The Foraminifera (or "forams") are single-celled eukaryotes, or protists. Foraminiferans are a very large group, in two senses. First, they are physically quite sizable for single-celled organisms; most species can be seen by the naked eye, and some may be several centimeters across. Second, there are many species in the group: more than 4,000 genera, living and fossil.
How do we know a cell is a foram?
All forams possess a unusual morphological feature: reticulopodia, which are a complex network of slender, microtubule-based pseudopodia. Forams use the dynamic and agile reticulopods to build their distinctive shells (or tests), find and eat prey, move, and structure the environment around them. The reticulopodial network of a foram can easily explore a volume of sediment or seawater two orders of magnitude larger than the cell body.
What do forams do with their tests?
The test is an important component of the survival strategy of many foraminiferans. It can be made of many materials: it might be nothing more than a thickened glycocalyx, or it may be constructed from flexible organic materials, sand grains or other things the cells pick up from the environment ("agglutinated" tests), or minerals like calcium carbonate or aragonite. For a large collection of images of foram tests, please visit the star*sand image gallery.
The shape of the test is also highly variable, and some of the shapes seem to help forams eat. For example, so-called planktonic (floating) species, all of which have thin, calcium-carbonate based tests, trap air within the test to make themselves buoyant. Many species also create long spines for support of the reticulopodia while floating. Benthic species, which live on the seafloor or other solid substrates, have developed many specialized test structures. One agglutinated genus, Notodendrodes, creates elaborate tree-like structures that lift the reticulopodia as much as two centimeters above the seafloor. It is thought that this may help the foram trap floating organic matter. Several groups that prey on diatoms, such as members of the genus Nonionella, develop stalactite-like "teeth" near the aperture, which is the opening in the test from which the reticulopodia emerge. These structures may help the foraminiferan break open the diatom frustule.
How long have there been forams?
For a very long time. Forams evolved before fish or dinosaurs. There are forams throughout the fossil record, all the way back to the Cambrian (~550 million years ago), and DNA-based analyses suggest that they may have existed for 800 million to 1.2 billion years.
They are also one of the first protist groups to have been studied by scientists: the 17th century microscopists Robert Hooke and Anthony van Leeuwenhoek both left sketches of foram species in their notebooks. Interestingly, most early researchers thought that forams were tiny cephalopods. It wasn't until 1835 that Dujardin demonstrated that forams were protozoans rather than animals.
Where are forams found?
Most known foram species are marine, but several have been identified in freshwater environments. Environmental DNA analyses suggest that there may be many more freshwater forams, but little is known about them.
Individual species have particular preferred habitats. For example the foram Miliammina fusca is famous for living in water that is only somewhat salty, like a salt marsh, and species like Sorites orbiculus are found in warm shallow water environments like coral reefs. The presence of different species' distinctive tests in fossilized sediment helps geologists figure out what climates were like in the past.