R.pachyptilia (Figure 3) a large vestimentiferan tube worm is one of the largest organisms found at pacific hydrothermal vent system, constituting a large percentage of the cumulative biomass in vent communities (Pond et al 2008). Similar to a number of surface dwelling polychaetes (to which now R.pachyptila is thought to be related (Sandra et al 2009)) the majority of the worm is protected by a robust tube, protecting it from predation (Resenblatt & Cohen 1986) e.g. Zoarcidae species and possibly offering some form of structural support. This species relies on a symbiotic relationship with an endo-chemautotrophic bacteria species, which resided in a specialized organ called a trophosome. A large red flume located at the anterior end of the worm is exposed to the direct flow from the vent. A series of blood vessels, close to the surface of the plume, allow a diffusion gradient of key nutrients. R.pachyptilia Haemoglobin has a high affinity for nutrients such as CO2, O2 and sulphides (Hahlbeck et al 2005). The enriched haemoglobin is then transported towards the trophosome, were the end products (carbohydrates) are produced by the chemoautotrophs.
Due to the large biomass formed by R.pachyptilia it is also logical to think of this species as a major component of the ecosystem. Research has recently been put forward that the Riftia tubes themselves form a biological microhabitat. Offering a substrate for benthic invertebrates to inhabit, much like in kelp communities found in temperate sub-tidal locations(Schaal et al 2009), limpet (Lepetodrilus elevatus) and mobile gastropod (Cyathermia naticoides) species are present feeding on the epifauna of the tubes. The tubes not only act as a hard substrate for the molluscs to occupy but also due to generalized location of the vent worms (within the opening to the vents) their outer surfaces will be rich in the particulate attached bacteria talked about in a previous section, and finally acting as some form of protection from the thermal stresses of living within such a hostile environment. The comparisons between the temperatures of the vent fluid often exceeding (190c) and the ambient temperature of the sea water at such depth (10C) suggests there may be a strong thermal stress for organisms living within such a community. Perhaps the R.pachyptilia tubes themselves offer some form of thermal insulation, acting as a buffer from any extreme changes in thermal conditions, however little research has been initiated in such areas and this is just conjecture.
As with all mega fauna species found at vent system, Juvenile dispersal is a key issue in terms of species success. Due to the extreme specialization of organisms and the sporadic placement of vent systems, largely occurring in areas of tectonic spreading (Tarasov et al 2005), it is extremely important individuals find suitable settlement sites. Due to competition for space on vent sites a wide dispersal method is required to spread offspring. Offspring of R.pachyptilia are most likely passively released into the water column, and the high pressure of the vent itself would be used as a dispersal tool (figure 4). As with most vent organisms juveniles of R.pachyptilia are lecithotrophic (Brooke & Young 2009) meaning lipid rich. High lipid concentration is a known deep sea adaptation for increasing buoyancy. Increased buoyancy is believed to be an aid in transportation of juveniles, giving a greater probability that a small number of them will settle in a suitable location.