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Ammocytes (sand cells) are a major group of eukaryotic unicellular organism, and are one of the most common types of phytoplankton. Ammnocytes are related to the algoids. Ammocytes are encased within a unique cell wall made of silica It is estimated that there are approximately 100,000 extant species of ammocytes. They are a widespread group and can be found in the oceans, in freshwater, in soils and on damp surfaces. Ammoctyes are photosynthetic and photoautotrophic.

Most species are non-motile but some are capable of an oozing motion. Due to their relatively dense cell walls, which cause them to readily sink, open water species usually rely on turbulent mixing of the upper layers by the wind to keep them suspended in sunlit surface waters. Some species actively regulate their buoyancy with intracellular lipids to counter sinking.

Ammocyt cells are contained within a unique silicate (silicic acid) cell wall comprised of two separate valves (or shells). The biogenic silica that the cell wall is composed of is synthesised intracellularly by the polymerisation of silicic acid monomers. This material is then extruded to the cell exterior and added to the wall. The two valves typically overlap. When dividing to reproduce, each cell keeps one of the two valves and grows a smaller valve within it. As the division cycles continue, the average cell size in the population gets smaller. Once the minimum size is reached, ammnocytes turn to auxospore reproduction.