Acetabularia acetabulum
A unicellular green alga with a parasol-like cap.
Acetabularia acetabulum, also called mermaid's wine glass, is a green alga belonging to the family Polyphysaceae. It lives in the Mediterranean Sea, typically one to two metres deep. During winter, only root-like rhizoids remain attached to the seabed. In spring, a single-celled thallus grows a slender stem from the holdfast, reaching about 5 cm tall. As the stem lengthens, whorls of dichotomously branching hairs encircle it at intervals; lower hairs eventually fall off, leaving circular scars. Once the stem is fully developed, a disc-shaped cap up to 1.2 cm wide forms at the tip, resembling a pale green parasol, with additional hair whorls sprouting from the cap’s upper surface.
Before the cap expands, the organism is a single cell with one nucleus at the stem’s base. When the cap grows, the nucleus divides once by meiosis, then multiple times by mitosis, producing thousands of haploid secondary nuclei. These migrate up the stem into the cap, where each becomes enclosed in a gametangium containing the nucleus and some cytoplasm. Inside each gametangium, the nucleus undergoes further mitotic divisions. By late summer, the cap disintegrates, releasing thousands of tertiary nuclei as gametes into the sea. Fertilisation happens in open water, and zygotes settle on the seabed.
Each frond is a single large cell containing millions of chloroplasts, which are constantly moving. During the day, they spread out to maximise light exposure, making the stem appear uniformly dark green. At night, they cluster together, turning the stem pale green. This effect can be seen by shining a blue light on a stem in darkness; after the light is switched off, a transient green band appears. The alga stores starch as granules within the chloroplast stroma. It has been a model organism in developmental biology due to its huge single cell, single nucleus, and complex structure. A known predator is the sea slug *Elysia timida*.
Quick Facts
- Genus
- Acetabularia
- Species
- acetabulum
Facts from the source article.
Lore & Background
This alga adheres to the substrate with rhizoids, which are the only part present in winter. In spring, a slender stem develops from the holdfast, growing vertically to about 5 cm. Growth is interrupted at intervals while whorls of hairs develop, branching dichotomously; as the stem lengthens, lower hairs drop off, leaving circular scars. When fully developed, a disc-shaped cap up to 1.2 cm wide grows at the tip, resembling a pale green parasol, with further whorls of hairs from the cap's upper surface.
Before cap expansion, the organism is a single cell with a single nucleus at the stem base. As the cap expands, the nucleus divides once by meiosis, then multiple times by mitosis, producing thousands of haploid secondary nuclei. These migrate into the cap, each becoming enclosed in a gametangium. Inside, the nucleus undergoes further mitotic division, so that by summer's end, when the cap disintegrates, thousands of tertiary nuclei are released as gametes. Fertilisation occurs in open water, and zygotes settle on the seabed.
Chloroplasts are in constant motion: in daytime they move to maximize light exposure, making stems uniformly dark green; at night they aggregate into clusters, turning the stem pale green. This effect can be demonstrated by shining blue light on a stem in the dark and extinguishing it, forming a transient green band. Predators include the sea slug Elysia timida.
Reader's Guide
Acetabularia acetabulum has been used as a model organism in developmental biology, valued for the enormous size of its single cell with a single nucleus, and its complex cell structure and development. Its unicellular nature, despite forming a macroscopic frond with a stem and cap, makes it a classic subject for studying nuclear-cytoplasmic interactions, morphogenesis, and cell polarity. The alga's life cycle, involving a single meiotic division followed by multiple mitotic divisions and the migration of thousands of nuclei into the cap, provides a clear system for investigating nuclear division and gamete formation. The daily movement of chloroplasts in response to light offers a visible example of intracellular motility and phototaxis. The main storage polysaccharide is starch, found as granules within the chloroplast's stroma. The alga is also part of the food web, being preyed upon by the sea slug Elysia timida. Its presence in the Mediterranean Sea at shallow depths makes it accessible for study, and its distinctive parasol-like appearance has contributed to its common name, mermaid's wine glass.
Did You Know?
- The alga is unicellular, with each frond formed from a single large cell containing several million chloroplasts.
- Before cap expansion, the organism has a single nucleus located at the base of the stem.
- Chloroplasts move to maximize light exposure during the day and aggregate into clusters at night.
Frequently Asked Questions
What is Acetabularia acetabulum?
It is a single-celled green alga in the family Polyphysaceae, widely known by the nickname 'mermaid's wine glass' because of its distinctive silhouette. It is one of the largest organisms known to consist of just one cell.
Where does Acetabularia acetabulum live in the wild?
It is native to the Mediterranean Sea, where it anchors itself to the seabed at depths of roughly one to two metres.
What does Acetabularia acetabulum look like?
A slender stem about 5 cm tall rises from a holdfast, ringed at intervals by whorls of dichotomously branching hairs, and is crowned by a disc-shaped cap up to 1.2 cm wide that evokes a tiny parasol. As the lower hair whorls shed, they leave neat circular scars along the stem.
How does Acetabularia acetabulum change through the seasons?
During winter only root-like rhizoids remain attached to the substrate, but in spring a single cell sprouts a new stem that steadily elongates and eventually produces its cap. The whole structure is built from starch granules stored within the chloroplast stroma as its energy reserve.
Why is Acetabularia acetabulum important to biology?
Because a single cell can stretch to several centimetres, it has long served as a classic model for studying regeneration, nuclear-cytoplasmic control, and the limits of unicellular life.
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