Foraminifera are a large group of amoeboid protists with reticulating pseudo pods fine strands of cytoplasm that branch and merge to form a dynamic net. They typically produce a test or a shell, which can have either one or multiple chambers, some becoming quiet elaborate in structures. These shells are made up of calcium carbonate (CaCO3) or agglutinated sediment particles. About 275000 species are recognized, both living and fossil. They are usually less than 1 mm in size, but largest recorded specimen is about 19 cm. Foraminifera have typically been included in the group Protozoa. The form and the composition of test is the primary means by which foraminifera are identified and classified. Foraminifera are abundant all over the ocean and found in all marine environments. They either live on the sea bottom (benthic) or float in the upper water column (planktonic). A few benthic species have been recorded from terrestrial environments including ground water.
The generally accepted classification of foraminifera is based on that of Loeblich and Tappan (1964). The order Foraminiferida (informally foraminifera) belongs to the:
Of the approximately 4000 living species of foraminifera the life cycles of only 20 or so are known. The life cycle is characterized by an alteration between two generations: a gamont generation which reproduces sexually, and an agamont generation which produces asexually.
The life cycle may be completed within a year in tropical latitudes; it can take two or more years at higher latitudes. An asexually produced haploid generation commonly forms a large proloculus (initial chamber) and is therefore termed megalospheric. Sexually produced diploid generations tend to produce a smaller proloculus and are therefore termed microspheric.
Foraminifera are small, abundant, widely distributed and often extremely diverse and have intricate morphology in which evolutionary changes can be readily traced. As previously mentioned, foraminifera have been utilised for biostratigraphy for many years, and they have also proven invaluable in palaeoenvironmental reconstructions most recently for palaeoceanographical and palaeoclimatological purposes.
Planktonic foraminifera have become increasingly important biostratigraphic tools, especially as petroleum exploration has extended to offshore environments of increasing depths. The first and last occurrence of distinctive "marker species" from the Cretaceous to Recent (particularly during the Upper Cretaceous) has allowed the development of a well-established fine scale biozonation. Since CaCO3 is affected by dissolution in elevated pCO2 conditions, foraminifera are widely used in ocean acidification studies.
Benthic foraminifera have been used for palaeobathymetry since the 1930's and modern studies utilise a variety of techniques to reconstruct palaeodepths. Variations in the water temperature inferred from oxygen isotopes from the test calcite can be used to reconstruct palaeoceanographic conditions by careful comparison of changes in oxygen isotope levels as seen in benthic forms (for bottom waters) and planktonic forms(for mid to upper waters).
Foraminifera from Kongsfjorden (Arctic Ocean), as described in Shetye et al 2011