Chlorophyta, Dasycladales; Family: Polyphysaceae.

Etymology

Acetabularia means bowl and refers to the algal umbel top.

Description

The alga resembles an umbrella or a small mushroom. A central axis and an umbel or disk are evident at the top. The top is made of a number of rays, whose shape and size facilitate identifying the species. The thallus is calcified, with less calcification occurring in young individuals.

Size

Individuals found in this region are very small. The top is less than 1 cm in diameter, and the foot is 1 cm long, at most. Individuals found elsewhere in the world are usually larger.

Colour

The alga is very light green, sometimes nearly transparent.

Special features

Although quite rare and not often accessible, the Acetabularia is easy to identify thanks to its umbrella-like shape.

Habitat

The Acetabularia grows in shallow lagoons in the intertidal zone. It grows in the upper, hidden regions of potholes, attached to stones. In deep water, it may be found in additional places. The algal rarity along Israeli coasts, its small size and its preference for dark hidden places impedes the description of its habitat. The example shown above was photographed in Cyprus.

Biology and reproduction

The Acetabularia has been a subject of research and education, and it is described in many books on nature. However, the species that grows in our region is very small which does not facilitate experimentation. The

Acetabularia’s life cycle has been studied in detail. A complete life cycle extends over several years, during which the Acetabularia undergoes three forms: resting cyst to sterile erect ‘stalk’ (diskless) to fertile disk (umbel). Upon germinating, the zygote develops into a rhizoid that attaches the alga to a substrate, and a ‘stalk’ grows, covered in assimilating hair. At this point in the vegetative development, a large cellular nucleus is located in the rhizoid that (due to its size) has been and is subject to much research. Following three years as a ‘stalk’, the characteristic umbel begins to develop. During this time, the large nucleus divides into secondary cellular nuclei. These wander into the umbel’s rays, and each nucleus gets wrapped in a cytoplasm and in a membrane, becoming a cyst. The cysts disperse in the water; the nuclei in the cells divide; and at the end of the process, each cyst releases several dozens of reproductive cells that contain zoospores. The zoospores merge; the zygotes settle; and the cycle recurs.

Seasonality and distribution

The Acetabularia genus is common in tropical and subtropical seas. The species found in the Eastern Mediterranean seems to be more available in summer, but the paucity of observations in winter may be the result, not so much of lower water temperature, but rather its disappearance.

Additional species

The Acetabularia genus includes about twenty species, some as small as the local ones, and some are several centimetres in size. Here are a few of the species reported in the region:

Acetabularia acetabulum (Linnaeus) Silva

= Acetabularia mediterranea Lamouroux Acetabularia parvula Solms-Laubach = Acetabularia moebii Solms-Laubach Acetabularia wettsteinii Schussnig.

 

Experiments by the Danish biologist Joachim Hammerling with the Acetabularia genus in the first half of the twentieth century are worth mentioning. These were a breakthrough in the understanding of biological species and the influence of the nucleus in determining an individual’s traits. The Acetabularia serves research purposes at present, as well, and is mentioned in many books on nature. One species, the Acetabularia major, is used in North Eastern Europe to treat gallstones.