338 



EXPOSED 



Littorina 



THE ALGAE 



SHELTERED 



Supra-littoral fringe 



Chthalamus 



Mytilus 



Centroceras 



Ulva 



Chaetomorpha 



Red algal turf 



Dictyopteris 

 Lithothamnia 



Littorina 

 Chthalamus 

 Ulva 



Chaetomorpha 

 Red algal turf 



Littorina 

 Pylaiella 

 Ulva 

 Dictyopteris 



Littorina 



Pylaiella 



Ulva 



Mid- 

 littoral 



Dictyopteris 



Red algal turf Sub- 

 Sargassum littoral 



In the warm waters of the world the sub-Httoral is of greater im- 

 portance. On the shore exposure, even for a relatively short time, 

 involves considerable desiccation and few species seem able to 

 tolerate it. The sub-Httoral flora depends primarily upon the type 

 of substrate, whether sand, rock or coral. Workers in the Mediter- 

 ranean believe that with those clear waters a number of different 

 belts can be recognized. It remains to be seen whether the same is 

 true of colder waters. The importance of the Mediterranean sub- 

 Uttoral is illustrated by Table 9 which summarizes the various 

 zones and belts proposed by workers for this region. Even in this 

 sea there is a pronounced difference in the summer and winter 

 aspects of the flora (see also North Carohna). Boreal Atlantic 

 species such as Ulothrix flacca, U. suhflaccida, Bangia fusco-pur- 

 purea and Porphyra spp. dominate the flora in winter, whilst in 

 summer it is the tropical and sub-tropical species such as Siphono- 

 cladus pusillus, Acetabularia mediterranean Pseudohryopsis myura, 

 Liagora viscida, etc., which form the dominant species. In com- 

 paring the behaviour of the Mediterranean vegetation with that of 

 the Boreal Atlantic one may distinguish several types of algal 



periodicity : 



(i) Algae with a summer vegetation period in both the Enghsh 

 Channel and the Mediterranean. These algae usually occur 

 at a considerable depth where there is little or no temperature 

 variation, e.g. Sporochnus pedunculatus, Arthrocladia villosa. 



(2) Algae with a winter and spring vegetational period in both 

 the Mediterranean and the English Channel, e.g. species of 

 cold waters such as Ulothrix flacca. 



(3) Algae appearing in the winter and spring in the Mediter- 

 ranean but during the stimmer in the English Channel. With 



