• Unicellular
• Fine pseudopodia that can branch and rejoin
• A shell made of stuck together particles or secreted minerals

Forams

The Foraminifera are one of the groups of protists that produce long, thin extensions of the cell (fine pseudopodia) to collect food and to drag themselves across surfaces. There is no general agreement about where the foraminfera should “start” and here only the “classic” forams are dealt with. Forams have a shell which may have one chamber, or several chambers of increasing size. These shells come in all sorts of shapes ranging from discs to spirals to collections of spheres to rambling irregular structures. In some forams the shell is made of particles such as sand grains or sponge spicules stuck together with an organic cement. These forams may be several centimetres across, among the largest unicellular protists of all. Most forams have shells made more like the shells of molluscs, with a secreted mineral layer, but these only get up to a few millimetres across. Under the microscope beautiful patterns of tiny holes, ridges and spines may be seen. Most forams are somewhere between white and brown, if coloured.

Forams are found in basically all marine environments. Planktonic forms tend to be globular and buoyant. A diverse community of occasionally large and often flattened forams live on surfaces, including those of large seaweeds and animals (e.g. mollusc shells). Most are slowly mobile although a few cement themselves down. Other forams live within the top layers of sediments, some at great depths and often form a significant proportion of the biomass in these environments. Surface dwelling forms in particular can be extremely abundant on coral reefs. On many reefs, foraminiferan shells make up most (up to 90%) of the sediments and are consequently thought to be an important factor in shaping the reef environment.

Forams mostly eat microbes that they encounter with their pseudopodia (either actively or passively). Surface-dwelling forams tend to eat diatoms and bacteria. Some forams can eat small animals and objects such as fish eggs. It is thought that surface and sediment-dwelling forams may take up a lot of dissolved organic matter. Many forams, especially those on coral reefs, have photosynthetic protist symbionts which supply them with energy. In some species thesymbionts alone are able to supply more energy than the foram needs to live. Other forams can ‘farm’ the chloroplasts (photosynthetic structures) of their prey for several weeks.

Most foraminifera are probably eaten by grazing or sediment-scouring animals such as snails, sea-cucumbers, polychaete worms etc. Elephant-tusk shells (Scaphopods) seem to be specialists at eating sediment-dwelling forams. Occasionally large forams will eat small ones, or fish will eat them by accident with seaweeds. The importance of forams as a food source is not well known, partly because the shells break down slowly (even if they have been eaten) and it can be difficult to distinguish a between living and dead shells.

Individual forams can live a long time for protists, up to a couple of years. Species with multichambered shells add on larger and larger chambers as they grow. Forams reproduce by breaking themselves up into many small cells without shells, which usually grow flagella and swim away. It is thought that most forams have alternating sexual and asexual generations. In other words, every second generation swimming cells from different parents pair up and fuse before growing a shell. Adults form sexual and asexual generations have slightly different body forms.

Forams are generally regarded a free-living and solitary. Many seem to like living on surfaces of particular types of seaweeds or animals. A few are parasites, mostly on other forams. As mentioned above, many forams have internal photosynthetic symbionts. Many of these symbionts are diatoms, but dinoflagellates, red algae and green algae are all common as well.

Forams are very plentiful and fossilise well. Entire deposits of sedimentry rocks can be composed of old foram shells. Well preserved foram fossils are of great use to geologists in determining ages of samples, and locating fossil fuel deposits etc. Otherwise people interact with forams by making then into necklaces or, occasionally, enormous monuments.