A game I like to play while scuba diving or snorkelling on a tropical coral reef is to slowly glide up to a boulder of coral and peer as closely as I can at clusters of Christmas tree worms without scaring them. These tiny creatures look remarkably like tiny fir trees, although the artificial kind made of brightly coloured plastic and tinsel. They can grow in rainbow mixtures of reds, yellows, oranges and blues, and all of them are the same single species, Spirobranchus giganteus.
Get too close and, in the blink of an eye, the reclusive worms disappear into their tubes, which are burrowed into the coral, and slam shut little lids, operculums, behind them. Then it’s a wait of a minute or longer before the worms decide it’s safe to come back out.