Foraminifers

Characteristics

Modern foraminifers are primarily marine, although they can survive in brackish conditions, and a few species survive in fresh waters. They have two modes of life, planktic (marine floaters) or benthic (seafloor dwellers). About 275,000 living and fossil species have been recognized. The foraminifers are usually less than 1 millimeter in size, but some are much larger. The largest recorded specimens reached 19 centimeters. They are closely related to the Radiolaria, which also includes amoeboids with complex shells.

The foraminifers typically produce a shell, or test, some becoming quite elaborate in structure. The test can have either one or multiple chambers, with the multichambered forms being more frequent. Adjacent chambers are separated by septa. Most have calcareous tests, composed of calcium carbonate (calcite, aragonite), but others may have organic, agglutinated, or silica tests. The organic tests are composed of protinaceous mucopolysac-charide. The term agglutinated refers to tests formed from foreign particles (coccoliths, diatoms frustules, microfragments of mollusk and echino-derm, sponge spicules, quartz, mica, magnetite, or garnet), glued together with a variety of cements (organic, calcareous, or siliceous). The calcareous and siliceous tests are secreted. Calcareous tests may be again subdivided into three major types: microgranular (composed of equidimensional sub-spherical calcite crystals smaller than 5 microns), porcellaneous (composed of thin inner and outer veneers enclosing a thick middle layer of crystal laths), and hyaline (composed of lamellas of calcite crystals larger than 5 microns, which may be equidimensional or radiate).

The name foraminifera is derived from the foramen, the connecting hole trough the septa between each chamber. The living foraminifers have reticulating pseudopods (reticulopodes) used for locomotion, [Page 528]for anchoring, and in capturing food. The opening by which the pseudopods access the outside is called the aperture. A bidirectional cytoplasmic flow along these reticulopodes carries granules, which may consist of mitochondria, digestive vacuoles, vacuoles containing waste product, and even symbiotic dinoflagellates. For this reason, the pseudopods of foraminifera are also called grano-reticulopodes. Foraminifers normally feed on diatoms, bacteria, and copepods. Many species have unicellular algae as endosymbionts, such as dinoflagellates, green algae, red algae, golden algae, and diatoms. Some foraminifers are kleptoplastic, retaining chloroplasts from ingested algae to conduct photosynthesis.

Uses in Science and Technology

The reason why foraminifers are so useful in micropaleontology is that their tests are easily fos-silizable and very abundant in the fossil record. Because of their diversity, abundance, and complex morphology, fossil foraminiferal assemblages are useful not only in biostratigraphy and geo-chronology, but also in paleoenvironmental reconstruction, paleoclimatology, and paleoceanography. The oil industry relies heavily on foraminifers in finding potential oil deposits.

Classification

The traditional classification of foraminifera is based on the composition and morphology of their tests. The primary characters used in these classifications are mainly the following: wall composition and structure, chamber arrangement, the shape and position of the apertures, and surface ornamentation. At species level, other morphologic features are also used, such as the test shape, chamber shape, number of chambers, and so on. The commonly accepted classification of the foraminifera is based on that of the 1987 classification by Loeblich and Tappan, although molecular data and cladistic analysis suggest that this classification may vary substantially in the future. Foraminifers were initially considered to be an order in the class Rhizopoda, phylum Protozoa, of which 12 suborders were distinguished. Today, they are considered as class Foraminiferea, of which 14 orders are distinguished: Allogromiida, Astrorhizida, Lituolida, Trochamminida, Textulariida, Fusulinida, Spirillinida, Carterinida, Miliolida, Lagenida, Robertinida, Globigerinida, Buliminida, and Rotaliida.

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