannounced.
Outbreaks occur after hurricanes, El Niño-Southern Oscillation events, ocean warming, removal of predators, discharges of nutrients and mass dispersal events.
Hurricanes and El Niño events carry extra sediments and nutrients into the reef. These nutrients then provide the juvenile COTS with an ample amount of food.
Over-harvesting of predators, such as the giant triton snail, sweetlip emperor, humphead Maori wrasse, and starry puffer fish allow for further growth of COTS at all life stages. After a triggering event occurs, the normally stable population spikes and begins to overtake the coral reefs.
Crown Of Thorns Sea Star Ecology
The crown of thorns sea star, Acanthaster planci, inhabits the waters from the Red Sea of Africa to the western portion of Panama. They are one of the largest species of tropical sea star reaching ½ a meter in diameter! The differences between COTS and other coral reef sea stars include a disc-shape appearance, 13-16 arms, venomous spines and have a flexible nature.
Unfortunately, the crown of thorns sea star’s primary food source is the polyps and soft tissue of corals. The consumption of this tissue layer allows the sea star to ingest nutrients from the coral. These polyps are home to zooplankton that are responsible for providing the coral with 90% of its nutrients and its vibrant colors. Stripping the coral of these zooplankton leads to mortality.
Currently, there are different strategies in place to control the crown of thorns sea star population.
The first step is to monitor the population of COTS in the dense coral reefs. When the population levels reach a point where the sea star is consuming the coral faster than it can grow, then an outbreak is occurring.
When these outbreaks occur the next step is to cull the excess sea stars. The common culling method is using an injection of a chemically safe mixture to kill the sea stars without harming the environment.
The final step is to continue to monitor the reef health to ensure that another outbreak doesn’t occur. While these are only short-term treatments, further research is currently being conducted to pinpoint the source of these outbreaks to further prevent them.
“Crown-of-Thorns Seastar (Acanthaster Planci).” NOAA PIFSC Hawaiian Monk Seal Research, www.pifsc.noaa.gov/cred/crown-of-thorns_seastar.php.
“Crown-of-Thorns Starfish.” Oceana, oceana.org/marine-life/corals-and-other-invertebrates/crown-thorns-starfish.
“What Is the Short-Term Strategy?” Australia Government – Great Barrier Reef Marine Park Authority, www.gbrmpa.gov.au/about-the-reef/animals/crown-of-thorns-starfish/what-is-the-short-term-strategy.