Siboglinidae

Evolution

The family Siboglinidae has been difficult to place in an evolutionary context. After examination of genetic differences between annelids, Siboglinidae were placed within the order Polychaeta by scientific consensus. The fossil record along with molecular clocks suggest the family has Mesozoic (250 – 66 Mya) or Cenozoic (66 Mya – recent) origins. However, some fossils of crystallized tubes are attributed to early Siboglinidae dating back to 500 Mya. The oldest definitive specimens referred to the family came from Early Jurassic (Pliensbachian-Toarcian) Figueroa Sulfide deposits from the San Rafael Mountains, found to be similar to modern Ridgeia. These tubes, known as 'Figueroa tubes', along with the 'Troodos collared tubes' (Cyprus, Turonian) were placed among modern vestimentiferans.

Molecular work aligning five genes has identified four distinct clades within Siboglinidae, These being Vestimentifera, Sclerolinum, Frenulata, and Osedax. Vestimentiferans live in vent and seep habitats. Separation of vestimentiferans into seep and deep-sea-dwelling clades is still debated due to some phylogenies based on sequencing data placing the genera along a continuum. Sclerolinum is a monogeneric clade (which may be called Monilifera) living on organic-rich remains. Frenulates live in organic-rich sediment habitats. Osedax is a monogeneric clade specialized in living on whale bones, although recent evidence shows them living on fish bones as well.

The following are cladograms of Siboglinidae:

Genera

• Clade Frenulata
  • Birsteinia
  • Bobmarleya
  • Choanophorus
  • Crassibrachia
  • Cyclobrachia
  • Diplobrachia
  • Galathealinum
  • Heptabrachia
  • Lamellisabella
  • Nereilinum
  • Oligobrachia
  • Paraescarpia
  • Polybrachia
  • Siboglinoides
  • Siboglinum
  • Siphonobrachia
  • Spirobrachia
  • Unibrachium
  • Volvobrachia
  • Zenkevitchiana
• Clade Monilifera
  • Sclerolinum
• Clade Vestimentifera
  • Alaysia
  • Arcovesia
  • Escarpia
  • Lamellibrachia
  • Oasisia
  • Ridgeia
  • Riftia
  • Tevnia
• incertae sedis
  • Osedax

Vestimentiferans

Like other tube worms, vestimentiferans are benthic marine creatures. Riftia pachyptila, a vestimentiferan, is known only from the hydrothermal vent systems.

Cladogram based on a maximum likelihood tree of concatenated 16-gene nucleotide data:

Anatomy of vestimentiferans

Vestimentiferan bodies are divided into four regions: the obturaculum, vestimentum, trunk, and opisthosome. The main trunk of the body bears wing-like extensions. Unlike other siboglinids that never have a digestive tract, they have one that they completely lose during metamorphosis.

The obturaculum is the first anterior body part. It is possible that the obturaculum is actually an outgrowth of the vestimentum rather than a separate body segment which would distinguish it from other siboglinids.

The vestimentum, from which the group's name is derived, is a wing-like body part with glands that secrete the tube. In a ventroanterior position in the vestimentum is the brain which is postulated to be simpler than relatives that maintain a gut in the adult form. The opisthosome is the anchoring rear body part.

Vestimentiferan ecology

Their primary nutrition is derived from the sulfide-rich fluids emanating from the hydrothermal vents where they live. The sulfides are metabolized by symbiotic hydrogen sulfide- or methane-oxidizing bacteria living in an internal organ, the trophosome. One gram of trophosome tissue can contain one billion bacteria. The origin of this symbiotic relationship is not currently known. The bacteria appear to colonize the host animal larvae after they have settled on a surface, entering them through their skin. This method of entry, known as horizontal transmission, means that each organism may have different species of bacteria assisting in this symbiosis. However, these bacteria all play similar roles in sustaining the vestimentiferans. Endosymbionts have a wide variety of metabolic genes, which may allow them to switch between autotrophic and heterotrophic methods of nutrient acquisition. When the host dies, the bacteria are released and return to the free-living population in the seawater.

Discovery of the hydrothermal vents in the eastern Pacific Ocean was quickly followed by the discovery and description of new vestimentiferan tubeworm species. These tubeworms are one of the most dominant organisms associated with the hydrothermal vents in the Pacific Ocean. Tubeworms anchor themselves to the substratum of the hydrocarbon seep by roots located at the basal portion of their bodies. Intact tubeworm roots have proven very difficult to obtain for study because they are extremely delicate, and often break off when a tubeworm is removed from hypothermal vent regions. How long the roots of the tube worms can grow is unknown, but roots have been recovered longer than 30 m.

A single aggregation of tubeworms can contain thousands of individuals, and the roots produced by each tubeworm can become tangled with the roots of neighbouring tubeworms. These mats of roots are known as "ropes", and travel down the tubes of dead tubeworms, and run through holes in rocks. The diameter and wall thickness of the tubeworm roots do not appear to change with distance from the trunk portion of the tubeworm's body.

Like the trunk portion of the body, the roots of the vestimentiferan tubeworms are composed of chitin crystallites, which support and protect the tubeworm from predation and environmental stresses. Tubeworms build the external chitin structure themselves by secreting chitin from specialized glands located in their body walls.