Acetabularia is a genus of green algae in the family Polyphysaceae, order Dasycladales, placed within the class Ulvophyceae of the phylum Chlorophyta. The genus is remarkable for being unicellular yet reaching 4 to 10 centimetres (1.6–3.9 in) in height — making its members among the largest single-celled organisms known. Each cell has three anatomical regions: a branching rhizoid at the base that anchors the alga to hard substrates and houses the single nucleus; a long, slender stalk; and an apical umbrella of branches that can fuse into a characteristic disc-shaped or branched cap. The cap's form gives the genus its common name: mermaid's wineglass.
What makes Acetabularia especially valuable to science is that its single nucleus can be surgically manipulated. If the cap is removed, the cell regenerates a new one; if a stem segment is isolated from the nucleus entirely, it still grows a replacement cap. Caps can even be exchanged between two different species. In the 1930s–1950s, biologist Joachim Hämmerling exploited these properties to show, for the first time in a eukaryote, that genetic information controlling cell form resides in the nucleus — a foundational result in molecular biology. Later work established that the alga transports messenger RNA from the nucleus to its apical tips, where translation occurs, and that it maintains an endogenous circadian rhythm in photosynthetic and respiratory activity even after the nucleus is removed.
Members of the genus are typically found in warm subtropical marine waters, attached to rock in shallow coastal habitats. They grow slowly and are grazed readily by herbivorous fish. A small number of species occasionally appear in the aquarium trade, best suited to macroalgae display tanks rather than reef or FOWLR systems.
Etymology
The name Acetabularia comes from the Latin acetabulum, a broad, shallow cup used at Roman tables for dipping bread — a reference to the alga's distinctive upturned cap. This form is also responsible for the popular name "mermaid's wineglass." A 19th-century attempt to apply the same name Acetabularia to a genus of fungi (now Cyphellopus) was ruled invalid; the algal name takes precedence.
Distribution
Acetabularia species are typically found in subtropical marine waters, anchored to hard substrates in shallow coastal zones. The Mediterranean Sea is well documented as a key range (with A. acetabulum studied extensively there), and the genus occurs broadly across warm-temperate to tropical seas.
Ecology
Acetabularia grows attached to rock or coral rubble in warm, shallow marine habitats. It is slow-growing and readily consumed by herbivorous fish, limiting its abundance where grazing pressure is high. The alga carries out oxygenic photosynthesis and exhibits a robust endogenous circadian rhythm in both respiratory and photosynthetic activity — a rhythm that persists even when the nucleus is experimentally removed, indicating cytoplasmic regulation. The genus occasionally enters the reef-aquarium trade but is considered challenging to maintain; it fares best in dedicated macroalgae display tanks with low herbivore pressure.
History
Acetabularia became central to 20th-century genetics through the experiments of Joachim Hämmerling conducted between the 1930s and 1950s. By transplanting caps and nuclei between A. acetabulum (then often called A. mediterranea) and A. crenulata — species whose caps differ in shape — Hämmerling showed that the nucleus, not the cytoplasm, determines cap form. This was the first demonstration that genes encoded in nuclear DNA direct eukaryotic development, complementing contemporaneous prokaryote work by Oswald Avery and others.
