|Xenophyophore in the Galapagos Rift|
Xenophyophores are giant unicellular organisms found throughout the world's oceans, at depths of up to 10,641 meters (6.6 miles). They were first described by Brady in 1883 as primitive Foraminifera, and later they were placed within the sponges. In the beginning of the 20th century they were considered as an independent class of Rhizopoda, and later as a new eukaryotic phylum of Protista. A recent genetic study suggests that xenophyophores are a specialized group of monothalamous (single-chambered) Foraminifera.
Xenophyophores have been described at depths greater than 500 meters and are more abundant on abyssal plains. Fourteen genera and approximately 60 species have been described; one particular species, Syringammina fragilissima, is among the largest known single-celled organisms, as it can grow up to 20 centimetres in diameter.
Xenophyophores are giant, multinucleate Foraminifera that are confined exclusively to depths below 500 metres. Their name Xenophyophora, which means "bearer of foreign bodies", comes from the Greek and refers to the sediments, called xenophyae, which are agglutinated (cemented together) to construct their tests. They are an important component of the deep sea-floor, as they have been found in all four major ocean basins. However, so far little is known about their biology and ecological role in deep-sea ecosystems.
They form delicate and elaborate agglutinated tests that range from a few millimetres to 20 centimetres. Species of this group are morphologically variable, but the general structural pattern includes a test enclosing a branching system of organic tubules together with masses of waste material (stercomata).
They are often found in areas of enhanced organic carbon flux, such as beneath productive surface waters, in sub-marine canyons, in settings with sloped topography (e.g. seamounts, abyssal hills) and on continental slopes.
As benthic detritivores, xenophyophores root through the muddy sediments on the sea floor. They excrete a slimy substance while feeding; in locations with a dense population of xenophyophores, such as at the bottoms of oceanic trenches, this slime may cover large areas. These giant protozoans seem to feed in a manner similar to amoebas, enveloping food items with a foot-like structure called a pseudopodium. Most are epifaunal (living atop the seabed), but one species (Occultammina profunda), is known to be infaunal; it buries itself up to 6 cm deep into the sediment.
Local population densities may be as high as 2,000 individuals per 100 square meters, making them dominant organisms in some areas. Xenophyophores may be an important part of the benthic ecosystem due to their bioturbation of sediment, providing a habitat for other organisms such as isopods. Research has shown that areas dominated by xenophyophores have 3-4 times the number of benthic crustaceans, echinoderms, and molluscs than equivalent areas that lack xenophyophores. The xenophyophores themselves also play commensal host to a number of organisms—such as isopods (e.g., genus Hebefustis), sipunculan and polychaete worms, nematodes, and harpacticoid copepods—some of which may take up semi-permanent residence within a xenophyophore's test. Brittle stars (Ophiuroidea) also appear to have a relationship with xenophyophores, as they are consistently found directly underneath or on top of the protozoans.
Xenophyophores are difficult to study due to their extreme fragility. Specimens are invariably damaged during sampling, rendering them useless for captive study or cell culture. For this reason, very little is known of their life history. As they occur in all the world's oceans and in great numbers, xenophyophores could be indispensable agents in the process of sediment deposition and in maintaining biological diversity in benthic ecosystems.
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