Siboglinidae

Siboglinidae is a family of polychaete annelid worms whose members made up the former phyla Pogonophora and Vestimentifera (the giant tube worms).[1][2] They are composed of about 100 species of vermiform creatures and live in thin tubes buried in sediments (Pogonophora) or in tubes attached to hard substratum (Vestimentifera) at ocean depths from 100 to 10,000 m (300 to 32,800 ft). They can also be found in association with hydrothermal vents, methane seeps, sunken plant material, or whale carcasses.

Siboglinidae
Temporal range: 189.6–Recent Ma
Riftia pachyptila
Scientific classification
Kingdom: Animalia
Phylum: Annelida
Class: Polychaeta
Infraclass: Canalipalpata
Family: Siboglinidae
Caullery, 1914
Genera

See text.

The first specimen was dredged from the waters of what is now Indonesia in 1900. These specimens were given to French zoologist Maurice Caullery, who studied them for nearly 50 years.

Anatomy

Most siboglinids are less than 1 millimetre (0.04 in) in diameter, but 10–75 centimetres (3.9–29.5 in) in length. They inhabit tubular structures composed of chitin and fixed to the bottom. The tubes are often clustered together in large colonies.[3]

Their bodies are divided into four regions. The anterior end is called the cephalic lobe, which bears from one to over 200 thin branchial ciliated tentacles, each bearing tiny side branches known as pinnules. Behind this is a glandular forepart, which helps to secrete the tube. The main part of the body is the trunk, which is greatly elongated and bears various annuli, papillae, and ciliary tracts. Posterior to the trunk is the short metamerically segmented opisthosoma, bearing external paired chaetae, which apparently help to anchor the animal to the base of its tube.[3]

The body cavity has a separate compartment in each of the first three regions of the body, and extends into the tentacles. The opisthoma has a coelomic chamber in each of its five to 23 segments, separated by septa. The worms have a complex closed circulatory system and a well-developed nervous system, but as adults, siboglinids completely lack a mouth, gut, and anus.[4]

Siboglinids are dioecious, with one gonad on each side of the trunk, within the body cavity. The fertilised eggs develop within the tubes, and hatch to produce small, ciliated, worm-like larvae.[3]

Evolution

The family Siboglinidae has been difficult to place in an evolutionary context.[5] By examining genetic differences between annelids, a scientific consensus now exists that the Siboglinidae are a family within the order Polychaeta.[6] The fossil record along with molecular clocks suggest the family has Mesozoic (250 – 66 Mya) or Cenozoic (66 Mya – recent) origins.[5] However, some fossils of crystallized tubes are attributed to early Siboglinidae dating back to 500 Mya.[5] Examination of molecular work aligning five genes has made clear that four distinct clades exist within the Siboglinidae.[7][8][9] The clades are vestimentiferans, Sclerolinum, frenulates, and Osedax.[8] These clades represent the four tracks evolution has followed within the Siboglinidae.[8] Vestimentiferans live in vent and seep habitats.[8] Separation of vestimentiferans into seep- and deep-sea-dwelling clades is still debated due to some phylogenies based on sequencing data placing the genera along a continuum.[10] Sclerolinum is a monogeneric clade living on organic-rich remains.[5] Frenulates live in organic-rich sediment habitats.[11] Osedax is a monogeneric clade specialized in living on whale bones, although recently, evidence shows them living on fish bones, as well.[12]

Vestimentiferans

Like other tube worms, vestimentiferans are marine and benthic. Riftia pachyptila, a vestimentiferan, is known only from the hydrothermal vent systems.[5] Their bodies are divided into four regions; the obturaculum, vestimentum, trunk, and opisthosome. The obturaculum is the first anterior body part. The main trunk of the body bears wing-like extensions, the vestimentum, from which their name is derived. Also, unlike other siboglinids that never have a digestive tract, they have one that they completely lose during metamorphosis.

Their primary nutrition is derived from the sulfide-rich fluids emanating from the hydrothermal vents where they live. The sulfides are metabolized by symbiotic hydrogen sulfide- or methane-oxidizing bacteria living in an internal organ, the trophosome. One gram of trophosome tissue can contain one billion bacteria. How the worms instigate their relationship with the bacteria is not completely known. The bacteria appear to colonize the host animal larvae after they have settled on a surface, entering them through their skin.[13] This method of entry, known as horizontal transmission, means that each organism may have different species of bacteria assisting in this symbiosis. However, these bacteria all play similar roles in sustaining the vestimentiferans. Endosymbionts have a wide variety of metabolic genes, which may allow them to switch between autotrophic and heterotrophic methods of nutrient acquisition.[14] When the host dies, the bacteria are released and return to the free-living population in the seawater.[15]

Discovery of the hydrothermal vents in the eastern Pacific Ocean was quickly followed by the discovery and description of new vestimentiferan tubeworm species. These tubeworms are one of the most dominant organisms associated with the hydrothermal vents in the Pacific Ocean. Tubeworms anchor themselves to the substratum of the hydrocarbon seep by roots located at the basal portion of their bodies.[16] Intact tubeworm roots have proven very difficult to obtain for study because they are extremely delicate, and often break off when a tubeworm is removed from hypothermal vent regions. How long the roots of the tube worms can grow is unknown, but roots have been recovered longer than 30 m.

A single aggregation of tubeworms can contain thousands of individuals, and the roots produced by each tubeworm can become tangled with the roots of neighbouring tubeworms.[17] These mats of roots are known as "ropes", and travel down the tubes of dead tubeworms, and run through holes in rocks. The diameter and wall thickness of the tubeworm roots do not appear to change with distance from the trunk portion of the tubeworm's body.

Like the trunk portion of the body, the roots of the vestimentiferan tubeworms are composed of chitin crystallites, which support and protect the tubeworm from predation and environmental stresses. These tubeworms build the external chitin structure themselves by secreting chitin from specialized glands located in their body walls.

Genera
  • Osedax
  • Frenulata[2]
    • Birsteinia
    • Bobmarleya[18]
    • Choanophorus
    • Crassibrachia
    • Cyclobrachia
    • Diplobrachia
    • Galathealinum
    • Heptobrachia
    • Lamellisabella
    • Nereilinum
    • Oligobrachia
    • Paraescarpia[19]
    • Polybrachia
    • Siboglinoides
    • Siboglinum
    • Siphonobrachia
    • Spirobrachia
    • Unibrachium
    • Volvobrachia[20]
    • Zenkevitchiana
  • Sclerolinum[2]
  • Vestimentifera[2]

References
  1. Kojima, S.; Hashimoto, T.; Hasegawa, M.; Murata, S.; Ohta, S.; Seki, H.; Okada, N. (July 1993). "Close phylogenetic relationship between vestimentifera (tube worms) and annelida revealed by the amino acid sequence of elongation factor-lα". Journal of Molecular Evolution. 37 (1): 66–70. doi:10.1007/BF00170463. PMID 8360920.
  2. Rouse, G. W. (2001). "A cladistic analysis of Siboglinidae Caullery, 1914 (Polychaeta, Annelida): formerly the phyla Pogonophora and Vestimentifera". Zoological Journal of the Linnean Society. 132 (1): 55–80. doi:10.1006/zjls.2000.0263.
  3. Barnes, Robert D. (1982). Invertebrate Zoology. Philadelphia, PA: Holt-Saunders International. pp. 862–863. ISBN 0-03-056747-5.
  4. Ross Piper (30 August 2007). Extraordinary animals: an encyclopedia of curious and unusual animals. Greenwood Publishing Group. ISBN 978-0-313-33922-6. Retrieved 3 December 2011.
  5. Hilário, Ana; Capa, María; Dahlgren, Thomas G.; Halanych, Kenneth M.; Little, Crispin T. S.; Thornhill, Daniel J.; Verna, Caroline; Glover, Adrian G. (2011). Laudet, Vincent (ed.). "New Perspectives on the Ecology and Evolution of Siboglinid Tubeworms". PLoS ONE. 6 (2): e16309. doi:10.1371/journal.pone.0016309. PMC 3038861. PMID 21339826.
  6. Struck, T. H.; Schult, N.; Kusen, T.; Hickman, E.; Bleidorn, C.; McHugh, D.; Halanych, K. M. (2007). "Annelid phylogeny and the status of Sipuncula and Echiura". BMC Evolutionary Biology. 7: 57. doi:10.1186/1471-2148-7-57. PMC 1855331. PMID 17411434.
  7. Black, M. B.; Halanych, K. M.; Maas, P. A. Y.; Hoeh, W. R.; Hashimoto, J.; Desbruyeres, D.; Lutz, R. A.; et al. (1997). "Molecular systematics of vestimentiferan tubeworms from hydrothermal vents and cold-water seeps". Marine Biology. 130 (2): 141–149. doi:10.1007/s002270050233.
  8. Glover, A. G.; Kallstrom, B.; Smith, C. R.; Dahlgren, T. G. (2005). "World-wide whale worms? A new species of Osedax from the shallow north Atlantic". Proceedings of the Royal Society B: Biological Sciences. 272 (1581): 2587–2592. doi:10.1098/rspb.2005.3275. PMC 1559975. PMID 16321780.
  9. Vrijenhoek, R. C.; Johnson, S. B.; Rouse, G. W. (2009). "A remarkable diversity of bone-eating worms (Osedax; Siboglinidae; Annelida)". BMC Biology. 7: 74. doi:10.1186/1741-7007-7-74. PMC 2780999. PMID 19903327.
  10. Bright, Monika; Lallie, François (2010-05-12), Gibson, R; Atkinson, R; Gordon, J (eds.), "The Biology of Vestimentiferan Tubeworms", Oceanography and Marine Biology, CRC Press, 20103650, pp. 213–265, doi:10.1201/ebk1439821169-c4, ISBN 9781439821169
  11. Rodrigues, C. F.; Hilário, A.; Cunha, M. R.; Weightman, A. J.; Webster, G. (2011). "Microbial diversity in Frenulata (Siboglinidae, Polychaeta) species from mud volcanoes in the Gulf of Cadiz (NE Atlantic)". Antonie van Leeuwenhoek. 100 (1): 83–98. doi:10.1007/s10482-011-9567-0. PMID 21359663.
  12. Rouse, G. W.; Goffredi, S. K.; Johnson, S. B.; Vrijenhoek, R. C. (2011). "Not whale-fall specialists, Osedax worms also consume fishbones". Biology Letters. 7 (5): 736–739. doi:10.1098/rsbl.2011.0202. PMC 3169056. PMID 21490008.
  13. Nussbaumer, Andrea D.; Fisher, Charles R.; Bright, Monika (2006-05-18). "Horizontal endosymbiont transmission in hydrothermal vent tubeworms". Nature. 441 (7091): 345–348. doi:10.1038/nature04793. ISSN 1476-4687. PMID 16710420.
  14. Reveillaud, Julie; Anderson, Rika; Reves-Sohn, Sintra; Cavanaugh, Colleen; Huber, Julie A. (2018-01-27). "Metagenomic investigation of vestimentiferan tubeworm endosymbionts from Mid-Cayman Rise reveals new insights into metabolism and diversity". Microbiome. 6 (1): 19. doi:10.1186/s40168-018-0411-x. ISSN 2049-2618. PMC 5787263. PMID 29374496.
  15. Klose, Julia; Polz, Martin F.; Wagner, Michael; Schimak, Mario P.; Gollner, Sabine; Bright, Monika (2015-09-08). "Endosymbionts escape dead hydrothermal vent tubeworms to enrich the free-living population". Proceedings of the National Academy of Sciences of the United States of America. 112 (36): 11300–11305. doi:10.1073/pnas.1501160112. ISSN 1091-6490. PMC 4568656. PMID 26283348.
  16. Halanych, K.; Maas, P.; Hoeh, W.; Hashimoto, J.; Desbruyeres, D.; Lutz, R.; Vrijenhoek, R. (1997). "Molecular systematics of vestimentiferan tubeworms from hydrothermal vents and cold-water seeps". Marine Biology. 130 (2): 141–149. doi:10.1007/s002270050233.
  17. Julian, D.; Gaill, F.; Wood, E.; Arp, A.; Fisher, C. (1999). "Roots as a site of hydrogen sulphide uptake in the hydrocarbon seep vestimentiferan Lamellibrachia sp". The Journal of Experimental Biology. 202 (Pt 17): 2245–57. PMID 10441078.
  18. Hilário, A.; Cunha, M. R. (2008). "On some frenulate species (Annelida: Polychaeta: Siboglinidae) from mud volcanoes in the Gulf of Cadiz (NE Atlantic)". Scientia Marina. 72 (2): 361–371. doi:10.3989/scimar.2008.72n2361.
  19. Southward, E. C.; Schulze, A.; Tunnicliffe, V. (2002). "Vestimentiferans (Pogonophora) in the Pacific and Indian Oceans: a new genus from Lihir Island (Papua New Guinea) and the Java Trench, with the first report of Arcovestia ivanovi from the North Fiji Basin". Journal of Natural History. 36 (10): 1179–1197. doi:10.1080/00222930110040402.
  20. Smirnov, R. V. (2000). "A redescription of Spirobrachia leospira Gureeva (Pogonophora), with the erection of a new genus and a revision of the Spirobrachiidae". Ophelia. 53 (2): 151–158. doi:10.1080/00785236.2000.10409445.
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