08/23/2026
Happy Seaweed Sunday! This month let’s snorkel in the Caribbean to find an umbrella-shaped green alga called Acetabularia. This seaweed grows in protected, shallow water on tropical to subtropical shores throughout the world. The thallus of Acetabularia is a single, large cell composed of rhizoids at the base, a stalk, and a cap at the apex. The cell walls of Acetabularia may be lightly to heavily calcified, giving the seaweed a milky green appearance and structural support to keep the stalk and cap upright to gather sunlight.
As a juvenile, the thallus first develops the rhizoid and then a slim stalk, upon which it periodically produces whorls of finely branched hairs. These hairs eventually fall off, leaving behind scars on the stalk surface. After reaching a species-specific length, the stalk apex forms a whorl of branches called cap rays (or gametophores), which adhere to each other to form the umbrella-like cap.
Unlike other siphonous greens, which are multinucleate throughout the vegetative phase, Acetabularia has a single, large diploid nucleus located in the rhizoid. Once the cap is formed, the nucleus undergoes meiosis followed by mitosis to form many haploid secondary nuclei that migrate up the stalk and into the gametophores. Once in the ray, a thick wall surrounds each nucleus, resulting in many round cysts. Inside the cysts, the nuclei divide again multiple times, each eventually forming a gamete. The gametes are released and fuse to create zygotes that develop into juveniles.
Acetabularia evolved around 38 million years ago; large fossil deposits have been found from around 12 mya in present-day Romania.
This species has co-evolved with the sacoglossan sea slug Elysia timida, which pierces and sucks the alga’s cytoplasm, retaining its chloroplasts. This gives the slug a distinctive green color and the ability to photosynthesize.
