Foraminifera
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Allogromiida |
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 produce a shell, or test, which can have either one or multiple chambers, some becoming quite elaborate in structure. Their tests are, in the modern day, most often made of calcium carbonate (calcareous), but can also be composed of organic material, or made up of small pieces of sediment cemented together (agglutinated). Foraminiferans are exclusively marine, although they can survive in brackish conditions, and are very common in the meiobenthos, although a few are planktonic. Some are relatively large, with the largest recored specimen 19cm in size, but extant forms rarely exceed 2cm.
Clockwise from top left: Ammonia beccarii, Elphidium excavatum clavatum,
Buccella frigida, and Eggerella advena.
About 250 000 species are recognized, both living and some fossil. The form of the test is the primary means by which foraminferans are identified and classified. Fossil foraminifera have been recovered from as far back as the Cambrian period but, as a group, did not begin to extensively radiate until the Carboniferous. Nummulitic limestone, which makes up the pyramids of Egypt, is composed almost entirely of ancient foraminifera.
A few other amoeboids produce reticulose pseudopods but lack elaborate tests, and these have been considered possible relatives of the foraminiferans, grouped together as the Granuloreticulosa. The name comes from the presence of characteristic granules moving both ways along the pseudopods of foraminiferans, although some others lack them. However, with the exception of Reticulomyxa, shown by genetic studies to be a foraminiferan that has lost its shell, the other reticulose amoeboids appear to lie elsewhere. Several are placed among the Cercozoa, a diverse group closely related to the true foraminiferans, grouped together among the Rhizaria.
A number of foraminifera have developed endosymbiotic interactions with unicellular algae. Their endosymbionts span divergent lineages such as the green algae, red algae, dinoflagellates, chrysophytes, and diatoms. Some even sequester chloroplasts of the algae they feed upon, and gain food from photosynthesis in these stolen chloroplasts, a condition known as kleptoplasty.
In the nutrient-poor marine waters of the tropics, the symbiont-bearing foraminifera can grow quite large, and make a significant contribution to the overall deposition of calcium carbonate on coral reefs. Remarkably in some tropical lagoons the sands consist largely of dead foraminiferal tests.
External links
The University of California Berkeley's Museum of Paleontology (http://www.ucmp.berkeley.edu/index.html) website has a good Introduction to the Foraminifera (http://www.ucmp.berkeley.edu/foram/foramintro.html)
The Smithsonian Institution's National Museum of Natural History (http://www.nmnh.si.edu/paleo/index.html) has the largest type collection of foraminifera (http://www.nmnh.si.edu/paleo/foram/) in the world.
The University College London's micropaleontology site (http://www.ucl.ac.uk/GeolSci/micropal/foram.html) has a great overview of foraminifera, including many beautiful high-quality SEMs.de:Kammerlinge fr:Foraminifera nds:Foraminifera pt:Foraminifera