posidonia and cymodocea 289 



The Cymodocea Community 



Unlike those of Posidonia, the rhizomes of Cymodocea are thin and 

 afford no strong support for larger algae, which may explain the paucity 

 of the latter in the community. Occasionally, however, one meets with 

 individuals of Hypnea musciformis or some calcareous crusts of Melo- 

 besiaceae. Caiderpa prolifera grows in the substratum supporting the 

 Cymodocea and this important alga, together with other sand-binding 

 species such as Pterosiphonia pennata, is one of the first forms in the cycle 

 of substratum colonization by Cymodocea. 



As previously mentioned, Cymodocea nodosa tolerates organic pollu- 

 tion in its environment. The amount of organic matter in the substratum 

 seems to affect the balance between shoot and rhizome sj'stems in exact 

 accord with the observations made by ]VIolinier and Picard (1952, p. 

 162). The Cymodocea growing near the Citadel where there is much 

 organic matter has the rhizome and root system much more developed 

 than the leaves. The Cymodocea prairies shown in Fig. 1 develop leaves 

 exceeding 150 cm in length but have reduced rhizome and root systems. 

 This well-developed leaf system enables plants growing in a substratum 

 relatively poor in organic content to compensate for this shortage through 

 carbon assimilation. 



Whether in shallow or deep water, only the upper parts of the leaves 

 carry epiphytic algae, which differ in quantity and quality according to 

 the depth of water above them. The epiphytes on the leaves are more 

 numerous in shallow water and include several of the more tolerant algae 

 which occur on Posidonia. Cymodocea leaves in calm, shallow water, 

 particularly in the spring, carry also a heavy load of diatoms composed 

 chiefly of Licmophora spp., Climacosphenia and Grajnmatophora, most 

 of which are colonial forms growing in long chains. The following are 

 the more important epiphytes on Cymodocea from shallow water: 



Chondria tenuissima (abundant) Hypnea musciformis 



PolysipJionia 'variegata (common) Spyridia filamentosa 



Ectocarpus irregularis Dasya arbuscula 



Ectocarpus mitc/ielli Ceramium tenuissimum 



Acrocliaetium spp. Cladophora spp. 



Colpomenia sinuosa Enteromorpha compressa 



Hydroclathrus clatliratus Blue green algae 



In deep water the epiphytes are scarce. Occasionally one meets with 

 Ectocarpus confervoides, Jcrochaetium, Colpomenia, and Hydroclathrus 

 in the Cymodocea prairies at Abu Qir. 



