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Xenophyophorea is a clade of . Xenophyophores are multinucleate unicellular organisms found on the ocean floor throughout the world's , at depths of . 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 genus and 75 species have been described, varying widely in size. The largest, Syringammina fragilissima, is among the largest known , reaching up to in diameter.
A 2013 molecular study using SSU rRNA found Syringammina and Shinkaiya to form a monophyletic clade closely related to Rhizammina. 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.
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:
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.
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. 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.'' sp. from the Porcupine Abyssal Plain in the NE Atlantic, from a depth of about 4800m.]]
Xenophyophores have been found between depths of 500 and 10,600 metres. Most are epifaunal (living atop the seabed), but one species ( Occultammina), is known to be infaunal; it buries itself up to deep into the sediment.
Xenophyophore densities are highest on soft sediments; however, they may still be found on rocky substrates including , canyon walls, and Manganese nodule crusts.
A 2021 study that utilised isotopic labeling to examine the question of xenophyophore feeding confirmed rapid uptake of both and dissolved organic matter in the form of glucose. This study found no evidence to support a bacterial farming function for the test, and instead proposed that it aided to function in the collection of phytodetritus by increasing surface area. These authors argued that xenophyophores fill a major role in ocean-floor biogeochemical cycling.
Some researchers have suggested that the enigmatic Graphoglyptidae Paleodictyon, known from the early Cambrian through recent times, could represent the remains of xenophyophores, and noted the similarity of the extant xenophyophore Occultammina to the fossil. Supporting this notion is the similar abyssal habitat of living xenophyophores to the inferred habitat of fossil graphoglyptids; however, the large size (up to 0.5m) and regularity of many graphoglyptids as well as the apparent absence of xenophyae in their fossils casts doubt on the possibility. Modern examples of Paleodictyon have been discovered; however, no evidence of tests, stercomares, grannelares, or xenophyophore DNA was found, and the trace may alternately represent a burrow or a Hexactinellid
Certain Carboniferous fossils have been suggested to represent the remains of xenophyophores due to the concentration of barium within the fossils as well as supposed morphological similarity; however, the barium content was later determined to be due to Diagenesis alteration of the material and the morphology of the specimen instead supported an algal affinity.
Starfish, , and Molpadiidae sea cucumbers have all been observed feeding on xenophyophores; specifically, the monoplacophoran Neopilina galatheae has been proposed as a specialised predator of the group.
Despite this abundance, the relatively low amount of protoplasm per unit of test means that xenophyophores often contribute little to total biomass.
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.
Scientists in the submersible DSV Alvin at a depth of 3,088 metres at the Alaskan continental margin in the Gulf of Alaska collected a spatangoid, Cystochinus loveni, about 5 cm diameter, which was wearing a cloak consisting of over 1,000 protists and other creatures, including 245 living xenophyophores, mainly Psammina species, each 3–6 mm. The fragility of the xenophyophores suggests that the urchin either very carefully collected them, or that they settled and grew there. Among several possible explanations for the urchin's behaviour, perhaps the most likely are chemical camouflage and weighing itself down to avoid being moved in currents.
Different xenophyophore ecomorphs are found in different settings; reticulated or heavily folded genera such as Reticulammina and Syringammina are more common in areas where the substrate is sloped or near canyon walls, while more fan-shaped forms like Stannophyllum are more common in areas with quieter water and/or lower primary productivity.
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