Seaweeds on the Abrasion Platforms of the Intertidal Zone of Eastern Mediterranean Shores

Over millions of years of evolution, marine macroalgae (commonly referred to as seaweeds) have remained within a narrow and restricted niche, compared to the extensive area covered by oceans and seas. This narrow fringe is the intertidal zone, in which seaweeds are intermittently exposed to harsh conditions such as high irradiance, desiccation and high temperatures. What were the adaptive strategies and physiological needs of these plants to thrive and complete their life cycles over millions of years in these harsh environments? Seaweed’s first records date at least 300 million years, and within this period of time they went through several episodes of environmental change. Today, marine macroalgae comprise about 20,000 species of which a large number can be found within the intertidal zone. During evolution macroalgae diverged into three major categories or divisions: green (Chlorophyta), brown (Phaeophyta) and red (Rhodophyta) seaweeds. The present Mediterranean flora has a history of about five million years. After the isolation of the Mediterranean from the Atlantic, biota surviving the late cooling Miocene re-colonized the vacant basin and established the early Pliocene biota. Then, the Mediterranean Sea lost its coral reefs and its tropical character in general (Luning, 1990). The dramatic climate changes (glacial periods), which took place in this area in the Pleistocene, may have allowed a number of cold-temperate species to invade the area and to form disjunctive populations in cooler parts of the Mediterranean after the glaciations (Hoek and Breeman, 1990). Empty niche space and the climate changes in the late Pliocene and the Pleistocene may have promoted speciation and origin of endemic species. Today, the Mediterranean coasts are inhabited by a rich seaweed flora, including endemic, tropical, warm and cold-temperate species (Orfanidis, 1992).