CHAPTER IV 



Special Problems of Shore Life 



While every well-marked type of environment has special 

 features of its own, which tend to give its fauna a cha- 

 racteristic " facies," nowhere are such features so varied or 

 so far-reaching in their effects as on the sea-shore. Not all 

 the important factors affecting littoral animals are entirely 

 peculiar to the sea-shore. During periods of storm, for 

 instance, the margins of large fresh-water lakes and the 

 animals living there may be pounded with a violence equal 

 to that of marine waves ; moreover, the waters of the same 

 lakes are subject to fluctuations of level known as " seiches," 

 which though very slight may possibly have results on the 

 fauna comparable to those caused by ocean tides. These two 

 phenomena, however, wave-impact and the fluctuation of 

 the water level, are so much more important as they occur 

 on the sea-shore, that the problems they present can 

 legitimately be regarded as " special " to the shore area. 

 Since factors of this type, together with others of a kind 

 unparalleled elsewhere, give the shore fauna its cha- 

 racteristic impress, it becomes necessary to examine them 

 thoroughly as regards both their nature and effects. From 

 the point of view of the organism, each of the features we 

 are about to discuss affords a particular problem the solu- 

 tion of which is imperative if the animal is to survive. As 

 one might expect, the same problem is met in various ways. 



the problem of the tides 



An outstanding feature of the tidal zone is that, while 

 its upper limit marks the permanent boundary between two 



60 



