POSIDOXIA AND CYMODOCEA 285 



factor is the accumulation of dead Posidonia leaves, detached algae, and 

 other sediments in the lee of the reef. In this sense, a phenomenon com- 

 parable to that of ecological succession in land plants may be ascribed 

 to these sea-grasses. The ecological factors which produce this succession 

 may be summarized as follows: 



1. The direction and strength of currents and waves, which deter- 

 mine the shape of the area occupied by Posidonia on rocky shores. 



2. The ability of Posidonia rhizomes to raise the substratum and 

 eventually build up a reef. 



3. The failure of Cymodocea to do the same, or to grow under the 

 influence of strong currents. 



4. The action of the Posidonia leaves as a buffer against the intensity 

 of the current, creating behind the reef calm conditions similar to those 

 in lagoons. 



5. The inability of Posidonia leaves to grow continuously above 

 water, causing the growth of the reef to stop at a certain minimum height 

 below sea level. 



6. The action of Caulerpa prolifera and other algae in binding the 

 substratum back of the reef, thus paving the way for Cymodocea. 



7. Cymodocea encroachments on Posidonia, by occupying depressions 

 and gullies formed by erosion or by biological factors. 



The lagoon shown in fig. 1 is protected to the west by the ridge and 

 to the north by emerged and submerged rocks. The water flowing between 

 these rocks creates a current directed toward the channel intersecting 

 the ridge; and the areas lying under the influence of this current are 

 largely inhabited by Posidonia oceanica. Toward the shore, the lagoon 

 is much shallower (20 to 50 cm), the water is little disturbed, and the 

 temperature much higher, as much as 5°C in summer, than in the open 

 sea. Here Cymodocea nodosa grows in patches among tropical algal 

 communities. 



The most conspicuous algal community in the lagoon, especially in 

 the shallow coastal region, is one dominated by Acanthophora delilei and 

 Spyridia aculeata. Asparagopsis delilei is abundant during the spring. 

 Branches of Acanthophora reach a height of 50 cm ; and Padina pavonia, 

 Hydroclathrus clathratus, and Colpoinenia sinuosa, found usually as 

 separate individuals or in small patches, also attain considerable size. 

 Of particular interest is Laurencia paniculata, which grows on rocks 

 covered with sand and shell debris, at a depth of about one meter, and is 

 covered in summer by dense growths of Jania rubens and Dictyota 



