The crown-of-thorns starfish (Acanthaster planci), the world's second-largest sea star, is endemic to the coral reefs of the Pacific. Because of its voracious appetite for live hard coral, it is also blamed for damaging large portions of reefs, notably Australia's Great Barrier Reef.
Crown-of-thorns starfish eat most types of coral, but prefer the branching tubular types of the genus Acropora, such as table coral, elkhorn coral and staghorn coral. Acropora corals are a major builder of the immense calcium carbonate substructure that supports the thin living skin of a reef.
At normal densities on healthy reefs (<10 per hectare), this nocturnal feeder can actually benefit biodiversity. By feeding on fast-growing corals it gives slower-growing corals a chance to thrive.
But they are also prone to infestations (population of >30 per hectare), and then they are a serious threat. After an outbreak, it may take a decade or more for the coral cover to return to its original level. And as many reefs are in close proximity to others, outbreaks can last up to 20 years as the starfish move from reef to reef.
Curiously though, major infestations were not observed until the 1960s, despite the fact crown-of-thorns have been present on reefs for millions of years.
One theory to explain these infestations is overharvesting of one of the few predators of crown-of-thorns, the Triton's trumpet snail (Charonia tritonis), for sale as souvenirs to tourists. Other predators are the starry puffer fish and the Titan trigger fish.
Another theory is that infestations are caused by increasing use of tropical coastlines for agriculture using fertlisers. These fertilisers are flushed into the sea, causing phytoplankton to bloom, which in turn causes zooplankton to bloom, providing more food for the larvae of the crown-of-thorns. Perhaps adding to this chain of events is the huge growth in tourism to the coastal tropics.
As of 2017, parts of the reef off Biri are exhibiting infestation levels of crown-of-thorns, making this a top priority for Biri Initiative.
Working with Biri Resort & Dive Center, we provide fishermen and divers with incentives to keep crown-of-thorns under control. But these hunts must be done with care. Crown-of-thorns are the only known venomous species of sea star, and wounds from their spines are painful and slow to heal.