Phaeophyta, Brown Algae
Dictyota dichotoma (Hudson) Lamouroux
Phaeophyta, Order: Dictyotales; Family: Dictyotaceae. The family includes about twenty genera and one hundred species.
Dictyota, derived from the Greek word “diktyon” that means ‘a net’. The species name, dichotoma, means ‘splits into two’.
The flat thallus of the alga is branched dichotomously several times at an angle of 15-30 degrees. Most branches are flat and slightly rounded at the edges. The branches usually grow on a plane, although some branches may be rolled around.
The alga reaches a length of 30 cm, but is usually smaller, no more than 10 cm.
The Dictyota dichotoma is light brown, tending towards olive-green when immersed in water.
The Dictyota dichotoma is particularly varied, and defining species is not unequivocal. Literature offers descriptions of many sub-species. Species are distinguished by the lengths of branches, their width and their manner of branching.
Most species individuals grow in shallow water, but they are sometimes found in depths of 5-8 m. Algae also develop on a wide variety of substrates. Many species are common in pools.
Biology and reproduction
The thallus is comprised of three layers. The inner layer contains large cells and the outer layers have smaller cells. The sporangium, the asexual reproductive cells found upon the sporophyte, are scattered upon both sides of the entire alga, except for its base and its edges. The gametophyte, the second stage, is dioecious, and male plants can be distinguished from female plants. The female clusters are very dark, and the male ones are shiny and relatively large, but colourless. They can be seen with the aid of a microscope or magnifying glass. The thallus’ development is from a single cell or two apical cells.
Some representatives of the Dictyotales order display a cyclical development of gametes that germinate in accordance with the lunar cycle. This phenomenon has been carefully investigated with regard to Dictyota dichotoma. In some regions, the gametes germinate once in two weeks, while in others this occurs once a month. Along the English coast, the gametes germinate a day or two after the spring tide, which occurs twice a month - at full moon and at the new moon (Williams, 1897; 1905). In the Gulf of Naples, the gametes germinate at a neap tide.
Seasonality and distribution
The alga begins to appear at the beginning of winter, increasing dramatically during winter and spring and disappearing towards summer and autumn. The species’ global distribution is in warm seas, the Mediterranean, the Atlantic Ocean, the Indian Ocean and the Pacific Ocean.
Several species of Dictyota grow in the Mediterranean. As mentioned, the species differ in colour, branching, general size, branch length, the relationship between branch length and width and the shape of the branch edges. Most of these characteristics are affected by the ecological conditions of the habitat too, a factor that hinders identification. Additional species mentioned in literature from this region include:
Dictyota divaricata (Huds.) Lamour. and Dictyota linearis
(J. Agardh) Greville. The Dictyota dichotoma is considered the most common in its family around the world.
The Pacific Island natives traditionally used to eat various species of Dictyota. In some research projects, including one centred in the Mediterranean, the thallus of the Dictyota dichotoma was found to contain antibiotic ingredients active against bacteria and fungi.