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Brief Summary

Riftia pachyptila is a giant tube-dwelling annelid in the family Siboglinidae. Siboglinids are important members of deep-sea chemosynthetic communities, which include hydrothermal vents, cold seeps, whale falls, and reduced sediments. As adults, these worms lack a functional digestive system and rely on microbial endosymbionts for their energetic needs. (Hilário et al. 2011)

Riftia pachyptila was discovered on hydrothermal vents at the Galapagos Rift in 1977. It is now known to be a widely distributed inhabitant of vents along the East Pacific Rise and Galápagos Rift (Coykendall et al. 2011). Larvae are estimated to disperse more than 100 km over a 5-week period (Marsh et al. 2001). Studies of vent community succession have shown that R. pachyptila is among the first species to colonize a new vent once suitable conditions are established. Within two years its numbers can grow to several thousand adult individuals, but changes in vent flow or overgrowth by mytilid mussels can lead to its replacement as the dominant species. This process can take months to years, depending on the location. (Coykendall et al. 2011 and references therein)

Coykendall et al. (2011) studied genetic variation among R. pachyptila populations at one mitochondrial and three nuclear loci. They found low rates of genetic variation, especially in southern populations, which exhibit lower occupancy (i.e., percentage of active vents occupied) than do more northern populations. They suggested that the observed geographic pattern of genetic variation is likely explained at least in part by geographic variation in rates of local extinction and (re)colonization. In the Eastern Pacific in general, vent habitats are highly ephemeral, persisting for a few years to several decades before fluid conduits are blocked, magma supplies shift, or lava flows extirpate local communities. On the source side, earthquakes can open fluid conduits, re-activating old vents, and magmatic eruptions spawn new vents.

Hilário et al. (2011) wrote of Riftia pachyptila: “[Riftia pachyptila] became the poster-child of deep-sea discovery, the ‘lost world’ of unknown animal lineages that scientists on the Challenger deep-sea expedition 100 years previously had so wanted, but failed, to find. Arguably, this single species of worm launched the careers of a generation of deep-sea biologists.” At one time, R. pachyptila was placed in its own phylum, the Vestimentifera, although this status was short-lived as a result of new phylogenetic investigations (for review, see Pleijel et al. 2009 and Hilário et al. 2011).

Like other siboglinids, adult R. pachyptila lack a gut, mouth, anus, and conventional feeding ability and possesses bacterial symbionts. (Hilário et al. 2011). Adult R. pachyptila are nourished entirely by sulfur-oxidizing endosymbiotic bacteria (Coykendall et al. 2011). Although the larvae of R. pachytila are symbiont-free and possess a transient digestive system, these digestive structures are lost during development, resulting in adult animals that are nutritionally dependent on their bacterial symbionts. Thus, each generation of tubeworms must be newly colonized with appropriate symbionts. (Nussbaumer et al. 2006)

In the deep sea, aggregations of vestimentiferan tubeworms at hydrothermal vents and hydrocarbon seeps host diverse assemblages of smaller invertebrates. At deep sea hydrothermal vents in the eastern Pacific, R. pachyptila form large and dense aggregations in a spatially and temporally variable environment. The density and diversity of smaller invertebrates is higher in association with aggregations of R. pachyptila than on the surrounding basalt rock seafloor. (Govenar and Fisher 2007 and references therein)

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