Wikipedia
In taxonomy, the Dasycladales is an order of large unicellular green algae in the class Ulvophyceae. It contains two families, the Dasycladaceae and the Polyphysaceae.
These single celled algae are from 2 mm to 200 mm long. They live on substrates in shallow warm marine waters, usually less than 20 meters deep. They are very large cells. They are able to attain these sizes without numerous internal cell wells because they build calcium carbonate shells around themselves.
They contain only one nucleus in their vegetative stage, which remains in the bottom of the cell in the holdfast at the substrate. Only when they are ready to produce gametes does the nucleus undergo meiosis and then numerous mitoses into many nuclei which then migrate into the gametangia at the top of the algae.
Because the nucleus is safely hidden in the holdfast, the cells easily regenerate if the top portions are broken off.
These algae are notable for having an intracellular network of 10 nm proteinaceous filaments, possibly for the storage and transport of ribonucleoprotein particles.
Because of all these properties, and the fact that they are easy to manipulate they have been favorite organisms in the study of the role of the nucleus vs the unnucleated cytoplasm in the behavior of cells.