Spirobranchus giganteus

Christmas tree worm
File:6798 aquaimages.jpg
Spirobranchus giganteus embedded in a brain coral in a reef in Bonaire.
Conservation status
Least Concern  (IUCN 3.1)
Scientific classification
Kingdom:	Animalia
Phylum:	Annelida
Class:	Polychaeta
Order:	Canalipalpata
Family:	Serpulidae
Genus:	Spirobranchus
Species:	S. giganteus
Binomial name
Spirobranchus giganteus Pallas, 1766

Spirobranchus giganteus, commonly known as christmas tree worms are small, tube-building polychaete worms belonging to the family Serpulidae.

Anatomy and morphology

The worm is aptly named; Both its common and latin names refer to the two, chromatically-hued spiral structures that are most commonly what is seen of the worm by divers. In actuality, these multicolored spirals are merely the worm's highly-derived respiratory structures.

S. giganteus appears like most tube-building polychaetes. It has a tubular, segmented body lined with chaeta, small appendages that aids the worm with its mobility. As it does not move outside its tube, this worm does not have any specialized appendages for movement or swimming.

The worms's most distinct features are the two "crowns" that are shaped like Christmas-trees. These "crowns" are actually highly modified prostomial palps which are specialized mouth appendages of the worm. Each spiral is actually composed of feather-like tentacles called radioles, which are heavily ciliated which allows any prey that are trapped in them to be transported straight towards the worm's mouth. While they are primarily feeding structures, S. giganteus also uses its radioles for respiration. It is because of this that the structures are commonly called "gills".

One of the major differences between christmas-tree worms and the closely-related sabellid fan worms is that the latter do not have any specialized body structures to plug the holes of their tubes with when they withdraw into them. S. giganteus, like the other members of its family possess a modified radiole, usually called the operculum, that it uses to secure its hole when withdrawn into its tube.

As an annelid, S. giganteus possesses a complete digestive system. It has a well-developed closed circulatory system. Like other annelids, these worms possess well-developed nervous systems with a central brain and many supporting ganglia, including pedal ganglia, which are unique to the Polychaeta. Like other polychaetes, S. giganteus excrete with fully-developed nephridia. When they reproduce, they simply shed their gametes straight into the water where the eggs (and spermatozoa) will become part of the zooplankton and can be carried by the currents.

Range and distribution

Christmas-tree worms are widely distributed throughout the world's tropical oceans. They have been known to occur from the Caribbean to the Indo-Pacific.

Ecology

Spirobranchus giganteus is commonly found embedded in entire heads of massive corals. Like members of its family, it can secretes a calcareous tube around its body. This tube serves as the worm's home and protection. S. giganteus usually bores a hole into an existing head of living coral before secreting its tube, thereby increasing its level of protection.

As sedentary inhabitants of coral reefs, christmas tree worms feed primarily by filter feeding. They use their brightly-colored radioles to filter microorganisms from the water, which are then deposited straight into the worm's digestive tract.

Few organisms are known to feed on tube-borne polychaetes and S. giganteus is no exception.

Importance to humans

While the worm itself has no commercial fishery importance, it is of interest to marine aquarists and divers everywhere. The variously-colored worm crowns make extremely popular underwater photographic subjects for sport divers. Many aquarists that have miniature reef aquariums purposely include heads of coral that S. giganteus specimens inhabit. The worms make a very good addition to the aesthetics of a marine aquarium.

Conservation status

As the species is widespread and relatively common, there are no conservation efforts focusing on this species (or Polychaetes in general).

Etymology and taxonomy

Spirobranchus essentially translates to "spiral gills", referring to the worm's unique crown.

There are two subspecies recognized by the ITIS, S. gigantea corniculatus and S. gigantea gigantea.

Bibliography

• "Spirobranchus giganteus". Integrated Taxonomic Information System. 24 January. {{cite web}}: Check date values in: |date= and |year= / |date= mismatch (help)

• "Spirobranchus giganteus, Christmas Tree Worm - MarineBio.org". Retrieved January 24. {{cite web}}: Check date values in: |accessdate= (help); Unknown parameter |accessyear= ignored (|access-date= suggested) (help)

• Ruppert, Edward E. (2004). Invertebrate Zoology: A Functional Evolutionary Approach, (7th edition ed.). Belmont, CA: Brooks/Cole - Thomson Learning Inc. pp. pp. 963. ISBN 0-03-025982-7. {{cite book}}: |edition= has extra text (help); |pages= has extra text (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)

References

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  Christmas Tree Worm South China Sea
• 
  Christmas Tree Worm South China Sea
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  Christmas Tree Worm Papua New Guinea
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  Christmas tree worms (Spirobranchus giganteus) from East Timor.