Foram Test Construction

As we mentioned on the previous web page What is a Foram? forams are single-celled organisms with a protective test (shell). What is the test made from and how does the foram make it? There are two basic kinds of tests, soft tests made from an organic material called tectin (a complex carbohydrate plus protein material) and hard tests made from minerals. Hard, mineralized tests are much more common.

The evolution of a hard outer shell likely provided forams with additional protection against predators, physical damage, chemical changes, and as a means to control buoyancy. Mineralized foram tests are sorted into four major classes based on the method the foram used to construct them.

  1. Agglutinated
  2. Microgranular
  3. Porcellaneous
  4. Hyaline

1. Agglutinated

This test style was the first to evolve (more than 500 million years ago) and consists of tiny cemented grains gathered from the sea floor. Particles are glued to a tectin base with an organic, calcite or iron-bearing cement. Remember the organic tests we first talked about? This is simply an organic test with particles glued on top. At times these particles appear to have been gathered at random and at other times the grains are similar in size, composition or shape.

Bigenerina nodosaria, Agglutinated test from Puerto Rico, (SEM) image -120X magnification (Credit: Heidi Crevison Souder; used by permission from Foraminifers as Bioindicators website, University of South Florida, St. Petersburg) (Photo courtesy Kristin Gribble, WHIO)

2. Microgranular

Test walls are composed of tiny, closely-packed grains of calcium carbonate, but with no obvious cement. For many years this group was lumped into the agglutinated class because it was thought that these tests were built from scavenged grains. In fact, some species do have some agglutinated grains thereby adding to the confusion. However, scientists are now convinced this group of forams actually secrets the tiny grains.

Parafusulina sp. Sample collected near Marathon, Texas. Black scale bar is 1 cm (~0.4 inches) indicating that the larger specimen is slightly less than one inch. This foram species was abundant at the end of the Paleozoic geologic time period. (Photo credit: J.H. Lipps, University of California, Davis. Copyright permission granted by Cushman Foundation for Foraminifera Research, Inc.)

3. Porcellaneous

This test is composed of microscopic calcite needles formed inside the foram and then moved to the outside of the cell. Tests are generally white in color, but may be pink or colorless with an appearance similar to porcelain, hence the name "porcellaneous". Test walls are generally built of three layers - a central layer sandwiched between two outer layers. Calcite needles in the middle layer are arranged in a poorly-organized mesh-like network, while needles in the outer layers are arranged parallel to one another, providing the outside of the test with a smooth surface.

Discorbis mira, Hyaline test from the Florida Keys, (SEM) image -150X magnification (Credit: Heidi Crevison Souder; used by permission from Foraminifers as Bioindicators website, University of South Florida, St. Petersburg)

4. Hyaline

To build this type of test, the foram secrets one or more layers of calcium carbonate over soft tectin material that serves as a template for the final shape. Rhizopodia, thin organic filaments connected to the foram, assist with hyaline test construction by delivering liquid calcium carbonate to different patches of the exposed tectin surface. The patches of calcium carbonate eventually merge and harden into a solid shell. Hyaline tests are often translucent to light, and commonly have a glassy or milky iridescent appearance.

Reussella, Hyaline benthic foram from Puerto Rico, SEM image 150X (Credit: Heidi Crevison Souder; used by permission Foraminifers as Bioindicators website, University of South Florida, St. Petersburg)

Related Links

Foraminifers as Bioindicators - Beautiful scanning electron microscope images of forams and information relating to the use of forams in monitoring coral reef environments.


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