The Polythalamea are unicellular marine organisms with significant global ecological importance, especially in seas and marine coasts. They have a shell made of agglutinated particles or more commonly of calcium carbonate. This shell has a highly variable structure, generally with one or more chambers where the cell resides, from which it constructs larger chambers as it grows. Some species can live suspended in marine waters, becoming very abundant, although most live on a substrate.

They are crucial in both the calcium chemical cycle and the carbon cycle, as they remove large amounts of carbon dioxide from the atmosphere by using it along with calcium to build their shells, which, upon death, fall to the seafloor and accumulate. These shell accumulations can form thicknesses of up to hundreds of meters, with their fossils being common in many parts of the globe today.

Some of the species are photosynthetic, but this is because they live alongside cells of a group of green algae (Phylum Chlorophyta), and not due to their own capacity.