Xenophyophore

Xenophyophores are marine protozoans, giant single-celled organisms found throughout the world's oceans, but in their greatest numbers on the abyssal plains of the deep ocean. They were first described as sponges in 1889, then as testate amoeboids, and later as their own phylum of Protista. A recent genetic study suggested that the xenophyophores are a specialized group of Foraminifera. There are approximately 42 recognized species in 13 genera and 2 orders; one of which, Syringammina fragillissima, is among the largest known protozoans at a maximum 20 centimetres in diameter.

Abundant but poorly understood, xenophyophores are delicate organisms with a variable appearance; some may resemble flattened discs, angular four-sided shapes (tetrahedra), or like frilly or spherical sponges. Local environmental conditions&mdash;such as current direction and speed&mdash;may play a part in influencing these forms. Xenophyophores are essentially lumps of viscous fluid called cytoplasm containing numerous nuclei distributed evenly throughout. Everything is contained in a ramose system of tubes called a granellare, itself composed of an organic cement-like substance.

As benthic deposit feeders, xenophyophores tirelessly root through the muddy sediments on the sea floor. They excrete a slimy substance whilst feeding; in locations with a dense population of xenophyophores, such as at the bottoms of oceanic trenches, this slime may cover large areas. Local population densities may be as high as 2,000 individuals per 100 square meters, making them dominant organisms in some 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.

Their glue-like secretions cause silt and strings of their own fecal matter, called stercomes, to build up into masses (called stercomares) on their exteriors. In this way, the organisms form structures which project from the sea floor; this characteristic also explains their name, which may be translated from the Greek to mean "bearer of foreign bodies". A protective, shell-like test is thereby agglutinated around the granellare, which is composed of scavenged minerals and the microscopic skeletal remains of other organisms, such as sponges, radiolarians, and other foraminiferans.

Xenophyophores may be an important part of the benthic ecosystem by virtue of their constant bioturbation of the sediments, 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 which lack xenophyophores. The xenophyophores themselves also play commensal host to a number of organisms&mdash;such as isopods (e.g., genus Hebefustis), sipunculan and polychaete worms, nematodes, and harpacticoid copepods&mdash;some of which may take up semi-permanent residence within a xenophyophore's test. Brittle stars (Ophiuroidea) also appear to have some sort of 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.