foraminiferans

Foraminiferans, commonly called forams, are a large group of single-celled eukaryotes in the phylum Foraminifera, within the Rhizaria. Most species are marine, with some inhabiting freshwater, soil, or brackish environments. They range from microscopic to several millimeters in size and are noted for their tests, or shells, which are often chambered and form the main body wall.

Tests are typically made of calcium carbonate, though some species construct their shells from organic material or by cementing together sediment grains (agglutinated tests). The test openings accommodate reticulopodial pseudopodia, a network of thin, branched projections extended for feeding and sometimes locomotion. Through these pseudopodia, forams capture bacteria, algae, detritus, and other small particles.

Most forams are benthic, living on or within marine sediments, while planktonic species float in the upper water column. Some species harbor symbiotic algae, which provide photosynthates. Reproduction varies widely and can be both sexual and asexual; several species exhibit complex life cycles with multiple developmental stages.

Fossil foraminifers have an extensive record dating back to the early Cambrian and are a major component of marine sediments. They are widely used in biostratigraphy to date rocks and in paleoceanography to infer past temperatures, salinity, and productivity. Large extinct groups, such as fusulinids, are important Paleozoic index fossils. Foraminifers remain a key proxy in studying Earth’s history and marine ecosystems.