The Process of Chamber Formation in the Foraminifer Rosalina floridana (Cushman)

SYNOPSIS. The foraminifer Rosalina floridana builds a chambered, calcareous test which is periodically enlarged by the addition of new chambers. R. floridana begins to form a chamber by constructing an algal growth cyst which covers the dorsal side of the animal and is cemented to the substrate. The pseudopods that build the cyst coalesce to form a cytoplasmic template or anlage on which the chamber walls will be secreted. Electron micrographs reveal the anlage cytoplasm to be a “froth” of nearly empty vesicles which contain mitochondria, fibrillar material, and electron dense granules. The organic lining of the new chamber is secreted on the anlage by pseudopods extending thru it. After the organic lining is completed, cytoplasm from within the test flows into the forming chamber and forces the anlage cytoplasm out thru the new aperture. This “frothy” anlage cytoplasm forms a sheath over the dorsal surface of the test; while it is in place, a layer of calcite is deposited on the walls of the new chamber and over the rest of the test. When calcification is completed, the sheath breaks up and is incorporated into newly formed pseudopods as the foraminifer gradually moves out of the growth cyst and begins normal feeding. The production of vesiculated cytoplasm in normal pseudopods and in the anlage is viewed as a method of greatly increasing cytoplasmic volume with a resultant very small loss in cytoplasmic mass. In the anlage during the production of the organic lining the vesiculated cytoplasm apparently acts only as a support for the membranes being secreted by the pseudopods and presumably does not take part in the secretory process. This same cytoplasm forms the sheath that is present during calcification. The view is advanced that, altho the sheath could be active during calcification with the mitochondria within the vesicles actively transporting calcium to sites of crystal growth, its more probable function is to form a partition between the parts to be calcified and the environment. The foraminifer could then secrete CaCO₃ from pooled reserves in the cytoplasm into the area between the sheath and the chamber membranes. The basal membrane is considered the nucleating agent during calcification and is responsible for the ordering of the crystals so their C axes are perpendicular to the surface of the chamber.