Xenophyophorea is a clade of foraminiferans. Xenophyophores are multinucleate unicellular organisms found on the ocean floor throughout the world's oceans, at depths of 500 to 10,600 metres (1,600 to 34,800 ft). They are a kind of foraminiferan that extract minerals from their surroundings and use them to form an exoskeleton known as a test. They were first described by Henry Bowman Brady in 1883. They are abundant on abyssal plains, and in some regions are the dominant species. Fifteen genera and 75 species have been described, varying widely in size. The largest, Syringammina fragilissima, is among the largest known coenocytes, reaching up to 20 centimetres (8 in) in diameter.
Naming and classification The name Xenophyophora means "bearer of foreign bodies", from the Greek. This refers to the sediments, called xenophyae, which are cemented together to construct their tests. In 1883, Henry Bowman Brady classified them as primitive Foraminifera. Later they were placed within the sponges. In the beginning of the 20th century they were considered an independent class of Rhizopoda, and later as a new eukaryotic phylum of Protista. Phylogenetic studies suggest that xenophyophores are a specialized group of monothalamous (single-chambered) Foraminifera. A 2013 molecular study using small subunit rDNA found Syringammina and Shinkaiya to form a monophyletic clade closely related to Rhizammina algaeformis. Further molecular evidence has confirmed the monophyly of xenophyophores. This study also suggested that many individual genera are polyphyletic, with similar body shapes convergently evolving multiple times. Historically xenophyophores have been divided into the agglutinated Psamminida and the flexible, proteinaceous Stannomida. However, cladistic analyses based on molecular data have suggested a high amount of homoplasy, and that the division between psamminids and stannomids is not well supported.
Anatomy
Xenophyophores are unicellular, but have many nuclei. Many form delicate and elaborate agglutinated tests—shells often made of calcium carbonate (CaCO3) and other foreign mineral particles glued together with organic cements—that range from a few millimetres to 20 centimetres across. The softness and structure of tests varies from soft and lumpy shapes to fans and complex structures. Some xenophyophores—notably Psammina—have compartmentalized tests consisting of multiple chambers. 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. A number of unique terms are used to refer to anatomical aspects of the group:
Individual waste pellets are referred to as stercomes or stercomata; pellets that are bundled together in long strings are referred to as stercomares. Stercomares also include small, yellow-red spherical bodies known as xanthosomes. Xenophyophores also commonly have abundant crystals of barite called granellae within their cytoplasm. This is not to be confused with the granellare, which refers to the plasma body and its tube. Linellae are long (several mm in length), threadlike structures found outside of the granellare in some xenophyophores (genera traditionally grouped together as "stannomida"); they are flexible and form part of the test. Xenophyae, for which the group is named, are the agglutinated particles from which the test is constructed. They vary by species; they can contain sediment particles, sponge spicules, radiolarian tests, and even the tests of smaller foraminifera. The protoplasm of xenophyophores contributes less than 1% of the total mass of the organism. They select certain minerals and elements from their environment that are included in its tests and cytoplasm, or concentrated in excretions. The selected minerals vary with species, but often include barite, lead and uranium. The granellare of Shinkaiya have been found to contain high concentrations of mercury. Studies have found unusually high concentrations of radioactive nuclides in xenophyophores; this was first reported in Occultammina but has since been found to be true of many other xenophyophore species from different parts of the ocean.
Growth and reproduction Very little is known about xenophyophore reproduction. It is assumed that an alternation of generations takes place, as in other foraminifera; however, this has not been confirmed. Gametes form in a specialised part of the granellare that may look like swollen side-branch (in Psammetta) or a stalked bulb (in Cerelasma). Gametes are reportedly about 20 μm in diameter, with two flagella; after this, an amoeba-like stage seems to be present. It is also possible that the amoeboid stage represents amoeboid gametes, found in other foraminifera. These amoeboid structures are also sometimes found inside the granellare. Juveniles have occasionally been found in association with adults; in Psametta they are horseshoe-shaped and already covered in xenophyae. The location of the initial plasma can sometimes be pointed out in adult xenophyophores. In some species this is denoted by a sharp change in the type of xenophyae; in others, the juvenile is regular and the adult is irregular; still others flip this pattern, so that the juvenile is irregular and the adult is regular. Growth is episodic; one observational study taking place over a period of eight months saw a three-to-tenfold growth in specimens of Reticulammina labyrinthica. This growth occurred in phases lasting 2–3 days each; each phase was separated by a resting period of approximately two months. These growth phases were approximately synchronous between specimens, but it is unclear if this is biologically or developmentally controlled; some evidence suggests the synchrony may have been due to chance.
Each episode of growth occurred in three phases: first, the base becomes wider and flatter, causing the surface texture to become smoother; then, the original shape of the organism is regained (albeit larger); and finally, the surface texture is rebuilt. The rapid rate of growth observed suggests that xenophyophores may not be as long-lived as previously hypothesised.
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