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The Foraminifera, often called forams, are an ancient taxonomic group (sometimes considered a phylum, sometimes subphylum or class of Kingdom Rhizaria) of amoeboid, single-celled Eukaryotes.  They have a fossil record back to the earliest Cambrian, 570 million years ago.  Most Foraminifera live in benthic environments (ocean floors) although planktonic forams appeared in the fossil record starting about 170 million years ago and about 1% of known extant species live in the water column.  Foraminifera are traditionally considered marine organisms, indeed, in many marine environments they are the most abundant shelled organism.  Expert scientists estimate approximately 6000 described and about 1500 yet unknown marine foram species (Appeltans et al. 2012).  However, recent molecular studies challenge the idea that Foraminifera are almost entirely marine, through descriptions of multiple new foram groups representing multiple colonizations of freshwater environments (Holzmann et al. 2003), and new species that suggest diverse and abundant foraminiferan life in terrestrial soils (Lejzerowicz et al. 2010).

Most forams have an external shell, the composition and shape of which is the primary determiner of foraminiferan taxonomy.  Because the organism’s protoplasm often covers it, the shell is referred to as a test.  Foram tests are composed of one of three different materials: secreted calcium carbonate or silica; secreted polysaccharides; or glued-together particles, such as quartz sand grains, sponge spicules, or other available building blocks that are the right size.  In most species the test is multi-chambered, and Foraminifera get their name from the tiny openings (foramina) between the chambers.  As they grow, Foraminifera build on more chambers, expanding to live in all of them except the one or two most recently built.  A broad diversity of chamber arrangement and many types and placements of aperture (the terminal opening to the outside of the test) define the test morphology, which can be quite beautiful and complex.  Some resembling mollusk and bivalve shells caused early taxonomists to believe foraminifera shared a lineage with the mollusks (Korsun et al. 2001; Olney 2002; Wetmore 1995).

While most foraminiferan species have a maximum size between 0.05-0.7 mm, some species grow much larger.  One of the largest extant species, Cycloclypeus carpenteri, (Nummulitidae) measures over 10 cm across its disc-like test; another extant species, Syringammina fragilissima (Xenophyophorea) is documented up to 20 cm and looks like a porous beach ball made of sand.  The fossil record shows a diversity of even larger species.  Even the largest Foraminifera are single-celled, however they may have multiple nuclei (Krüger 1997; Pawlowski 2003; Song et al. 1994).

Some of the largest species form symbiotic associations with algal cells.  Xenophyophoreans are thought to cultivate bacteria for food.  Other species are suspension feeders, opportunistic omnivores that eat detritus, smaller protists and even multicellular organisms from the substrate.  To reduce issues of high with surface area to volume ratios, forams have thin pseudopodia (called reticulopodia) which stretch out of tiny pores in the test, to dramatically extend their surface area.  Some stretch their long pseudopodia to form a large external feeding net, and they can also use their pseudopodia to burrow through the sediment or to attach themselves to rocks or plants efficiencies (Korsun et al. 2001; Olney 2002; Wetmore 1995).  

Foraminiferan species are found all over the world and species tend to be particular to their specific environment, for example, deep sea trenches, intertidal pools, coral reefs, brackish estuaries.  They can be extremely abundant, in some places in the deep sea the sediment on the sea floor is composed almost entirely of shells from planktonic species.  Forams are an important basic link in the marine food chain, as food for small invertebrates and fish.  Because they are so wide-spread and ancient in origin, fossil Foraminifera are useful for analyzing changes throughout time in ocean environments and temperatures and predicting climate changes.  They are used as bioindicators of the health of marine environments, including coral reefs.  The oil industry analyses forams deposits as they give precise indicators of age and conditions of rock formation important in guiding drilling for opimum oil well efficiencies (Korsun et al. 2001; Olney 2002; Smithsonian NMNH 2013; Wetmore 1995). 

A recent study compared Foraminiferan species diversity in deep sea environments (>1500m) around the world and found that species numbers, composition and genetic makeup was far more consistent among deep sea sampling locations than numbers and composition of foram species in different shallow habitats (<200 m), revealing greater stasis of species over space and time at depth.  Along with observations that deep sea species have a longer duration in the fossil record than do shallow species, they propose that a model where new species evolve in shallower areas and then migrating to deeper seas seems to be consistent with marine foram samplings (Buzas et al. 2013).

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