Spirobranchus giganteus, also known as Christmas Tree Worms, is a species of polychaete worms that inhabits reefs in warm waters of the Caribbean. The worms live on multiple species of coral, where they build calcareous tubes for protection and predator avoidance from lobsters, fish, sea urchins, and sea stars. The species S. giganteus occurs in a variety of colors, including red, violet, orange, brown, and yellow. S. giganteus has free swimming larvae, and larval settlement patterns are poorly understood. We predicted that S. giganteus would prefer coral host species similar in color to themselves for camouflage. We conducted a study off the coast of Calabash Caye, Belize during May 2016. Our group established five randomly selected 9m2 quadrats and recorded the abundance and color of S. giganteus and the color and species of host corals. The majority of S. giganteus observed were orange. Orange and brown worms primarily occurred on orange crenellated fire coral (Millepora alcicornis) and yellow maze coral (Meandrina meandrites). Most red worms occurred on boulder coral (Colpophyllia natans), which is yellow, and most violet worms occurred on fire coral, which is orange. Our hypothesis that S. giganteus would select coral hosts the same color as themselves for camouflage from predators was not supported. Most S. giganteus were located on fire coral hosts, which release toxins from cnidocytes for defense. We suggest that S. giganteus benefits from living on fire coral, which is detrimental to most species of S. giganteus predators.
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