(Hydrothermal vent and cold seep worms)
Number of families 8
Segmented worms that have an unusual anatomy and rely on symbiotic bacteria for nutrition. They are nearly always found in deep waters, some as members of hydrothermal vent communities and others found in association with reducing sediments such as “cold seeps”
Photo: This photo of Riftia pachyptila comes from a depth of 8,200 ft (2,500 m) at 9° 50 N. on the East Pacific Rise, and was shot on a dive in the submersible Alvin.
Evolution and systematics
A number of fossil hydrothermal vent and seep systems have been discovered containing tubular fossils that appear to be those of vestimentiferan tubes. These tubes have been found in sulfide ores dating to the Carboniferous and Cretaceous, and possibly to the Silurian period.
The varied and complex taxonomic history of Vestimentifera, containing 15 described species, represents one of the more fascinating tales in animal systematics. It has traditionally been regarded as a distinct phylum, probably due to its complex anatomy and bizarre lifestyle. Most scientists now consider it to be part of the polychaete family Siboglinidae (formerly phylum Pogonophora), within the phylum Annelida. This recent classification is based on its clear annelid features and on molecular sequence data. The more conservative classification is followed here.
Vestimentifera encompasses 10 genera, namely Alaysia, Ar-covestia, Escarpia, Lamellibrachia, Oasisia, Paraescarpia, Ridgeia, Riftia, Seepiophila, and Tevnia. With the exception of Escarpia (two species) and Lamellibrachia (four species), each of these genera contains only one species.
Vestimentiferans have elongated cylindrical worm-like bodies and are always found living in tubes. In nearly all cases, the tube must be attached to a hard surface rather than lying free in the sediment. The animal secretes the tube and occupies most of it, extending the anterior portion of its body, the plume, out into the water. If disturbed, this vulnerable plume can be quickly retracted back into the tube. The plume is comprised of hundreds of branchial filaments that are clustered into lamellae. These filaments are filled with blood vessels, and the red hemoglobin in the blood gives the plumes their bright color. The plume and obturaculum are equivalent to the head of other annelid worms and there are no known sensory organs such as eyes.
A short region immediately behind the head is called the vestimentum and this has lateral flaps that fold over the top of the worm. The front part of the vestimentum forms a collar. It is this region that may be used for secretion of the tube. The next body region is generally referred to as the trunk and comprises most of the body. The trunk contains the reproductive organs and is also largely filled with an expanded gut tissue called the trophosome. There is no gut lumen as such and, in adults, there is no mouth or anus. The trophosome lies between the ventral and dorsal blood vessels and is filled with special cells called bacteriocytes that contain symbiotic chemoautotrophic bacteria. The remainder of the body is a short multi-segmented region called the opisthosoma. The anterior segments of the opisthosoma have rows of hook-like chaetae that act as anchors for the worm to retract into the tube.
Vestimentifera contains some of the largest of the annelids. Riftia pachyptila and Ridgeia piscesae grow to more than 4.9 ft (1.5 m) in length and live in tubes more 8.2 ft (2.5 m) long. Other vestimentifera such as Lamellibrachia satsuma reach 16 in (40 cm) in length as adults.
The tubes of Vestimentifera are whitish to gray-brown. The plume is usually bright red and surrounds a central white obturaculum. The body within the tube is generally green to brown and there are often large red blood vessels visible through the body surface.
The particular requirements of vestimentiferans mean that they are restricted to deep-sea environments, with most found at depths >0.6 mi (>1 km), though one species was observed at <328 ft (<100 m) depth. Hydrothermal vents are found on active spreading ridges between continental plates. The major ones around the world where vestimentiferans have been found are the east Pacific Rise, the mid-Atlantic Ridge, and the Galapagos Rift. Hydrothermal vents and vestimentiferans are also found at sea-floor spreading centers in what are known as “back-arc basins” of the western Pacific, including the Okinawa Trough, Mariana Trough, and the Lau, Manus, and North Fiji Basins. Cold seeps are mainly located along subduction zones or continental margins, and vestimentiferans have been found at seeps in the Gulf of Mexico, off the coasts of North and South America, Spain, and in the Mediterranean Sea. Undoubtedly, many more Vestimentifera will be discovered as the deep sea is further explored.
Most of the deep seafloor is soft sediment; vestimentifer-ans need a hard surface to attach their tubes to. Some vesti-mentiferans settle and grow on the chimneys of hydrothermal vents where the water temperature is around 68°F (20°C). They are also found on lava flows associated with vents. Often, they form large clusters, with younger worms making their tubes on those of larger worms. At cold seeps, the worms also tend to form clusters, with tubes growing on tubes.
Two dramatically different life histories are apparent in this group. It has been shown that Riftia pachyptila could colonize a new hydrothermal vent site, grow to sexual maturity, and have tubes of 4.9 ft (1.5 m) in length, in less than two years. This may represent the fastest growth rate of a marine invertebrate; this rapid growth appears to be essential because their habitat is ephemeral and lasts for only a few years or decades. In contrast, cold seeps such as the Louisiana slope (Gulf of Mexico) provide a stable supply of sulfide over centuries. Lamellibrachia luymesi that live in this environment grow very slowly and, while reaching more than 6.5 ft (2 m) in tube length, may take more than 100 years to do so.
Apart from hydrothermal vents, cold seeps, and whale carcasses, vestimentiferans have not been associated with any other habitat.
Vestimentiferans form dense aggregations of both sexes at both hydrothermal and cold seep sites with worms at all stages of life. No other social organization is apparent.
Little is known of the mating system in vestimentiferans. It seems there is no contact between sexes and that sperm spawned by males makes its way to the tubes of females where they fertilize the eggs. There does not appear to be mate selection or parental behavior.
Feeding ecology and diet
Nutritional requirements for vestimentiferans are met through their symbiotic relationship with chemoautotrophic bacteria in the trophosome. A transitory mouth appears to be the pathway for bacteria to occupy the trophosome. The bacteria require carbon dioxide and either sulfide or thiosulphate,
all of which are supplied by the host. In return, the host obtains nutrition from the bacteria, or digests them (Southward 1993). All the chemoautotrophic bacteria found in vestimen-tiferans belong in the Proteobacteria and, where it has been studied, only one kind of bacterium is found in a given ves-timentiferan species.
Little is know about predators of Vestimentifera. Possible predators include buccinid snails.
All vestimentiferans studied to date appear to have separate sexes with gametes that are produced in the trunk. No courtship behaviors have been documented to date. Males produce masses of sperm or sperm bundles that are spawned into the water and end up in the tubes of females. Fertilization appears to occur in or just outside the oviducts.
Females produce large numbers of eggs that are around 0.0039 in (0.1 mm) in diameter when mature.
Vestimentiferans face the problem of their favored habitats often being ephemeral. Thus, they need to have mechanisms that allow them to have some larvae continue to colonize the site where their parents are located, but also have others that are capable of dispersing long distances to other vents or seeps. How this is achieved is the subject of intense study.
The larvae of vestimentiferans are similar to that of many other polychaetes in that they have enough yolk to develop into small juveniles. The length of time this process takes is not presently known. Early larvae swim with the aid of a band of cilia at the front end. At a certain stage, to gain the nutrition the larvae need to continue development, symbiotic bacteria must colonize them. This seems to happen via the transitory mouth that appears in late-stage larva, allowing bacteria to enter the body.
Vestimentiferans probably release early embryos into the plankton. No larvae have ever been found in the tubes of females, nor is there any parental care. This is in contrast to other pogonophores in which brooding of larvae in the female tube is common.
No species of Vestimentifera are listed by the IUCN. Since their first discovery in 1969, vestimentiferans have been found on a regular basis as exploration of the deep sea continues. Presumably, there are many more species and genera to be discovered and named. At present, there is no known obvious threat to any vestimentiferans. When they are found, they tend to occur in large numbers, but the nature of their habitats means that they have restricted distributions and high levels of endemism.
Significance to humans
Vestimentifera attracted significant attention in 1977 when they were first discovered at hydrothermal vents and were shown to be a large and significant part of the animal communities found there. While they have no commercial value to humans, they have iconic status as deep-sea animals.
1. Lamellibrachia luymesi; 2. Hydrothermal vent worm (Riftia pachyptila) in tube; 3. Hydrothermal vent worm (Riftia pachptila) without tube.
No common name
Lamellibrachia luymesi van der Land and Norrevang, 1977, Guyana Shelf.
OTHER COMMON NAMES
Body reaches 1.6 ft (0.5 m) in length, with tubes to 6.5 ft (2 m) in length. Plume dark red with ivory obturacula.
Off Guyana, Gulf of Mexico.
Forms dense thickets on silty sediments where cold seeps occur. Presumably, one or more animals attach to a stone or piece of rock and gradually the tubes of the worms provide settling places for other worms. This allows for the large discrete thickets to form. Otherwise, little is known.
FEEDING ECOLOGY AND DIET
Symbiotic bacteria in trophosome provide nutrition for worm. Host provides bacteria with sulfide via its plume and carbon dioxide as a byproduct of its own respiration. Cold seeps provide a stable supply of sulfide over centuries. Worms found in this environment grow very slowly and adults may be 100 or more years old.
Separate sexes, with males spawning into the water and females releasing eggs after fertilizing them. Eggs 0.0039 in (0.1 mm) in diameter and larvae can disperse for several weeks without the need to feed.
Not listed by the IUCN.
SIGNIFICANCE TO HUMANS
Hydrothermal vent worm
Riftia pachyptila Jones, 1981, East Pacific Rise.
OTHER COMMON NAMES
English: Deep-sea tubeworm, giant tubeworm, vestimentiferan tubeworm; French: Vers geant.
Largest of Vestimentifera; body reaches up to 4.9 ft (1.5 m) in length and white tubes to 8.2 ft (2.5 m). Bright red plume and white obturaculum.
East Pacific Rise, Galapagos Rift, and Guaymas Basin.
Found at depths of around 1 mi (1.5 km) associated with hy-drothermal vents.
Forms dense thickets on hydrothermal vent “chimneys.” Otherwise, little is known.
FEEDING ECOLOGY AND DIET
Symbiotic bacteria in trophosome provide nutrition for the worm. The host provides bacteria with sulfide via its plume
and carbon dioxide as a byproduct of its own respiration. One of the fastest growing marine invertebrates, with tubes reaching 4.9 ft (1.5 m) in length in only 18 months.
Separate sexes, with males spawning into the water and females releasing eggs after fertilizing them. Larvae must be capable of dispersing to new vents since vents are ephemeral habitats.
Not listed by the IUCN.
SIGNIFICANCE TO HUMANS
“Posterchild” of hydrothermal vents.