Foraminifera
Foraminifera are single-celled organisms, members of a phylum or class of Rhizarian protists characterized by streaming granular ectoplasm for catching food and other uses; and commonly an external shell called a test of diverse forms and materials. Tests of chitin are believed to be the most primitive type. Most foraminifera are marine, the majority of which live on or within the seafloor sediment, while a smaller number float in the water column at various depths, which belong to the suborder Globigerinina. Fewer are known from freshwater or brackish conditions, and some very few soil species have been identified through molecular analysis of small subunit ribosomal DNA.
Foraminifera typically produce a foraminifera test, or shell, which can have either one or multiple chambers, some becoming quite elaborate in structure. These shells are commonly made of calcium carbonate or agglutinated sediment particles. Over 50,000 species are recognized, both living and fossil. They are usually less than 1 mm in size, but some are much larger, the largest species reaching up to 20 cm.
In modern scientific English, the term foraminifera is both singular and plural, and is used to describe one or more specimens or taxa: its usage as singular or plural must be determined from context. Foraminifera is frequently used informally to describe the group, and in these cases is generally lowercase.
History of study
The earliest known reference to foraminifera comes from Herodotus, who in the 5th century BCE noted them as making up the rock that forms the Great Pyramid of Giza. These are today recognized as representatives of the genus Nummulites. Strabo, in the 1st Century BCE, noted the same foraminifera, and suggested that they were the remains of lentils left by the workers who built the pyramids.Robert Hooke observed a foraminifera under the microscope, as described and illustrated in his 1665 book Micrographia:
I was trying several small and single Magnifying Glasses, and casually viewing a parcel of white Sand, when I perceiv'd one of the grains exactly shap'd and wreath'd like a Shell I view'd it every way with a better Microscope and found it on both sides, and edge-ways, to resemble the Shell of a small Water-Snail with a flat spiral ShellAntonie van Leeuwenhoek described and illustrated foraminiferal tests in 1700, describing them as minute cockles; his illustration is recognizable as being Elphidium. Early workers classified foraminifera within the genus Nautilus, noting their similarity to certain cephalopods. It was recognised by Lorenz Spengler in 1781 that foraminifera had holes in the septa, which would eventually grant the group its name. Spengler also noted that the septa of foraminifera arced the opposite way from those of nautili and that they lacked a nerve tube.
Alcide d'Orbigny, in his 1826 work, considered them to be a group of minute cephalopods and noted their odd morphology, interpreting the pseudopodia as tentacles and noting the highly reduced head. He named the group foraminifères, or "hole-bearers", as members of the group had holes in the divisions between compartments in their shells, in contrast to nautili or ammonites.
The protozoan nature of foraminifera was first recognized by Dujardin in 1835. Shortly after, in 1852, d'Orbigny produced a classification scheme, recognising 72 genera of foraminifera, which he classified based on test shape—a scheme that drew severe criticism from colleagues.
H.B. Brady's 1884 monograph described the foraminiferal finds of the Challenger expedition. Brady recognized 10 families with 29 subfamilies, with little regard to stratigraphic range; his taxonomy emphasized the idea that multiple different characters must separate taxonomic groups, and as such placed agglutinated and calcareous genera in close relation.
This overall scheme of classification would remain until Cushman's work in the late 1920s. Cushman viewed wall composition as the single most important trait in classification of foraminifera; his classification became widely accepted but also drew criticism from colleagues for being "not biologically sound".
Geologist Irene Crespin undertook extensive research in this field, publishing some ninety papers—including notable work on foraminifera—as sole author as well as more than twenty in collaboration with other scientists.
Cushman's scheme nevertheless remained the dominant scheme of classification until Tappan and Loeblich's 1964 classification, which placed foraminifera into the general groupings still used today, based on microstructure of the test wall. These groups have been variously moved around according to different schemes of higher-level classification. Pawlowski's use of molecular systematics has generally confirmed Tappan and Loeblich's groupings, with some being found as polyphyletic or paraphyletic; this work has also helped to identify higher-level relationships among major foraminiferal groups.
Taxonomy
The taxonomic position of the Foraminifera has varied since Schultze in 1854,who referred to as an order, Foraminiferida. Loeblich and Tappan reranked Foraminifera as a class as it is now commonly regarded.
The Foraminifera have typically been included in the Protozoa, or in the similar Protoctista or Protist kingdom. Compelling evidence, based primarily on molecular phylogenetics, exists for their belonging to a major group within the Protozoa known as the Rhizaria. Prior to the recognition of evolutionary relationships among the members of the Rhizaria, the Foraminifera were generally grouped with other amoeboids as phylum Rhizopodea in the class Granuloreticulosa.
Cavalier-Smith defines the Rhizaria as an infra-kingdom within the kingdom Protozoa.
Some taxonomies put the Foraminifera in a phylum of their own, putting them on par with the amoeboid Sarcodina in which they had been placed.
Although as yet unsupported by morphological correlates, molecular data strongly suggest the Foraminifera are closely related to the Cercozoa and Radiolaria, both of which also include amoeboids with complex shells; these three groups make up the Rhizaria. However, the exact relationships of the forams to the other groups and to one another are still not entirely clear. Foraminifera are closely related to testate amoebae.
| Taxonomy from Mikhalevich 2013 |
* Foraminifera d'Orbigny 1826
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Anatomy
The most striking aspect of most foraminifera are their hard shells, or tests. These may consist of one of multiple chambers, and may be composed of protein, sediment particles, calcite, aragonite, or silica. Some foraminifera lack tests entirely. Unlike other shell-secreting organisms, such as molluscs or corals, the tests of foraminifera are located inside the cell membrane, within the protoplasm. The organelles of the cell are located within the of the test, and the of the test allow the transfer of material from the pseudopodia to the internal cell and back.The foraminiferal cell is divided into granular endoplasm and transparent ectoplasm from which a pseudopodial net may emerge through a single opening or through many perforations in the test. Individual pseudopods characteristically have small granules streaming in both directions. Foraminifera are unique in having granuloreticulose pseudopodia; that is, their pseudopodia appear granular under the microscope; these pseudopodia are often elongate and may split and rejoin each other. These can be extended and retracted to suit the needs of the cell. The pseudopods are used for locomotion, anchoring, excretion, test construction and in capturing food, which consists of small organisms such as diatoms or bacteria.
Aside from the tests, foraminiferal cells are supported by a cytoskeleton of microtubules, which are loosely arranged without the structure seen in other amoeboids. Forams have evolved special cellular mechanisms to quickly assemble and disassemble microtubules, allowing for the rapid formation and retraction of elongated pseudopodia.
In the gamont, foraminifera generally have only a single nucleus, while the agamont tends to have multiple nuclei. In at least some species the nuclei are dimorphic, with the somatic nuclei containing three times as much protein and RNA than the generative nuclei. However, nuclear anatomy seems to be highly diverse. The nuclei are not necessarily confined to one chamber in multi-chambered species. Nuclei can be spherical or have many lobes. Nuclei are typically 30-50 μm in diameter.
Some species of foraminifera have large, empty vacuoles within their cells; the exact purpose of these is unclear, but they have been suggested to function as a reservoir of nitrate.
Mitochondria are distributed evenly throughout the cell, though in some species they are concentrated under the pores and around the external margin of the cell. This has been hypothesised to be an adaptation to low-oxygen environments.
Several species of xenophyophore have been found to have unusually high concentrations of radioactive isotopes within their cells, among the highest of any eukaryote. The purpose of this is unknown.