(To complete all classifications ETI has added the Kingdom and the Phyla of all the different taxa treated on this DVD-ROM without higher classification descriptions. Texts from Lynn Margulis and Karlene V. Schwartz, Five Kingdoms. CD-ROM. © 2002 ETI / Freeman & Co Publishers).
[Editor's note: in the book South Atlantic Zooplankton the Forminifera were classified within the "old" Phylum Sarcomastigophora instead of the Granuloreticulosa].
Granuloreticulosans are easily defined: these organisms bear reticulopods, cells that fuse to form networks in which bidirectional (two-way) streaming can be seen. Two classes make up this phylum: Reticulomyxida and Foraminifera (from the Latin foramen, little hole, perforation, and ferre, to bear). By far the better-known class, Foraminifera — affectionately known as forams — have pore-studded shells, or tests. In contrast, reticulomyxids are “snot shaped” slimy nets of messy, bactivorous soft masses that lack shells. Very few have been studied. These “naked forams,” reticulomyxids, are the presumed ancestral group; therefore, with their discovery, the former Phylum Foraminifera was renamed.
Forams are exclusively marine organisms. The smallest are some 10 mm in diameter, and the largest ones, visible to the naked eye, grow to several centimeters in diameter. The majority are tiny and live in sand or mud or attached to rocks, algae, or other organisms. Two families of free-swimming modern planktonic forams (Globigerinidae and Globorotalidae) are very important in the economy of the sea as food for many marine animals.
The pore-studded tests of forams are composed of organic materials, often reinforced with minerals. Some are made of sand grains; most are neatly cemented granules of calcium carbonate deposited from sea water. Some forams, by mechanisms that are still unknown, choose echinoderm plates (Subphylum Urochordata) or sponge spicules (Phylum Porifera) to construct their tests. The test and the organism itself may be brilliantly colored—salmon, red, or yellow brown. Whereas the simplest forams have single-chambered tests, most are multichambered. A typical test looks like a clump of blobs of partial spheres. Pores in the test permit thin cytoplasmic projections, the microtubule-reinforced filopodia, to emerge. Anastomosing (linked-up) filopodia form nets called reticulopodia. The filopodia are used for feeding, swimming, and gathering materials for tests. Forams are omnivorous: they eat algae, ciliates (Phylum Ciliophora), actinopods (Phylum Actinopoda), and even nematodes and crustacean larvae. Many forams, probably most that live in shallow water, harbor photosynthetic symbionts—dinomastigotes (Phylum Dinomatigota), chrysomonads (Phylum Chrysomonada, planktonic), and diatoms (Phylum Diatoms or Bacillariophyta).
Although some foram genera (for example, Textularia) have been seen reproducing only by asexual budding into multiple offspring, others that have been well studied—some dozen species—show a remarkably complex life cycle. The known cycles are variations on the theme of Rotaliella. Meiosis takes place in the agamont, a fully adult diploid organism that produces and releases smaller haploid forms called agametes. These agametes disperse and grow by mitotic cell divisions into a second kind of adult, called gamonts. The gamonts reproduce sexually, by fusion of haploid nuclei, to produce diploid offspring, which are agamonts.
The alternation of the diploid agamont and haploid gamont generations is obligatory in the forams that have been studied, just as alternation of generations is obligatory in plants, such as mosses (Phylum Bryophyta) and ferns (Phylum Filicinophyta). In fact, forams are the only heterotrophic protoctists that alternate morphologically distinct free-living adult generations. What complicates matters is that, unlike other organisms except ciliates (Phylum Ciliophora), forams show a striking nuclear dimorphism. The agamonts of Rotaliella roscoffensis, for example, contain four diploid nuclei. Three of these nuclei, the generative nuclei, reside in a chamber separate from that in which the larger somatic nucleus remains. The somatic nucleus never undergoes meiosis; it eventually becomes pycnotic (it stains heavily) and disintegrates. The three generative nuclei give rise to 12 haploid products by meiosis. These products become the nuclei of the small haploid agametes. Later, in the gamonts, pairs of haploid nuclei, apparently of opposite sex, fuse to form diploid zygotes. In effect, these organisms show programmed cell death (selective “death” of the somatic nucleus), and each gamont fertilizes itself, although neither egg nor sperm is formed.
Foram tests have contributed greatly to the sediment on the bottom of marine basins, especially since the Triassic period. There are fossil giant forams of great fame. Some, such as Lepidocyclina elephantina, had tests as thick as 1.5 cm. Camerina laevigata (also known as Nummulites, the “coin stone”) was a large (10 cm wide) foram that lived in warm shallow waters during the Cenozoic era from the Eocene to the Miocene epoch (some 38 million to 7 million years ago). Rocks bearing Miocene forams, many of them easily visible to the naked eye, abound on the shores of the Mediterranean. It is from such “nummulitic” limestone that the pyramids of Egypt were constructed.
The abundance of foram tests and their detailed architecture (the earliest ones appeared in the Cambrian) make them excellent stratigraphic markers. Geologists use the 40,000 or so fossil species to identify geographically separate sediment layers of the same age. Because the tests are often found in strata that cover oil deposits, recognition of foram morphology and knowledge of their distribution is helpful in petroleum exploration.