Riftia pachyptila (Jones 1981)

Feeding Strategy: Symbiosis
Hydrothermal vents are driven by chemoautotrophic bacterium which have the sulphur-oxidising capabilities needed to convert the high-sulphide content of these environments to energy (Zierenberg et al., 2000; van Dover et al., 2002). R. pachyptila have evolved to utilise these bacterium by becoming a host and living symbiotically with them (Cavanaugh et al., 1981). To house the symbiont, the tubeworm has developed a highly-vascularised – surrounded by lots of blood vessels – organ, called the trophosome. The trophosom takes up approximately 50% of its body (Cavanaugh et al., 1981), a the bacteria are densely packed inside the organ (Minic and Herve, 2004). The worm assimilates inorganic metabolites through its branchial plume at the top of its body (Fig. 1), including O2, NH3, and H2S (Minic and Herve, 2004).

Giant tube worm with Tevnia jerichonana in background
Figure 1: The bright red branchial plume of Riftia pachyptilla that makes this species so identifiable is the exchange site  for metabolites, such as hydrogen sulphide.

The worm has adapted so that it possesses three types of extracellular haemoglobin – found in both the vascular blood and coelomic fluid – which can attach to both O2 and H2S (Zal et al., 1996; Zal et al., 1998), thus enabling R. pachyptila to transport the hydrogen sulphide from the plume to the bacteria in the trophosome. Binding to haemoglobin also helps control the toxicity of the hydrogen sulphide on the worm whilst the presence of cysteine residues help maintain the detoxification of haemoglobin by the H2S (Zierenberg et al., 2000). Once the sulphide reaches the trophosome, the bacteria convert it to metabolic energy that both the bacteria and the worm can utilise (Cavanaugh et al., 1981).

As a result of the adaptation to this energy source, R. pachyptila lacks a digestive tract completely (Minic and Herve, 2004).

Glossary
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References
Batson, P. (2014) Riftia pachyptila (Figure 1). Available: http://www.arkive.org/giant-tube-worm/riftia-pachyptila/image-G78006.html [09/12/2014, 2014].

Cavanaugh, C., Gardiner, S., Jones, M., Jannasch, H. & Waterbury, J. (1981) Prokaryotic cells in the hydrothermal vent tube worm Riftia pachyptila jones – possible chemoautotrophic symbionts. Science, 213, 340-342.

Minic, Z. & Herve, G. (2004) Biochemical and enzymological aspects of the symbiosis between the deep-sea tubeworm Riftia pachyptila and its bacterial endosymbiont. European Journal of Biochemistry, 271, 3093-3102.

Van Dover, C., German, C., Speer, K., Parson, L. & Vrijenhoek, R. (2002) Marine biology – evolution and biogeography of deep-sea vent and seep invertebrates. Science, 295, 1253-1257.

Zal, F., Lallier, F., Wall, J., Vinogradov, S. & Toulmond, A. (1996) The multi-hemoglobin system of the hydrothermal vent tube worm Riftia pachyptila .1. reexamination of the number and masses of its constituents. Journal of Biological Chemistry, 271, 8869-8874.

Zal, F., Leize, E., Lallier, F., Toulmond, A., Van Dorsselaer, A. & Childress, J. (1998) S-sulfohemoglobin and disulfide exchange: The mechanisms of sulfide binding by Riftia pachyptila hemoglobins. Proceedings of the National Academy of Sciences of the United States of America, 95, 8997-9002.

Zierenberg, R., Adams, M. & Arp, A. (2000) Life in extreme environments: Hydrothermal vents. Proceedings of the National Academy of Sciences of the United States of America, 97, 12961-12962.

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