Introduction

Riftia pachyptila is a large red Annelid worm species (Rouse, 2001).  One to two meters in length, these tubeworms dominate the fauna of deep sea hydrothermal vent ecosystems (Figure 1).  R.pachyptila lives in dense groups close to the opening of vents called black smokers, exposed to high levels of sulphides which they rely on for life. Although being members of the Polychaete class, R.pachyptila is unusual, due to the fact that it possesses no mouth or digestive system (Corliss et al., 1979).

Colony of R.pachyptila and other fauna at the Galapagos Rift spreading center.  2011 NOAA Galapagos Rift Expedition

Figure 1. Colony of R.pachyptila and other fauna at the Galapagos Rift spreading center. 2011 NOAA Galapagos Rift Expedition

The deep sea environment in which R.pachyptila lives is extreme for a number of reasons.  The two obvious environmental factors any deep dwelling organism must cope with is the immense pressure and lack of light.  Other extreme environmental factors are more specific to the black smokers of hydrothermal vent environments: As R.pachyptila colonises areas of high vent liquid flow (as shown in figure 2), it is exposed to large fluctuations of temperature where dilute vent water between 15-20ºC (Hessler et al., 1988) mixes with ambient seawater temperature of around 2ºC (Johnson et al., 1988).  In dilute vent water, there is also low oxygen availability (oxygen levels reach 0mg/L over 11ºC) and exposure to toxic hydrogen sulphide of concentrations on the order of 1 to 3 millimolar in 15ºC water (Van Dover, 2000).  The chemical reactions that take place when super-heated seawater meets the earth’s crust cause venting fluid to have a high sulphide content as well as a low pH and high concentration of heavy metals such as iron, copper and zinc (Hannington et al., 1995).

Figure 2. A black smoker named Sully in the North East Pacific.  R.pachyptila individuals can be seen living close to vent openings.  Photo from University of Washington

Figure 2. A black smoker named Sully in the North East Pacific. R.pachyptila individuals can be seen living close to vent openings. Photo from University of Washington

Prior to the discovery of deep sea vents and their associated fauna in 1977, it was previously believed all life was derived from photoautotrophic primary producers at the base of food chains.  However large numbers of worms (along with other animals) were found living around hydrothermal vent sites along the Galapagos Rift spreading centre (Jones, 1981).  Subsequently R.pachyptila has been observed to be a common species found at sites along the Galapagos Rift and East Pacific Rise (Coykendall et al., 2011).

One initial theory attempting to address the source of primary productivity at vent sites suggested that the warm water flowing out of hydrothermal vents drive deep water convection currents, carrying photoautotrophic organisms to the bottom of the ocean and therefore providing food for filter feeders (Lonsdale, 1977).

Animals such as R.pachyptila, or the giant clam Calyptogena however, extend down to the bottom most areas of vents and so lateral advection would not provide them with food (Grassle, 1985).  Instead, the currently accepted theory is that chemoautotrophic bacteria oxidise hydrogen sulphide in order to gain energy, forming the base of the food chain (Lonsdale, 1977).

In 1977 when R.pachyptila was first discovered, it was classified in a now redundant phylum, the Pognophora.  Then in 1985 they were reclassified into a new phylum as Vestimentifera (Kojima et al., 1993).  Today however, since molecular evidence has been analysed, R.pachyptila is recognised as a polychaete worm in the phylum Annelida.  Riftia is a monospecific genus i.e. it only has one species; Riftia pacyhptila (Bright and Lallier, 2010).  This species is widely distributed along the Galapogos Rift and East Pacific Rise spreading centres in the eastern pacific ocean (Coykendall et al., 2011).

This blog will focus specifically on the biological adaptions of Riftia pachyptilaR.pachyptila does also exhibit adaptations to the physically harsh environment at vent sites:  In response to the immense pressure and low temperate of the deep ocean, tubeworms are able to increase the relative amount of rigid, double bonded unsaturated fatty acids in their lipid membranes. This prevents the membranes from packing densely together and ultimately solidifying (Cossins and Bowler, 1990).  R.pachyptila also maintains a constant extracellular pH of 7.3/7.4 (Van Dover, 2000) while living in an acidic environment, as low as pH 4.5 (Reysenbach et al., 2006) by means of a proton pumping mechanism (Goffredi et al., 1997).  R.pachyptila is able to excrete toxic heavy metals that are present in venting fluids by confining them in insoluble vesicles and also by binding them to soluble metalloproteins that are triggered when concentrations of metals in the tubeworm’s body are high (Cosson and Vivier, 1997).

The aim of this blog is to describe how Riftia pachyptila is able to live at hydrothermal vent sites which are far too extreme for the majority of marine organisms to survive in.  As the blog develops I will outline the key biological adaptations which allow R.pachyptila to live at vent sites. This will include; how the anatomy of the worm is suited in order to take advantage of its bacterial symbiosis, the role of chemosynthesis and how worms make the most of their symbiotic relationship with prokaryotic bacteria. I will also outline how the reproductive biology of tubeworms allows them to live in an ephemeral habitat.

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