The star*sand project catalogs information about the Foraminifera, which is a group of 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.
The Foraminifera are members of a slightly larger group of protists, known as the Granuloreticulosea. More than 99% of all known granuloreticulosean species are forams, but the group also contains a few species, such as the unusual freshwater "amoeba" Reticulomyxa filosa, that were not previously thought of as being related to foraminiferans. All granuloreticuloseans 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.
The test is an important component of the survival strategy of many foraminiferans. The test may be nothing more than a thickened glycocalyx, or it may be constructed from flexible organic materials, sand grains or other agglutinated material, or calcium carbonate or aragonite. The exact details of composition, such as the crystal structure and magnesium content of carbonate tests, have been shown to be useful tools for the proper classification of many species. The shape of the test is also highly variable, and specialized structures built into it allow innovative feeding strategies for some foraminiferans.
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 extremely diverse test morphologies and compositions. 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.
Foraminiferans are an ancient group; there is fossil evidence for them throughout the fossil record back to the Cambrian (~550 million years ago), and molecular 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.
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. Species have particular preferred habitats, and the presence of different species' distinctive tests in fossilized sediment provides important paleoenvironmental information.