Contrary to their name, Christmas tree worms (Spirobranchus) are not corals but fascinating marine annelids that form symbiotic relationships with hard corals. These tube-dwelling polychaete worms, belonging to the phylum Annelida, are widely distributed in tropical oceans, where their vibrant, spiral-shaped structures add a touch of whimsy to coral reefs. Their common name stems from their striking resemblance to miniature, underwater Christmas trees.