Riftia is a monotypic genus of polychaete annelid worms in the Siboglinidae family, which contains the sole species Riftia pachyptila, commonly known as the giant tubeworm or giant beardworm. R. pachyptila is found in association with geologically active regions of the deep sea. Riftia worms are sessile and are found clustered together in large colonies around deep-sea hydrothermal vents in the East Pacific Rise and the Galapagos Rift. The size of a patch of tube worms surrounding a vent can span tens of metres, and individuals can reach a length of 3 m (9 ft 10 in). R. pachyptila has the fastest growth rate of any known marine invertebrate, and have been known to colonize a new site, grow to sexual maturity, and increase in length to 4.9 ft (1.5 m) in less than two years. Adult Riftia worms lack a digestive system and rely on their symbiotic relationship with chemotrophic sulfur-oxidising bacteria to provide them with energy. The hydrothermal vent habitat in which Riftia lives provides a natural ambient temperature ranging from 2–30 degrees Celsius (36–86 °F) and emits large amounts of chemicals such as hydrogen sulfide that are utilised by the tube worm for bacterial chemosynthesis.
Taxonomy R. pachyptila was first encountered in 1977 during an expedition to the hydrothermal vents on the floor of the Galápagos Rift. It is the only species in the genus Riftia. The generic name alludes to the rift that formed the geothermal vents where the species inhabits, while pachyptila (pachy; thick + ptilon; feather) refers to the anterior plume of the worm. The holotype, USNM 59951, is held by the National Museum of Natural History (USNM). Historically, the genus Riftia was placed within the obsolete phyla Pogonophora. They are now understood to be annelids, and have been placed in the family Siboglinidae, with their closest relatives being other deep sea tube worms such as Escarpia and Lamellibrachia.
Description
Anatomy Like other siboglinade tube worms, R. pachyptila has a vermiform body that secretes a chitinous tube for protection and support. Riftia worms can reach a length of 3 m (9 ft 10 in), and their bodies have a diameter of 4 cm (1.6 in).
Riftia have a red feather-like structure called a branchial plume at the anterior end which is used to acquire chemicals for chemosynthesis. The plume is highly vascularised, and its red colour is due to the presence of hemoglobin. The structure of the plume maximises the surface area needed to absorb chemicals from the water efficiently. The red respiratory lamellae of the plume grow out of the obturaculum, a rigid, collagenous support structure that splits and flares out at the tip. If the tubeworm perceives a threat or is touched, it retracts the plume into its tube and seals the tube with the tip of the obturaculum, which functions as a kind of an operculum. Beneath the plume is the vestimentum, a muscular body region that contains the heart, the brain and the two genital openings. The middle region of the body houses the coelomic cavity, gonads, and the trophosome, a spongy tissue where symbiotic bacteria and sulfur granules are found. Adult Riftia have no mouth, digestive system or anus as they do not eat. Instead, they rely on bacteria in their trophosome to provide them with nutrition. The posterior part of the worm is the opisthosome, which anchors the animal to the tube and is used for the storage of waste from bacterial reactions.
Tubes
Riftia's tubes are composed of chitin, secreted from cup-microvilli-like structures within glands which form crystallite chitin layers over time. The tube is cylindrical, flexible, and closed at the posterior end. Riftia tubes can reach 2 m in length and 5 cm in diameter and are very thick compared to those of other tubeworms, especially at the base. The tubes are very resistant to enzymatic attack by bacteria, and can take years to decompose. The worms are able to adjust both the top and the base of their tubes, which allows for some adaptability in the highly competitive and crowded spaces they grow in, as a worm may need to adjust its position to maximise its access to vent fluid as more worms settle in the colony. Riftia tubes grow at a rate of 10 to 85 cm (3.9 to 33.5 in) per year.
Physiology
Metabolism
Deep-sea species which do not inhabit hydrothermal vent sites typically have low metabolic rates. In contrast, the enzyme activity relating to glycolysis, the citric acid cycle and electron transport in R. pachyptila is similar to that of shallow-living animals, suggesting a significantly faster metabolism than is typical of deep sea animals. Despite their toxicity, nitrate and nitrites are required for Riftia's biosynthetic processes: the chemosynthetic bacteria within the trophosome convert nitrate to ammonium ions, which are then converted to amino acids utilized by the tubeworms. To transport nitrate to the bacteria, R. pachyptila concentrates nitrate in its blood to a concentration 100 times higher than the surrounding seawater; the exact mechanism used to withstand and concentrate nitrate to this degree is still unknown. Hydrothermal vents create low-oxygen conditions (hypoxia). In hypoxic conditions, sulfur-storing organisms start producing hydrogen sulfide (H2S). In R. pachyptila, the production of H2S starts after 24 hours of hypoxia. H2S can be damaging for some physiological processes as it inhibits the activity of cytochrome c oxidase, impairing oxidative phosphorylation. Riftia pachyptila are able to bind H2S to haemoglobin in its blood to expel it in the surrounding environment, in order to avoid physiological damage. Additionally, Riftia hemoglobins are atypical in that they are able to carry oxygen in the presence of sulfide without being inhibited by it.
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