l6] vegetational types of seas 529 



Benthos 



Although only about 2 per cent, of the sea area of the world is 

 shallow enough to be occupied by benthic plants, the vegetation- 

 types of the eulittoral and sublittoral are much the most striking and 

 studied of all to which the term marine can be properly applied. 

 Numerous books and papers describe these zones in more or less 

 detail for different parts of the world, so a brief outline of the vegetation 

 will suffice for each of the four main regions, viz., tropical, warm- 

 temperate, cool-temperate, and polar. 



I. The marine vegetation of tropical seas is apt to be less luxuriant 

 than that of cooler regions, though some groups of Monocotyledons 

 and Green Algae are largely restricted to warm waters, where reef- 

 Algae mav show periodic correlation with the monsoons. Apart 

 from such manifestations as the Sargasso Sea which are, rather, 

 planktonic, and were accordingly treated above as such. Brown Algae 

 tend to be relatively poorly represented in tropical seas whereas 

 Red and Green Algae are abundant. Although Red Algae are par- 

 ticularly characteristic of the sublittoral zone, certain species of this 

 group often form a thick coating of a dirtv violet colour on the roots 

 and stem-bases of the mangrove-swamp dominants which form such 

 a characteristic feature of the eulittoral vegetation of tropical shores ; 

 but mangroves, being mainly aerial, were dealt with in Chapter XIV. 

 Small Green Algae also with low light requirements may form a 

 characteristic investment on the soft mud, which is rich in organic 

 material, between the roots of the dominant mangrove trees. In 

 a few instances Brown Algae have been reported to form fairly well- 

 developed littoral communities in the tropics. 



Unlike the situation in colder seas where the unstabilized bottoms are 

 often devoid of macroscopic vegetation, the eulittoral and sublittoral 

 in some tropical regions may have sand or gravel beds populated by 

 numerous Green x^lgae. These may even be found in deep water, 

 many being strongly calcified and in addition accumulating much 

 sediment. Also forming extensive communities in some places are 

 marine Monocotyledons such as species of Thalassia and Cymodocea 

 which may dominate characteristic consociations or associations 

 down to a depth of some 30 metres. In many parts of the Pacific 

 Ocean the so-called ' coral-reefs ' are largely built up by calcareous 

 Algae — in particular by Porolithon onkodes. Such virtually tide- 

 less seas as the Caribbean naturally have practically no inter- 

 tidal vegetation or girdling of Algae, the communities near the 



