Description: Thalli usually colonial, rarely solitary, forming 2- or 8-celled coenobia with cells radiating from a center and embedded in mucilaginous matrix. Cells elongate to linear with acute, obtuse or truncate cell apex. Cells 7-40 x 1-8 µm, connected only at base. Cells uninucleate; chloroplast single and parietal with pyrenoid. Asexual reproduction by autospores, 4-16 per sporangium, usually organized in one coenobium or disintegrated in single cells; spore release by dissolution of parental cell wall. Sexual reproduction unknown. Actinastrum planktonic in eutrophic freshwater ponds and lakes, reported world-wide except in arctic and subarctic climates. Relationships of genus unclear and taxon also assigned to Selenastraceae. Habit similar to some diatoms with asteroid cell arrangement