Giant Christmas Tree Worm, Spirobranchus giganteus
Giant Christmas Tree Worm, Spirobranchus giganteus. Underwater photographs taken in Zihuatanejo Bay, Guerrero, March 2018. Photographs courtesy of Ron Woheau, Zihuatanejo.
Giant Christmas Tree Worm, Spirobranchus giganteus. Underwater photograph taken in coastal waters off Bonaire, December 2019. Photograph and identification courtesy of Bob Hillis, Ivins, Utah.
Phylogeny: Giant Christmas Tree Worms, Spirobranchus giganteus (Pallas, 1766), like their less-attractive cousins the Earthworms, are in the phylum Annelida of segmented worms. They are in the Serpulidae Family of Christmas Tree Worms. The Spirobranchus genus is one of seventy-one genera in the Serpulidae Family, and there are forty-one species in the Spirobranchus genus. They are also known as the Caribbean Christmas Tree Worm, the Giant Spiraled Polychaete, and the Giant Spiral-gilled Fan Worm and in Mexico as Gusano de Arbol Navidad. The genus name Spirobranchus comes from the Greek words meaning “spiral gills” or “spiral branches”, referring to the radioles (branch-like structures).
Morphology: Giant Christmas Tree Worms inhabit calcareous tubes, made from a mix of calcium carbonate and mucopolysaccharide. The calcium carbonate is secreted by a calcium gland, located at the collar of the animal. Giant Christmas Tree Worms are recognized by a pair of cone-shaped tentacular crowns made up of feather-like radioles (branches). The radioles are arranged in 6 to 8 layers of whorls. One radiole in the structure is modified to serve as an operculum (trapdoor) that closes when the animal withdraws into its tube. The radioles attach to a lobe surrounding the mouth. A very close look will also reveal one to four, antler-like, spines around the aperture of the tube. The crowns may be blue, white, or reddish-orange in color. The soft body of these animals may have over 100 segments. The crowns only extend 1.0 cm (0.5 Inch) or so above the reef, but the animal averages around 3.8 cm (1.5 inches) in length. The similar looking Spirobranchus spinosus of the Eastern Pacific only has three layers of whorls. Christmas Tree Worms tubes that are no longer occupied by the worm may break loose and appear to be sea shells of the Vermetidae Family. The two can be distinguished by Worm tubes having a dull interior finish while Vermetids have a glossy interior finish.
Habitat and Distribution: Giant Christmas Tree Worms grow on rocks and living corals, with the coral eventually overgrowing the tube. This species is found subtidally to a depth of 60 m (197 feet). Giant Christmas Tree Worms are circumtropical in distribution. In Mexican waters they are found in coastal waters along both the Atlantic and Pacific Oceans. In the Atlantic they range from Tampico, Tamaulipas to Belize. They are found in all Mexican waters of the Pacific Ocean with the exception that they are absent from the West Coast of the Baja Peninsula and in the northern portions of the Sea of Cortez.
Diet: Giant Christmas Tree Worms are sessile suspension feeders, and they lack a buccal organ. The buccal organ consists of muscles and glands in the pharynx or proboscis. They are unable to reach out with mouth parts to grab food. Instead, they use their tentacles (radioles) to grab plankton from the water column, and then pass the food to their mouth. They eat phytoplankton, zooplankton, and detritus. Once caught, the plankton are swept down a groove, by currents generated by cilia, to the mouth.
Predators: Giant Christmas Tree Worms are consumed by crabs, fish, gastropods and starfish. They have the ability to quickly retreat into their tubes when they sense changes in water movement or light that they associate with a predator.
Reproduction: Giant Christmas Tree Worms are gonochoric (male or female for life). They can reproduce asexually by paratomy (budding from the posterior end), or sexually. Sexual reproduction is through broadcast spawning with external fertilization. The fertilized eggs turn into planktonic larvae within about twenty-four hours. The larvae settle on to hard substrates and begin tube building. From this point forward, they are confined to their tube. This species is known to live for over thirty years.
Ecosystem Interactions: Giant Christmas Tree Worms can be important contributors of calcium to coral reef building. In some areas other species in this genus seem to have a mutualistic relationship with corals of the genus Porites. The coral provides the worm with an attachment structure that is exposed to plankton rich currents. In return, the sharp spines of the tube protect the coral from predation by sea stars. It is thought that the spines irritate the starfish’s tube feet and stomach. Also, coral bleaching and algae growth seem to be reduced in areas near Christmas Tree Worms. Perhaps the movement of the radioles circulates water over the polyps, keeping them cooler. Some studies have found that Giant Christmas Tree Worms cause more damage to individual coral heads than benefit. Lastly, some cleaner gobies, such as the Yellowhead Goby Elactinus randalli, parasitize Christmas Tree Worms by stealing food from the open radioles.
Human Interactions: Giant Christmas Tree Worms can destructive by fouling boat hulls and pipelines. Otherwise, they have very limited impact on human activities. From a conservation perspective, Giant Christmas Tree Worms have not been evaluated though they are common with a very wide range and should be considered of Least Concern.
Synonyms: Cymospira bicornis, Cymospira cervina, Cymospira gigantea, Cymospira megasoma, Cymospira rubus, Olga elegantissima, Penicillum marinum, Pomatoceros oerstedi, Serpula (Cymospira) gigantea, Serpula (Galeolaria) gigantea, Serpula bicornis, Serpula gigantea, Spirobranchus (Cymospira) giganteus, Spirobranchus giganteus giganteus, Spirobranchus giganteus microceras, Spirobranchus giganteus tricornis, Spirobranchus megasoma, Spirobranchus tricornis, and Terebella bicornis.