Monothalamea is a class of foraminiferans, previously known as Xenophyophorea. Members of this class 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 extracts minerals from their surroundings and uses 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. Fourteen genera and approximately 60 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.
Haeckel, 1862, emend. Pawlowski et al., 2013
|Orders and subtaxa incertae sedis|
Monothalameans are an important part 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 seem to be unicellular, but have many nuclei. They form delicate and elaborate agglutinated tests (shells made of foreign mineral particles glued together with organic cements) 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). The softness and structure of tests varies from soft and lumpy shapes to fans and complex structures.
Monothalameans 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. 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 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. As of 2015, recent phylogenetic studies suggest that monothalameans are a specialized group of monothalamous (single-chambered) Foraminifera.
As benthic detritivores, Monothalameans root through the muddy sediments on the sea floor. They excrete a slimy substance while feeding; in locations with a dense population of Monothalameans, 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 centimetres (2.4 in) deep into the sediment.
Local population densities may be as high as 2,000 individuals per 100 square metres (1,100 sq ft), making them dominant organisms in some areas. Monothalameans 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 monothalameans have 3–4 times the number of benthic crustaceans, echinoderms, and molluscs than equivalent areas that lack monothalameans. The monothalameans 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 monothalamean's test. Brittle stars (Ophiuroidea) also appear to have a relationship with monothalameans, as they are consistently found directly underneath or on top of the protozoans.
Monothalameans 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, monothalameans 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 urchin, Cystochinus loveni, about 5 cm diameter, which was wearing a cloak consisting of over 1,000 protists and other creatures, including 245 living monothalameans, mainly Psammina species, each 3–6 mm. The fragility of the monothalameans 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.
The Allogromiida is an order of single-chambered, mostly organic-walled foraminiferans, including some that produce agglutinated tests (Lagynacea). Genetic studies indicate that some foraminiferans with agglutinated tests, previously included in the Textulariida or as their own order Astrorhizida, may also belong here. Allogromiids produce relatively simple tests, usually with a single chamber, similar to those of other protists such as Gromia. They are found as both marine and freshwater forms, and are the oldest forams known from the fossil record.Astrorhizata
Astrorhizata is a class within the foraminifera that comprises those species with primitive unilocular, (single chambered), tests composed of aggulinated material or microgranular calcite. Forms may be subspherical, elongated, or irregular and may appear to become multichambered during growth.
Two subclasses are included, the Astrorhizana which includes four orders, the Astrothizida, Dendrofryida, Hippocrepinida, and Saccamminida, and the Lagynana with the Allogromiida
The Monothalamea as emended by Pawlowski et al., 2013, is nearly equivalent to the Astrorhizata Mikhalevich, 1980; the primary difference being the inclusion of the superfamily Xenophyrophoroidea in the Monothalamea.Conicotheca nigrans
Conicotheca nigrans is a species of benthic Foraminifera that was first identified from samples collected from the Challenger Deep in the Mariana Trench. C. nigrans has a test or outer shell of agglutinated organic materials and minerals. As a species belonging to a basal clade of foraminifera, the order Allogromiida, C. nigrans may be informative about the evolution of the forams.Komokiacea
The Komokiacea are a small group of amoeboid protozoa, considered to be foraminifera, though there have been suggestions that they are a separate group, closely related to foraminifera. Komokiacea are rather large organisms, often exceeding 300 micrometers in maximum dimensions. Along with Xenophyophores they dominate the macro- and megabenthic fauna in the deep sea and are commonly referred to as "giants protists".The komokiacean body consists of a central tube with several branching tubules that contain diffuse protoplasm and numerous waste pellets (stercomata). They often incorporate fine grain material between the tubules. However, in other forms such as that of the genus Lana the body is a loose mass of branching tubules with no centre of organization. Komokiacea serve often as a substrate for benthic meiofaunal organisms such as foraminifera, fungi and other deep-sea taxa. They are fragile and often get fragmented during analysis in the laboratory, which leads to a large number of tube fragments difficult to identify. They are commonly abundant in oligotrophic areas and, like other soft-walled foramiferal taxa, they become increasingly important with depth, especially below the carbonate compensation depth. In terms of diversity approximately 40 species have been described worldwide. High numbers of morphospecies have been described from the Southern Ocean (50), and the North-east subequatorial Pacific (102), which suggests that they are a significant constituent of benthic foraminiferal diversity in the deep sea.Spiculosiphon oceana
Spiculosiphon oceana is a giant species of foraminifera (a phylum of unicellular eukaryotes). Its appearance and lifestyle mimics that of a sponge. It was discovered in 2013 in underwater caves 30 miles off the coast of Spain.Syringammina fragilissima
Syringammina fragilissima is a xenophyophore found off the coast of Scotland, near Rockall. It is the largest single-cell organism known, at up to 20 centimetres (8 in) across. It was the first xenophyophore to be described, after being discovered in 1882 by the oceanographer John Murray.
The cell grows into hundreds of branched and interconnecting tubes, which secrete an organic cement to collect particles of sediment and sand, forming a crusty structure called the test. As the test grows, the cell withdraws from parts of it, which are then colonised by other organisms, such as nematodes. The cell is multinucleate (it has multiple nuclei).It is not known how the organism feeds or reproduces. However, it has been shown to have high concentrations of lipids within its cytosol, which suggests that it may feed on bacteria from the sediment that makes up the "sand tubes."