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What are Foraminifera?

We are presenting a new micropaleo website devoted to Arctic foraminifera. The page contains circa 50 Scanning Electron Microscope (SEM) images for 18 common Arctic foram species. We’ll keep uploading new images and plan to expand on distribution charts and data tables. Comments and suggestions are greatly appreciated.

What are Foraminifera?

Foraminifera (foraminifers or, informally, just forams) are single-celled amoeboid protists. Modern taxonomies rank the group as a phylum or subphylum. The principal characteristics of the taxon are (1) threadlike anastomosing pseudopodia bearing granules that reveal constant bidirectional streaming of the cytoplasm (granuloreticulopodia); (2) the life history characterized by an alteration of sexual and asexual generations with meiosis associated with the asexual reproduction—a feature unique in heterotrophic eukaryotes; and (3) the presence of a test (shell). The test can be composed of biogenic calcium carbonate (calcareous), cemented foreign particles such as quartz grains (agglutinated) or an organic theca composed of polysaccharides. Some foraminifera have lost the test; the existence of a naked foraminiferal progenitor has not yet been identified. The phylogenetic affinity of Foraminifera is under debate; based on molecular evidence, they do not seem to have close relatives except possibly for some unstudied naked and testate rhizopods. Forams are abundant all over the ocean. They either live on the sea bottom (benthic) or float in the upper water column (planktonic). Of the estimated 4000 species living today, 40 are planktonic. A few benthic species have been recorded from terrestrial environments including ground water.

The size of the foraminiferal test typically ranges from 0.05 to 0.5 millimeters although some forams may be as large as several centimeters with a recorded maximum of 18 centimeters in diameter. Foraminifera remain single-celled despite the large sizes they reach. Among the protozoa, even the smaller forams stand husky and hence they face the problem of sustaining the surface/volume ratio. And this is where the reticulopodia, a highly efficient and multifunctional organelle, come into play. They not only provide surface for respiration, but also perform feeding, locomotion, test building, metabolite release, adhering, etc. These cytoplasmic threads, the thinnest of which are less than one micrometer across, may reach a distance of up to 25 times their test diameter. The agile network of foraminiferal pseudopodia covers most of the ocean bed. Nobody has calculated yet the total surface of foraminiferal cell membrane, but it is likely to be comparable to that of the bacteria in the ocean.

Foods of the foraminifera are variable: dissolved free amino acids, bacteria, unicellular algae and even metazoans, such as copepods. In tropical euphotic waters, where trophic resources are highly competitive and sunlight is plentiful, several families of benthic and planktonic foraminifera harbor unicellular algae. The latter provide the foraminiferal hosts with carbohydrates. Unlike the famous symbiont-bearers—the scleractinians and giant clams, which acquire only dinoflagellates—foraminifera host a variety of photoautotrophs—dinoflagellates, diatoms, green algae, red algae and eventually chrysophytes and prymnesiophytes. Owing to the diversity of endosymbionts—or, more precisely, their photopigments—the symbiont-bearing foraminifera are successful to utilize a wider range of the light spectrum and water depths.

Foraminifera are game for many small marine invertebrates and fish; however, there seem to be rather few groups specialized on forams, the best known of which are the scaphopod mollusks. Many scientists propose, that the foraminifera are a key group in the marine food chain: they feed on small prey mostly inaccessible for the macrofauna and are prey for the latter. Predation by macrofauna is not the only way the ecosystem utilizes foraminiferal production. The cytoplasm of survived adults is ultimately released as the offspring and thus become available to suspension-feeders or is simply included in the pool of organic detritus.

Forams have been found in rocks of marine origin since at least the Cambrian (~550 million years ago). Since then, forams have radiated and evolved to make up approximately 40,000 species in the rock record. Planktonic forams appeared ~200 million years ago. These sea-surface dwellers evolved faster than most of their benthic counterparts making them more suitable as biostratigraphic markers. Both planktonic and benthic species are sensitive to changes in food availability as well as physical environmental parameters, such as salinity and temperature. Because of this sensitivity, forams are useful indicators of environmental change, both on local and global scales.