664 Ecology 



tons per square inch in the depths. Furthermore, the daily tidal move- 

 ments, currents at times the size of rivers, tremendous waves, salinity 

 variations, and differences in substrate all influence the distribution of 

 animals and plants. 



Unlike the fresh-water habitat, the ocean is continuous. As a re- 

 sult some larger animals are world-wide in their range ; many fish range 

 from one community to another. Such facts as these make it difficult 

 to reconcile the concepts of ocean ecology with those of terrestrial or 

 fresh water. Perhaps this in part is due to the difficulty of study and 

 hence our comparatively limited knowledge. 



Since the salt concentration in the ocean water is close to that of 

 protoplasm, conditions are not so severe as in fresh water, and it is 

 possible for many invertebrates to have free-swimming larval stages. 

 One of the major differences in fresh-water and oceanic plankton is the 

 presence of these many larval forms. 



Despite the complexity of community interaction, it is possible to 

 recognize certain definite zones within the ocean. The most familiar 

 and best studied zone is the narrow intertidal zone. This is part of 

 the eulittoral zone which extends outward from the shore to about 50 

 meters in depth. This area merges gradually into the siiblittoral zone 

 of the continental shelf. At about 200 meters depth, the slope of the 

 continental shelf is very abrupt, and it becomes the archibcnthic zone 

 which extends to a depth of about 2,000 meters. Below this is the 

 abyssal-henthic zone which continues to great depths. The waters above 

 the sublittoral constitute the n critic province, while those above the 

 deep sea off the continental shelf are termed the oceanic province. 



Eulittoral Zone. — That area extending from high tide mark to a 

 depth of 50 meters is known as the eulittoral zone. The shoreward 

 portion of the eulittoral is alternately exposed and covered by the 

 tide, twice every day-night period. This results in drastic alterations 

 of temperature, moisture, light, friction, and salinity. Despite these 

 seemingly unfavorable conditions, the intertidal zone teems with life. 

 The animals dwelling there usually have some method by which they 

 can escape the effects of drying; some burrow, some retreat into shells, 

 and others crawl under rocks or other protecting materials. Depending 

 upon the substratum, three types of habitats may be recognized : ( 1 ) 

 the rocky shores, (2) the sandy shores, and (3) the muddy shores. 

 There may also be a fourth type which is really a combination of the 

 last two. 



