The Foraminifera, or forams for short, are a large group of amoeboid protists with reticulating pseudopods, fine strands that branch and merge to form a dynamic net. They typically produce a shell, or test, which can have either one or multiple chambers, some becoming quite elaborate in structure. About 250,000 species are recognized, both living and fossil. They are usually less than 1 mm in size, but some are much larger, and the largest recorded specimen reached 19 cm.

The Foraminifera are closely related to the Cercozoa and Radiolaria, 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.

Living forams

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Modern forams are primarily marine, although they can survive in brackish conditions. A few species survive in fresh water and one even lives in damp rainforest soil. They are very common in the meiobenthos, and about 40 species are planktonic. The cell is divided into granular endoplasm and transparent ectoplasm. The pseudopodial net may emerge through a single opening or many perforations in the test, and characteristically has small granules streaming in both directions.

The pseudopods are used for locomotion, anchoring, and in capturing food, which consists of small organisms such as diatoms or bacteria. A number of forms have unicellular algae as endosymbionts, from diverse lineages such as the green algae, red algae, golden algae, diatoms, and dinoflagellates. Some forams are kleptoplastic, retaining chloroplasts from ingested algae to conduct photosynthesis.

The foraminiferan life-cycle involves an alternation between haploid and diploid generations, although they are mostly similar in form. The haploid or gamont initially has a single nucleus, and divides to produce numerous gametes, which typically have two flagella. The diploid or schizont is multinucleate, and after meiosis fragments to produce new gamonts. Multiple rounds of asexual reproduction between sexual generations is not uncommon.

Tests

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The form and composition of the test is the primary means by which forams are identified and classified. Most have calcareous tests, composed of calcium carbonate, which generally takes the form of interlocking microscopic crystals, giving it a glassy or hyaline appearance. In other forams the test may be composed of organic material, made from small pieces of sediment cemented together (agglutinated), and in one genus of silica. Openings in the test, including those that allow cytoplasm to flow between chambers, are called apertures.

Tests are known as fossils as far back as the Cambrian period, and many marine sediments are composed primarily of them. For instance, the nummulitic limestone that makes up the pyramids of Egypt is composed almost entirely of them. Forams may also make a significant contribution to the overall deposition of calcium carbonate in coral reefs.

Genetic studies have identified the naked amoeba Reticulomyxa and the peculiar xenophyophores as foraminiferans without tests. A few other ameoboids produce reticulose pseudopods, and were formerly classified with the forams as the Granuloreticulosa, but this is no longer considered a natural group, and most are now placed among the Cercozoa.

Uses of forams

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Because of their diversity, abundance, and complex morphology, fossil foraminiferal assemblages can give accurate relative dates for rocks and thus are extremely useful in biostratigraphy. Before more modern techniques became available, the oil industry relied heavily on microfossils such as forams to find potential oil deposits. Common fossil foraminifera include Camerina, Dentalina, Elphidium, Endothyra, Globigerina, Hyperammina and Triticites.

Fossil foraminifera are also useful in paleoclimatology and paleoceanography. They can be used to reconstruct past climate by examining the stable isotope ratios of oxygen; see δ18O. Geographic patterns seen in the fossil records of planktonic forams are also used to reconstruct ocean currents.

For the same reasons they make useful biostratigraphic markers, living foraminiferal assemblages have been used as bioindicators in coastal environments, including indicators of coral reef health. Because calcium carbonate is subsceptible to dissolving in acidic conditions, Foraminifera may be particularly affected by changing climate and ocean acidification.

External links

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• The University of California Museum of Paleontology website has a good Introduction to the Foraminifera
• The star*sand project, part of the micro*scope system of websites, is a cooperative database of information about foraminifera.
• The Smithsonian Institution's National Museum of Natural History has the largest type collection of foraminifera in the world.
• Researchers at the University of South Florida developed a system foraminifera for monitoring coral reef environments.
• University College London's micropaleontology site has a great overview of foraminifera, including many beautiful high-quality SEMs.
• A glossary of terms used in foraminiferal research.
• CHRONOS has several foraminifera resources . See also the foram taxon search page and the micro-paleo section.
• eForams is the web site focused on foraminifera and new results on modeling of foraminiferal shells.

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