Invertebrates

Christmas Tree Worm

Spirobranchus giganteus · Segmented worm, family Serpulidae (calcareous tube worms); the familiar reef animals form the Spirobranchus giganteus species complex

Christmas tree worms are the reef's pop-up decorations: twin spiral crowns, two or three centimeters tall, in blue, orange, yellow, white, and maroon, sprouting in constellations from heads of living coral. The crowns are the working end of a tube worm whose body stays sealed inside the coral skeleton, filtering plankton and breathing with the same feathery radioles. Startle one (a shadow, a pressure wave, an exhale too close) and it vanishes faster than the eye can track, capping its burrow with a spiked trapdoor. Then the game begins: hold position and count the seconds until the spirals unfurl again. Individuals can occupy the same coral head for decades, outliving most of the fish around them. They make superb practice subjects for breath control and slow approaches, grading your technique instantly and in public.