The Crown of Thorns (CoT) sea star is native to coral reefs in the Indo-pacific region. It is one of the largest sea stars in the ocean. It has up to 21 arms and reaching an average diameter of 35 centimetres, but can grow up to 80 centimetres! The CoT is named for the venomous thorn-like spines covering its upper surface. It is the only known venomous sea star and possesses one of the highest fertility rates of any known marine invertebrate (animals lacking a backbone), releasing up to 65 million eggs over a single spawning season.
Due to its flexible nature, it is able to bend and twist around the contours of corals on which it feeds. To feed, it expels its stomach through its mouth on the underside and the stomach surface secretes digestive enzymes, allowing the CoT to absorb nutrients from the coral tissue. This leaves a white scar upon the coral skeleton. It feeds upon coral twice as fast as smaller sea star species and will feed faster in warmer temperatures. When conditions are ripe, they’re like a plague, exploding in numbers and devastating hard coral communities. These plague-like outbreaks have occurred approximately every 15-17 years over the past 50 years on the Great Barrier Reef. The Australian Institute of Marine Science released a study in 2012, saying CoTs are responsible for almost half of the coral cover lost on the reef in the last 27 years.
CoTs have few natural predators; Giant Triton Snails, the Humphead Wrasse, Starry Pufferfish and Titan Trigger Fish, which are unable to keep population blooms in check. Scientists inject CoTs with a lethal injection of bile salts (sodium cholate and sodium deoxycholate). They continue to research other techniques that will have a greater impact and be more time efficient, including replicating the chemical released by the Giant Triton Snail, which induces terror and dispersal in this thorny predator.
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