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Giant single-celled animals have recently been discovered living in the deepest place on Earth – the Mariana Trench in the western Pacific Ocean. Using a “dropcam” (a digital video camera encased in a glass bubble designed to resist the massive water pressures encountered on the ocean floor), researchers from the Scripps Institution for Oceanography at the University of California, San Diego found populations of mysterious “xenophyophores” thriving at greater depths than ever before reported for this group.
Xenophyophores have previously been observed in the New Hebrides Trench, in the south-west Pacific, at a depth of about 7500 km. These new observations take the record to more than 10,600 km beneath the Earth’s surface.
What is a Xenophyophore?
Xenophyophores were first described in the 1970s. Originally thought to be sponges, they were later identified as single, gigantic cells, and classified as members of the kingdom Protista along with other single-celled organisms and simple multicellular life lacking specialized tissues. Genetic studies have identified the xenophyophores as a type of foraminiferan – a group of amoebae that usually have shells, or tests, formed from calcium carbonate, the minuscule fossils of which are the major constituent of limestone.
Unlike other foraminifera, however, xenophyophores do not have calcium carbonate tests. This is because, at the extreme deep-sea pressures at which they live (more than one thousand times atmospheric pressure at sea level), calcium carbonate is soluble in water. Instead, xenophyophores have transparent tests made from an organic “glue” mixed with particles of clay, minerals, the skeletal remains of other organisms and other substances picked up as they move along the ocean floor. The Greek for “bearer of foreign bodies” gives the xenophyophores their name.
Giants of the Single-celled World
The Protista include many species of single-celled animals; what makes the xenophyphores remarkable is their size. They are often more than 10 cm in diameter, and individuals of the largest species, Syringammina fragilissima, have been found that are twice this size. In contrast, other foraminiferans are commonly less than 1 mm across.
Different species of xenophyophore vary widely in their appearance, ranging from flattened disks to spheres, and from angular to frilly. The individuals of one species exist as a series of branching tubes embedded beneath the ocean floor.
Like many deep-sea animals, xenophyophores are well adapted to the extreme cold and high pressure of ocean-trench life, but are fragile and difficult to bring back to the surface for closer study. As a consequence, little is known about their reproduction and other behaviors. They are, however, very abundant in their natural habitat – in some regions of the ocean floor, as many as 2000 xenophyophores have been counted per 100 square meters.
Xenophyophores feed on small particles of organic matter, which they obtain from the surrounding water or by burrowing in the ocean bed. Like other amoebae, they “swallow” their food by forming flexible extensions called pseudopods, which they wrap around food particles to absorb them into the cell.
By stirring up the seafloor, feeding xenophyophores bring nutrients to the surface on which other animals can feast; in this way, they function as important members of the deep-sea community. More enigmatically, these giant cells have been observed carrying worms, crustaceans, nematodes, brittle stars and other marine organisms, both on and in their tests. Smaller animals may be permanent residents, while others may hitch a lift on a xenophyophore to avoid predators, to feed or to breed.
Although xenophyophores are abundant and ecologically important in their deep-sea habitat, relatively little is known about these mysterious creatures. This may change in the future, however. According to ocean engineer Kevin Hardy, who worked on the glass sphere design used in the dropcam,
“Scripps researchers hope to one day capture and return novel living animals to the laboratory for study in high pressure aquariums that replicate the trench environment.”
The first steps have already been taken; during the Mariana Trench expedition, the Scripps researchers successfully tested an advanced seafloor vehicle that uses glass spheres to recover deep-sea microbes.
Gooday, A.J., Todo, Y., Uematsu, K., Kitazato, H. New Organic-walled Foraminifera (Protista) from the Ocean’s Deepest Point, the Challenger Deep (Western Pacific Ocean). Zoological Journal of the Linnean Society. Accessed December 1, 2011.
MSNBC. Giant Amoebas Discovered in Deepest Ocean Trench. Accessed December 1, 2011.
Scripps News. Researchers Identify Mysterious Life Forms in the Extreme Deep Sea. Accessed December 1, 2011.
The Offshore Directory. December 1, 2011.
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