Twin tapering (narrower at the top) spiral gill crowns in various colors, small operculum in between (usually pinkish) that seals off the tube when retracted. The tube is rarely visible since Christmas tree worms are embedded in live coral tissue.

Unassessed by the IUCN Red list, but a circumglobal genus that is found all around the equator, in tropical and subtropical water.

Red Sea; West Indian Ocean; Mediterranean Sea; Gulf of Mexico; Caribbean Sea and the Pacific Ocean. The grow on reefs, pref. with Acropora coral, especially on the reef crest (or algal ridge) in shallow water. The depth range however is quite large, from 0-60 meters.

Spirobranchus sp. are sexually dimorphic – having separate male and female morphologies with their own unique reproductive organs. Once mature, they reproduce by a mechanism called broadcast spawning. This involves both the male and female simultaneously releasing their gametes into the external environment water column to give the sperm (of the male) an opportunity to fertilize the eggs (of the female). The adult form has no further impact on the gametes once they are released. To increase the chances of fertilization success, adult Christmas tree worms tend to aggregate gregariously and spawn in synchrony with their co-specifics and in changing tides. Successful fertilization produces an embryo, which over the course of approximately 24 hours, develops into a larvae.

The life cycle consists of 2 phases. The first phase is the planktonic larval phase, which can last for hours to weeks depending on the environmental cues. It can consist of some or all of these different stages of growth (trochophore-metatrochophore-setigerous) until it becomes ready to settle. This phase is the dispersive period of the Christmas tree worm and is very important in terms of determining where the adult will reside for its entire life. Settlement is the process in which larvae chooses its substrate to begin its benthic life. Habitat selection is a decision made by the larvae in response to specific cues in the environment. Once the planktonic larva has settled onto site, it undergoes a drastic morphological and physiological change called metamorphosis to become a benthic juvenile. The second phase is the adult one. As a temporary dwelling, the juvenile produces a mucus tube in which the foundation of the carbonate tube can be developed. This juvenile takes on the same form as that of the adult, which continues to grow larger throughout its lifetime as it matures. The adult lifespan can extend to up to 30 years! Christmas tree worms build calcareous tubes up to 20cm long which become embedded into the coral skeleton by extending their tube towards the living tissue. Many people misinterpreted this behavior as a boring process. It obtains this carbonate which lines the tube by ingesting sand particles and other calcium derived particles and processing them in a specialized gland to purify them. The purified calcium carbonate would then be excreted from the body of the worm to develop the tube. Sometimes the worm does not extend the entire length of the tube. This is most likely a defensive mechanism, so in case damage to the host occurs, the worm is able to retract further into its burrow to ensure survival.