POSIDOXIA AND CYMODOCEA 283 



itself. The algae inhabiting these grottos share with those dwelling in 

 the rock-crevices the common characteristic of being shade-algae (Scia- 

 philes). The composition of such a community is modified by the degree 

 of exposure to wave and surf action. Three types of grottos distinguished 

 at Abu Qir are listed below, with their characteristic flora. 



1. Strongly exposed grottos. These are ordinarily open to the north- 

 west and are washed by strong and continuous waves. The platform 

 under which they are found is usually covered with calcareous algae 

 and Pterocladia capillacea, while the roof is inhabited by suspended 

 Cladophora pellucida. In addition to sponges and hydroids, the following 

 algae are found on the sides and inner walls: Botryocladia botryoides, 

 Valonia utricularis, RhodophyUis bifida, and Phyllophora nervosa. Rho- 

 dymenia ardissoni and Cryptonemia lomation are also frequently found. 



2. Moderately exposed grottos. The platforms covering these grottos 

 are usually covered with the same Cystoseira-Sargassum community as 

 one finds on the ridge. Cladophoropsis zoUingeri forms a narrow belt at 

 the base of this community (fig. 4). In the grottos are found algal groups 

 dominated by D'tctyopteris tnenibranacea and/or Taonia atornaria, asso- 

 ciated with Dictyota dichotoma, D. linearis, Udotea petiolata, Amphiroa 

 beauvoisii, Lithothamnion lichenoides, Pseudolithophyllum expansum, 

 and Peyssonnelia. Some of the species found in the strongly exposed 

 grottos are also present here, but less abundantly. The floors are usually 

 covered with Caulerpa prolifera, Halopithys pinastroides, and Dasycladus 

 clavaeformis. 



3. Calm-water grottos. Boring animals play an important role in 

 forming these small grottos, several of which are found on the lagoon 

 side of the ridge. They are inhabited chiefly by Codium spp., Ulva 

 lactuca, Hypnea musciformis, Spyridia filamentosa, Halimeda tuna, 

 Cordylecladia erecta, and Peyssonneli/i polymorpha. Asparagopsis delilei 

 is frequent in summer. Udotea petiolata and Valonia macrophysa are 

 sometimes found and, less frequently, Digenea simplex and Chrysimenia 

 ventricosa. 



Extending along the ridge at about a depth of one meter and sloping 

 gently to the west to about 1.5 to 2 meters, is a living Posidonia-Reei 

 (figs. 2-3), formed by the accumulation of years of dead rhizomes of 

 Posidonia, together with sand grains, calcareous tubes of worms, crustose 

 algae, dead shells of animals, all resulting in an elevation of the sub- 

 stratum. The surface of the reef is occupied by living Posidonia whose 

 leaves attain a length of 50 to 80 cm, reaching the surface of the water 

 near the ridge. It forms a triangle with its base oriented toward the north. 



