Overview

Brief Summary

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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Comprehensive Description

Description of Foraminiferida

Granuloreticulosea, with shells (loricae); among the more abundant and most conspicuous protozoa in most marine and brackish water habitats; shells may be durable and are an important component of marine sediments and fossilize well; good geological record extending back to the Cambrian, planktonic more commonly benthic; large protozoa (range 60 µm-12 cm), life-spans often proportional to their sizes (days-years); some monothalamic species reproduce by binary fission, budding, or cytotomy; most have complex life cycles, involving both sexual and asexual reproduction; there may be morphological differences between the sexual (gamont) and asexual (agamont, schizont) phases of the life cycles; the materials used in the test (e.g. organic, agglutinated, various types of mineralized calcareous), the geometry of chambers (in multichambered species) and their construction, the form of the aperture(s) are some of the important characters used in foraminiferan identification and classification.; planktonic foraminifera are often hosts for endosymbiotic algae.
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Physical Description

Diagnostic Description

Diagnostic Apomorphies of Foraminifera

The diagnostic apomorphies of Foraminifera include both morphological and molecular characters.  One of the most recognizable apomorphies that characterizes the derived foraminiferal subclades, is the presence of a test, or shell, that grows by terminal addition, either by the addition of discrete chambers (resulting in a multi-chambered test), or by accretionary growth whereby new material is added to the test at the apertural margin of a single-chambered tubular test (Goldstein 2002, Hottinger 2000). 

Additional apomorphies can only be observed in living taxa, and thus serve to distinguish Foraminifera from other crown clades (such as Gromida, Haplosporidia, Plasmodiophorida, and Radiolaria), within the more inclusive clade Rhizaria. The small subunit ribosomal RNA sequences of foraminiferans possess a unique helix in Domain III that is up to 350 nucleotides in length, and lies between Helices 41 and 39 in Domain III, and (Bowser et al. 2008: Fig. 5.2, Habura et al. 2004: Figs. 2, 3). The gene sequences that code for the two foraminiferal actin paralogs contain twenty unique spliceosomal introns (Flakowski et al. 2006: Fig. 2).

Foraminiferans possess a type 2 ß-tubulin isoform that is highly divergent relative to other eukaryotes (Habura et al. 2005: Figs. 3, 4). The reticulopodia of foraminiferans possess motility organizing vesicles (elliptical, fuzzy-coated vesicles) (Bowser and Travis 2002: Pl. 2, figs. 1, 2), and tubulin-containing helical filaments (Bowser and Travis 2002: Pl. 2, fig. 3; Pl. 3, Habura et al. 2005: Fig. 1).

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Ecology

Associations

Known predators

Foraminifera (Foraminifera (Formaifera??)) is prey of:
Paralichthyes albigutta
Strongylura marina
Urophycis floridana
Prionotus scitulus
Prionotus tribulus
Gobiosoma robustum
Microgobius gulosus
Lagodon rhomboides
Leiostomus xanthurus
Syngnathus scovelli
Hippocampus zosterae
Laridae
Cyprinodon variegatus
Anatidae
Fundulus confluentus
Fundulus similis
Adinia xenica
suspended particulate carbon
Sabellidae
Serpulidae
Zoarces viviparus
Pomatoschistus minutus
Pomatoschistus microps
Pleuronectes platessa
Platichthys flesus
Crangon crangon
Retusa obtusa

Based on studies in:
USA: Florida (Estuarine)
Scotland (Estuarine)

This list may not be complete but is based on published studies.
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Known prey organisms

Foraminifera (Foraminifera (Formaifera??)) preys on:
phytoplankton
bacterioplankton
Microprotozoa
POM

Based on studies in:
USA: Florida (Estuarine)
Scotland (Estuarine)

This list may not be complete but is based on published studies.
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Evolution and Systematics

Evolution

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 where their evolutionary rate slows seems to be consistent with marine foram samplings (Buzas et al. 2013).

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Names and Taxonomy

Taxonomy

Origin of the Name Foraminifera

The name Foraminifera is a latinization of the French name Foraminifères, first applied to a group of “microscopic cephalopods” by Alcide d’Orbigny in 1826. The name is derived from the Latin foramen (hole) and -fer (bearing), so called because the aperture and openings that connect successive chambers (intercameral foramina) in a multi-chambered foraminiferal test were thought to be homologous to the siphon in the chambered Nautilus. Earlier researchers considered foraminiferans to be molluscans and classified under them under various names, such as Polythalamiis (Breyn 1732:1), Nautilus (Fichtel and Moll 1798:12), Polythalamacea (de Blainville, 1824:175) and Asiphonoidea (de Haan, 1825:29). Dujardin’s (1835a-d) discovery that foraminiferans were not cephalopods, but single-celled organisms, lead to their reclassification as rhizopods (Rhizopodes or Rhizopoda), and the reinterpretation of their “tentacles” as pseudopodia, or threadlike extensions of the cell’s sarcode or cytoplasm (Dujardin, 1841).  Ehrenberg (1838) used the name Polythalamia for the group, but viewed foraminiferans and other single-celled eukaryotes as organisms complete with miniature circulatory, digestive, excretory, nervous, motor, and reproductive systems (Churchill 1989).

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