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Acanthaster planci

A zoomed-in image of the crown-of-thorns sea star.
(Klaus Jost, jostimages.com)

Scientists have been studying why populations of crown-of-thorns sea stars (Acanthaster planci) have mushroomed in recent decades. Coral reefs can suffer when the sea star's numbers explode because the echinoderm has a healthy appetite and few predators. And they are part of the reason why Australia's Great Barrier Reef is in decline.