Thalassodendretum ciliati in Sinai (northern Red Sea) with special reference to quantitative aspects

Thalassodendretum ciliati is one of the most common seagrass communities in the northern Red Sea. It ranges from just below low-water level to at least 30-m depth. This is a typically subtidal seagrass community, although at the shallowest levels the uppermost leaves are sometimes exposed during exceptionally low water and become "brunt". It is the seagrass community with the highest standing crop in the northern Red Sea. Highest densities occur at c. 6 m, but tallest vegetation and largest leaves occur at the deepest levels reached by the community. The high standing stock stems from the extensive woody tissues the plants contain on one hand, and from the many 'tannin cells' in their leaves on the other, which deter grazers and prevent consumption of the plant. Sciaphilic epiphytic algal assemblages develop on the vertical stems of the seagrass beneath the canopy, where they are sheltered from grazing fishes, and photophilic assemblages inhabit the exposed leaves. Both are rich in species, but contribute only a little to the high standing crop of the community.