A Christmas Tree Worm lives in a tube. The tube is like a shell that it builds on the surface of a coral. Then it sticks its head out of the tube. The twin "Christmas trees" you see are the worm's radioles. Radioles are like antennae on top of the worm's head. The worm uses these radioles to filter plankton out of the water for food and to exchange oxygen and carbon dioxide with the water so that the worm can breathe.
This Christmas Tree Worm is living on the surface of a Star Coral colony. You can see the worm's tube below the radioles. The bumps on the surface of the coral are the coral polyps (say "PAH-lips"). Each polyp is one animal. Since the coral only grows one quarter of an inch each year, the Christmas Tree Worm only has to increase the length of its tube as the coral grows, and soon it will be encased in the coral. When danger threatens, the worm can pull its head down into its tube in the blink of an eye. The worm will have excellent protection when it is completely encased in the coral's skeleton in a few years. We saw many Christmas Tree Worms on the corals at Black Forest Reef and other reefs near Grand Turk.
This close-up photograph was 3 inches from side to side, and the radioles are about 1 inch tall.
Tessa Dowell took this picture using a Nikonos V with 28mm lens, 1:2 extension tube, and SB105 strobe.