The most prolific reef builder doesn’t make just cups. Clapping worms are highly mobile little worms that swim by means of rows of tiny oars down their sides. They spend most of their waking hours poking out of tiny holes in their castles to catch plankton by “clapping” their hands and drawing it into their mouths. When not feeding or sleeping, they spend time building up castles with secretions from their hands, spreading it over the surface. The mineral-rich mucus hardens after only a few seconds, containing active cells to finish the process before dying. Inside each castle are common living spaces and a complex network of tunnels.