Cyclicity

Christmas tree worms are heterotrophic and with an obligate relationship with species of coral.  These organisms have a indirect biphasic lifecycle. This means that there are two phases that complete the lifecycle.

Larval stage

The first phase is the planktonic larval phase, which can last for hours to weeks depending on the environmental cues required (Conlin 1989; Qian 1999). Planktonic means that the larvae are able to feed on plankton during its larval phase in order to survive (Rupert E. E. 2004).

It can consist of some or all of these different stages of growth (trochophore-metatrochophore-setigerous) until it becomes ready to settle (Marsden 1987). 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 (Qian 1999). The factors which the larvae control in terms of settlement is habitat selection and selective mortality (Conlin 1989). Successful recruitment to appropriate sites is dependent on the numbers of competent larvae transferred to the site. This is affected by frequency of adults in a population, their reproductive cycles and output, and hydrodynamics of the environment (Connell, 1985; Roughgarden et al., 1988; Thorson, 1950; Butman, 1987).

Habitat selection is a decision made by the larvae in response to specific cues in the environment. The chemical settlement cues that can impact whether larvae will settle at a particular site include: juvenile hormones; free fatty acids; certain polysaccharides, proteins, inorganic ions or amino acids; and neurotransmitters. Physical factors including hydrology of the area, phototactic and geotactic organs, settlement site availability and the thickness of the deposit on these sites also influence the larvae recruitment (Marsden 1984; Qian 1999). These factors usually distinguish the different sites in which the larval worm settles. The health of the larvae will determine its ability to respond to these cues when required, with weaker larva having a reduced chance to settle and metamorphose successfully (Qian 1999).

Adult phase

Once the planktonic larva has settled onto site, it undergoes a drastic morphological and physiological change called metamorphosis to become a benthic juvenile (Fenaux & Pedrotti,

1988 quoted in (Qian 1999). 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 (Nishi 1999). 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 (Smith 1984; Nishi 1999). 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.  

Reproduction

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) (Rupert E. E. 2004). 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 (Rowley 2008). Successful fertilization produces an embryo, which over the course of approximately 24 hours, develops into a larvae (Cronin 2002).