Species Description: 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 the Indian 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, terminating in a single disk. The stalk is calcified and thickens in diameter toward the base (see “Age, Size, Lifespan”), 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.
Potentially Misidentified Species: Several species of umbrella algae appear quite similar at first glance and often can only be positively identified at the microscopic level.
In the Indian River Lagoon, A. calyculus could be mistaken for A. crenulata, A. c.f. farlowii, or A. schenckii (Littler et al. 2008). Both A. crenulata and A. schenckii are often whitish-green in color, setting them apart from the bright green color of A. calyculus. The remaining species, A. c.f. farlowii, is similar in color and appearance, with 20-42 rounded rays dividing each disk. Under the microscope, this species can be distinguished from A. calyculus based on structural differences such as the shape of corona (ring) at the center of the disk (Littler et al. 2008).
Regional Occurrence: The range of A. calyculus extends throughout the world in warm temperate to tropical locales, including from North Carolina to Florida in the United States; throughout the Gulf of Mexico; throughout the Caribbean and tropical Atlantic (Taylor 1979; Littler & Littler 2000; Littler et al. 2008); and the Eastern Atlantic, Mediterranean and Adriatic Seas (Orlando-Bonaca et al. 2008). Clusters are commonly attached to shell fragments and rocks from the lower intertidal zone down to 20 m (Taylor 1979), in seagrass beds and coral reefs, on tidal flats, and attached to mangrove prop roots (Littler & Littler 2000).
Age, Size, Lifespan: A. calyculus can occur as solitary individuals or as clusters spanning several centimeters (e.g. Littler & Littler 2000). The stipe or stalk of each individual thickens toward the base from 200 to 420mm in diameter, grows 1-7 cm tall and bears a single disk measuring 4-7 mm in diameter (Taylor 1979; Littler & Littler 2000; Littler et al. 2008).
Little information exists concerning factors that affect age and lifespan of A. calyculus. However, as mentioned below, individuals need about six months to develop after fertilization into a mature, reproducing plant (Puiseux-Dao 1970; Braga 1980). This time span is likely altered by factors such as light and nutrient availability (Braga 1980), salinity and temperature, and grazing by herbivores.
Reproduction & Embryology: Sexual reproduction is one method of propagation in A. calyculus. Mature ‘plants’ release gamete-filled cysts from the rays of the umbrella-shaped reproductive cap into the water column. Cyst production varies among species and in A. calyculus, approximately 80 spherical, bright green cysts measuring 80 to 160 μm in diameter are released from each ray (Littler & Littler 2000; Littler et al. 2008). These cysts include a small cap that opens when the gametes are ready to be released (Puiseux-Dao 1970). Each cyst supplies gametes of only one sex that combine to form the zygote. After settling to the benthos, a new individual begins to develop by producing a growing stalk, followed eventually by whorls of branched hairs and a new reproductive cap (Braga 1980). Under ideal conditions, this entire process of sexual reproduction from initial germination to mature adult lasts approximately six months (Puiseux-Dao 1970; Braga 1980).
Species of the genus Acetabularia can also reproduce asexually through either cysts, which release motile, flagellated reproductive cells called zoospores; or through regeneration from sections of the parent plant (Puiseux-Dao 1970). Although the latter form of reproduction is considered to be rare, the groundbreaking work done by Hämmerling in 1931 showed that even anucleate (without a nucleus) cuttings taken from the parent were able to regenerate and maintain anatomical features that were characteristic of the particular species for up to several weeks. These findings, together with the unique unicellular nature of the genus, have spurred a biological and physiological interest in Acetabularia that has continued for nearly 70 years (e.g. Brachet & Bonatto 1970; Woodcock 1997).
Temperature & Salinity: Little information is available concerning the temperature and salinity tolerances of A. calyculus. However, the natural range and habitats of the species suggests it prefers and/or requires warm waters. In addition, clusters can be found in estuaries where conditions are often quite variable, subjecting the species to salinities from brackish to hypersaline (e.g. Braga 1980).
Trophic Mode: Like most other species of algae and plants, A. calyculus is autotrophic, converting energy from the sun into usable food through photosynthesis.
Predators: Information regarding the common grazers of A. calyculus is scarce, but it is likely that the species is fed upon by a variety of organisms, including urchins, hermit crabs, sea turtles, and herbivorous fishes.
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