Xenophyophore
Encyclopedia
Xenophyophores are marine protozoa
, giant single-celled
organisms found throughout the world's oceans, at depths of up to 10,641 meters (6.6 miles). Xenophyophores are found in the greatest numbers on the abyssal plains of the deep ocean. They were first described as sponges
in 1889, then as testate amoeboid
s, and later as their own phylum of Protist
a. 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 fragilissima
, 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 frilly or spherical sponges. Local environmental conditions—such as current direction and speed—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 (highly branched) 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 while 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 mineral
s and the microscopic skeletal remains of other organisms, such as sponges, radiolarian
s, 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, echinoderm
s, and molluscs than equivalent areas which lack xenophyophores. The xenophyophores themselves also play commensal
host to a number of organisms—such as isopods (e.g., genus Hebefustis), sipuncula
n and polychaete
worms, nematode
s, and harpacticoid
copepod
s—some of which may take up semi-permanent residence within a xenophyophore's test. Brittle star
s (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.
Protozoa
Protozoa are a diverse group of single-cells eukaryotic organisms, many of which are motile. Throughout history, protozoa have been defined as single-cell protists with animal-like behavior, e.g., movement...
, giant single-celled
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
organisms found throughout the world's oceans, at depths of up to 10,641 meters (6.6 miles). Xenophyophores are found in the greatest numbers on the abyssal plains of the deep ocean. They were first described as sponges
Sea sponge
Sponges are animals of the phylum Porifera . Their bodies consist of jelly-like mesohyl sandwiched between two thin layers of cells. While all animals have unspecialized cells that can transform into specialized cells, sponges are unique in having some specialized cells, but can also have...
in 1889, then as testate amoeboid
Amoeboid
Amoeboids are single-celled life-forms characterized by an irregular shape."Amoeboid" and "amœba" are often used interchangeably even by biologists, and especially refer to a creature moving by using pseudopodia. Most references to "amoebas" or "amoebae" are to amoeboids in general rather than to...
s, and later as their own phylum of Protist
Protist
Protists are a diverse group of eukaryotic microorganisms. Historically, protists were treated as the kingdom Protista, which includes mostly unicellular organisms that do not fit into the other kingdoms, but this group is contested in modern taxonomy...
a. A recent genetic study suggested that the xenophyophores are a specialized group of Foraminifera
Foraminifera
The Foraminifera , or forams for short, are a large group of amoeboid protists which are among the commonest plankton species. They have reticulating pseudopods, fine strands of cytoplasm that branch and merge to form a dynamic net...
. There are approximately 42 recognized species in 13 genera and 2 orders; one of which, Syringammina fragilissima
Syringammina fragilissima
Syringammina fragilissima is a xenophyophore found off the coast of Scotland, near Rockall. It is the largest single-celled organism known, at up to across...
, 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 frilly or spherical sponges. Local environmental conditions—such as current direction and speed—may play a part in influencing these forms. Xenophyophores are essentially lumps of viscous fluid called cytoplasm
Cytoplasm
The cytoplasm is a small gel-like substance residing between the cell membrane holding all the cell's internal sub-structures , except for the nucleus. All the contents of the cells of prokaryote organisms are contained within the cytoplasm...
containing numerous nuclei
Cell nucleus
In cell biology, the nucleus is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these...
distributed evenly throughout. Everything is contained in a ramose (highly branched) system of tubes called a granellare, itself composed of an organic cement-like substance.
As benthic
Benthos
Benthos is the community of organisms which live on, in, or near the seabed, also known as the benthic zone. This community lives in or near marine sedimentary environments, from tidal pools along the foreshore, out to the continental shelf, and then down to the abyssal depths.Many organisms...
deposit feeders, xenophyophores tirelessly 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. 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
Greek language
Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...
to mean "bearer of foreign bodies". A protective, shell-like test is thereby agglutinated around the granellare, which is composed of scavenged mineral
Mineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...
s and the microscopic skeletal remains of other organisms, such as sponges, radiolarian
Radiolarian
Radiolarians are amoeboid protozoa that produce intricate mineral skeletons, typically with a central capsule dividing the cell into inner and outer portions, called endoplasm and ectoplasm. They are found as zooplankton throughout the ocean, and their skeletal remains cover large portions of the...
s, and other foraminiferans.
Xenophyophores may be an important part of the benthic ecosystem by virtue of their constant bioturbation
Bioturbation
In oceanography, limnology, pedology, geology , and archaeology, bioturbation is the displacement and mixing of sediment particles and solutes by fauna or flora . The mediators of bioturbation are typically annelid worms , bivalves In oceanography, limnology, pedology, geology (especially...
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, echinoderm
Echinoderm
Echinoderms are a phylum of marine animals. Echinoderms are found at every ocean depth, from the intertidal zone to the abyssal zone....
s, and molluscs than equivalent areas which lack xenophyophores. The xenophyophores themselves also play commensal
Commensalism
In ecology, commensalism is a class of relationship between two organisms where one organism benefits but the other is neutral...
host to a number of organisms—such as isopods (e.g., genus Hebefustis), sipuncula
Sipuncula
The Sipuncula or Sipunculida is a group containing 144-320 species of bilaterally symmetrical, unsegmented marine worms...
n and polychaete
Polychaete
The Polychaeta or polychaetes are a class of annelid worms, generally marine. Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called chaetae, which are made of chitin. Indeed, polychaetes are sometimes referred to as bristle worms. More than 10,000...
worms, nematode
Nematode
The nematodes or roundworms are the most diverse phylum of pseudocoelomates, and one of the most diverse of all animals. Nematode species are very difficult to distinguish; over 28,000 have been described, of which over 16,000 are parasitic. It has been estimated that the total number of nematode...
s, and harpacticoid
Harpacticoida
Harpacticoida is an order of copepods, in the Subphylum Crustacea. This order comprises 463 genera and about 3,000 species. Members of it are benthic copepods found throughout the world in the marine environment and in fresh water...
copepod
Copepod
Copepods are a group of small crustaceans found in the sea and nearly every freshwater habitat. Some species are planktonic , some are benthic , and some continental species may live in limno-terrestrial habitats and other wet terrestrial places, such as swamps, under leaf fall in wet forests,...
s—some of which may take up semi-permanent residence within a xenophyophore's test. Brittle star
Brittle star
Brittle stars or ophiuroids are echinoderms in the class Ophiuroidea closely related to starfish. They crawl across the seafloor using their flexible arms for locomotion. The ophiuroids generally have five long slender, whip-like arms which may reach up to in length on the largest specimens...
s (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
Cell culture
Cell culture is the complex process by which cells are grown under controlled conditions. In practice, the term "cell culture" has come to refer to the culturing of cells derived from singlecellular eukaryotes, especially animal cells. However, there are also cultures of plants, fungi and microbes,...
. 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.