Acetabularia, genus of single-celled green algae (family Polyphysaceae) found in subtropical seas. The algae are among the largest single-celled organisms and also feature an unusually large nucleus. Because part of one species can be grafted onto another, Acetabularia has been used to study the relative role of nucleus and cytoplasm in the genetic control of growth and development.
Acetabularia species are umbrella-like in appearance and are anchored to their substrate with rootlike rhizoids. At the top of the tall, slender stalk, 0.5 to 10 cm (0.2 to 3.9 inches) long, is a ring of branches that may be separate or fused to form a cap. Some species also have rings of hairlike structures along the stalk. Near the base of the stalk is a large nucleus that divides many times when the alga matures and reproductive structures form. Streaming cytoplasm carries the daughter nuclei to the saclike sporangium of each umbrella lobe, where the gametes are released for sexual reproduction.