180 MARINE ANIMALS 



coast line afford the richest development of the littoral fauna. Thus the 

 littoral selachians are nowhere so richly differentiated as in the island 

 archipelagoes of the Oriental region, with 33 species each in the Indian 

 and Malayan provinces. Marine mollusks are much the most richly 

 developed in the Indo-Pacific region; those of the Philippines are esti- 

 mated at 6000 species. The West Indies also have a great number of 

 species. Although only 7% of the total ocean surface lies within the 

 180-m. line, the great majority of benthic animals are found within 

 this narrow coastal strip, and their numbers are much greater than 

 those of the pelagic forms. 



A comprehensive characterization of the coastal area, either with 

 reference to its environmental conditions or to the animal life depend- 

 ent on them, is extremely difficult. Its extension in depth to about 

 200 m. produces great differences in lighting, temperature, motion of 

 the water, and oxygen supply, and the various combinations of the 

 conditioning factors produce a surprising multiformity in the whole. 

 The only natural division appears to be that based on the nature of the 

 substratum. Movable, more or less loose beach material and solid rocky 

 coast differ in many important respects in their influence on animal life. 



Below the low tide mark a rich plant life appears in the ocean, 

 which is strongly influenced by the substratum, like the animal life. 

 Where the bottom is argillaceous or sandy, flowering plants can estab- 

 lish their roots, and so take up the salts required by them. Where the 

 bottom is hard, such root functions are impossible, and the plant 

 growth is confined to algae and kelp. The distinction of loose bottom 

 from rocky coast is thus equally justified with reference to plants. 



Littoral shores with loose substratum, the depositing shores. — 

 Coastal areas with sandy or muddy bottom are grown over below the 

 low tide mark, in sheltered places, with "seaweed," i.e., with plants of 

 the family Potamogetonaceae. These are represented by various genera, 

 Zostera* in the North Atlantic, Posidonia and Cymodocea in the Medi- 

 terranean, and these with the addition of Halodule and Halophila in 

 the Red Sea. Such plants are wanting where there is too much wave 

 action, which stirs up and redeposits the sand. These meadows of sea- 

 weed extend to various depths, depending in general on the entrance of 

 sufficient amounts of light, Zostera in the Limfjord in Jutland reaches 

 only 5 m., but extends from 10 to 40 m. in the North Atlantic, while 

 Posidonia ranges to depths of 8 to 10 m. in the Mediterranean, with its 

 extreme limit occasionally at 20 m. 



* At present, 1936, the eelgrass is greatly reduced on both sides of the Atlantic, 

 apparently as the result of the attack of a protozoan parasite. As yet there is no 

 certainty whether this condition is temporary or permanent. 



