COTS are a species of starfish whose feeding behaviour differs greatly from all other types of starfish.  Their larvae settle mostly in deeper waters at the base of the reef. They feed on fast-growing corals, slowing down the coral species to be able to form colonies, thereby decreasing coral diversity. They are typically most active during the night except when their population is dense, which is when they are seen feeding during the day as well.  In densely populated areas, their growth is limited, which causes them to be smaller in size compared to those in sparse habitats. There are only a few species of fish that are known to prey on the COTS. These include triton shells, painted greens, green triggerfish, and the Napoleon wrasse.

 

The Crown of Thorn Starfish is affected in multiple ways by the pollution that enters the Great Barrier Reef. Terrestrial runoff from nearby agricultural areas carries high levels of nutrients and sediments into the reef. This creates plankton blooms, increasing the food for COTS larvae. Since elevated nutrient levels exacerbates their outbreaks, the timing of this occurring is important as if it occurs during the spawning season then it fuels the outbreak even more. The outbreak of this species is one of the biggest threats to the reef as the population density of the starfish increases to a point where they consume coral tissue faster than corals can grow; causing a population decline in coral cover. The increase of this population contributes to 42% of the total destruction of the coral reefs and since 1985, coral cover has declined by half along the Great Barrier Reef. The pollution driver COTS are responsible for over 40% of this loss.

 

Prior to human impact, the carrying capacity of the Crown of Thorn Starfish was limited as there was less food for the larvae to consume, as well as more competition from other predators of coral. Currently, the lack of predators of the COTS is a contributing factor to them being allowed to exceed their carrying capacity, as well as an abundance of coral for them to feed on. The exceeding of their carrying capacities allows the COTS to have an exponential growth, in turn causing a population decrease in the corals that they prey on. A theory is that fishing and shell collecting have led to less predators, allowing COTS population to grow ‘beyond natural levels’.    The human impact of pollution results in the greatest effect on the Great Barrier Reef Ecosystem. Runoff from agricultural areas create algal blooms, this being what the starfish larvae feed on. An overabundance of food for the larvae creates an overabundance of the larvae itself and exceeds the carrying capacity of the crown of thorn starfish. Often, groups of this species spawn at the same time. A single female can produce up to 60 million eggs during their breeding season. Therefore, the profusion of their food caused by terrestrial runoff from nearby areas can lead to the crown of thorn starfish exceeding their carrying capacity, and even more so during their breeding season.

 

Without intervention, the population of COTS will continue to grow exponentially and the coral population will continue to decline. Corals build the structure of the reef, where many fish and several hundred thousands species reside and give birth. It also endangers many other organisms who rely on the coral for food. However, once the coral goes extinct, the COTS will have a very limited food source and will plummet from the heights they were at previously. Crown of Thorn Starfish are difficult to control due to the fact that each limb is capable of regenerating an entire starfish if it is cut off. However, according to recent studies, the most effective way to control their growth is through the injection of sodium bisulphate. It has proven to kill the starfish without any damage being done to the reef.