Thalli microscopic or macroscopic, including filamentous, pseudoparenchymatous, tubular or blade-like forms, attached, free-floating, endophytic or shell-boring. Cells uni- or multinucleate, generally possessing a single parietal chloroplast with one to several pyrenoids. Life history variable, with isomorphic or heteromorphic diplohaplontic cell cycle. In some members, sexual life cycle unknown. Asexual reproduction by biflagellate or quadriflagellate zoospores. Sexual reproduction by biflagellate isogametes or anisogametes formed by unisexual or bisexual thalli, often parthenogenetic. Motile cells are naked, uncovered by body scales.
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