Owing to the unique shape of their disks, species belonging to the genus Acetabularia are also called umbrella algae. Unlike other algal genera, each individual in this group of green algae is one large cell with a single nucleus. These cells can occur individually or in large clusters. Acetabularia calyculus is one of a few species within this genus found in and around theIndian River Lagoon. The thallus, or body, is goblet-shaped and bright green in color (Littler & Littler 2000; Littler et al. 2008). Each individual is attached to the substratum via a holdfast at the base, leading to a long, slender axis or stalk, and up to a single disk. The stalk is calcified and thickens in diameter toward the base (see "Age, Size, Lifespan" below), bearing several complex branchlets in immature specimens, which are evidenced only by scars after maturity. The solitary disc at the top of each stalk is cup-shaped or occasionally flattened and divided into 22-30 rays. Upon close inspection, the outer margins appear distinctly notched.