Spirobranchus worms and their host corals are a beautiful addition to a reef aquarium if the hobbyist is able to provide proper husbandry to keep them thriving.
Initially, the worm was thought to obtain a permanent shelter from predation at a minor expense to the coral. However, experimental work has shown that the cost to the coral is minimal and that the worms can provide two quite significant benefits to the coral: First, the top of the calcareous operculum is secreted in the form of multiple sharp rays.
Symbiotic relationships are one of the most interesting parts of this hobby. A coral reef is a battlefield for real estate, however there are still these pockets of cooperation between completely different animals that is amazing.