It might look pretty, but the crown-of-thorns starfish is one of the main reasons the Great Barrier Reef is in serious trouble. Half the reef’s coral has been killed off in the last few decades, due to this beastie, plus storm damage and coral bleaching caused by global warming. Scientists at the Australian Institute of Marine Science took this footage as part of their monitoring of the reef. If you come across one of these on your next snorkelling holiday, do your worst.