Siboglinidae
Siboglinidae is a family of polychaete annelid worms whose members made up the former phyla Pogonophora and Vestimentifera (the giant tube worms).[1][2] The family is composed of around 100 species of vermiform creatures which live in thin tubes buried in sediment (Pogonophora) or in tubes attached to hard substratum (Vestimentifera) at ocean depths ranging from 100 to 10,000 m (300 to 32,800 ft). They can also be found in association with hydrothermal vents, methane seeps, sunken plant material, and whale carcasses.
The first specimen was dredged from the waters of Indonesia in 1900. These specimens were given to French zoologist Maurice Caullery, who studied them for nearly 50 years.
Most siboglinids are less than 1 millimetre (0.04 in) in diameter, but 10–75 centimetres (3.9–29.5 in) in length. They inhabit tubular structures composed of chitin which are fixed to rocks or substrates. The tubes are often clustered together in large colonies.[3]
Their bodies are divided into four regions. The anterior end is called the cephalic lobe, which ranges from one to over 200 thin branchial ciliated tentacles, each with tiny side branches known as pinnules. Behind this is a glandular forepart, which helps to secrete the tube. The main part of the body is the trunk, which is greatly elongated and bears various annuli, papillae, and ciliary tracts. Posterior to the trunk is the short metamerically segmented opisthosoma, bearing external paired chaetae, which help to anchor the animal to the base of its tube.[3]
The body cavity has a separate compartment in each of the first three regions of the body and extends into the tentacles. The opisthosoma has a coelomic chamber in each of its 5 to 23 segments, separated by septa. The worms have a complex closed circulatory system and a well-developed nervous system, but as adults, siboglinids completely lack a mouth, gut, and anus.[4]
The family Siboglinidae has been difficult to place in an evolutionary context.[5] After examination of genetic differences between annelids, Siboglinidae were placed within the order Polychaeta by scientific consensus.[6] The fossil record along with molecular clocks suggest the family has Mesozoic (250 – 66 Mya) or Cenozoic (66 Mya – recent) origins.[5] However, some fossils of crystallized tubes are attributed to early Siboglinidae dating back to 500 Mya.[5] Molecular work aligning five genes has identified four distinct clades within Siboglinidae.[7][8][9] The clades are Vestimentifera, Sclerolinum, Frenulata, and Osedax.[8] Vestimentiferans live in vent and seep habitats.[8] 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.[10] Sclerolinum is a monogeneric clade (which may be called Monilifera) living on organic-rich remains.[5] Frenulates live in organic-rich sediment habitats.[11] Osedax is a monogeneric clade specialized in living on whale bones, although recent evidence shows them living on fish bones as well.[12]
