Tidal Zone 101 



Tidal Zone 



Another situation that might be classified as amphibious is the 

 tidal zone. Plants and animals living on the seashore between tide 

 marks are subject to alternating droughts and floods twice each day. 

 Organisms living in the center of the tidal zone must be able to be 

 aquatic for about 6^4 hours when the tide is in, terrestrial for about 

 6% hours when the tide has ebbed, and so on, day in and day out. 

 The tidal zone is thus a very difficult habitat, and also a very compli- 

 cated one because of differences in the timing and height of tide. 

 Since low tide and high tide occur about 1 hour later each day, the 

 intertidal area comes to be exposed at all hours of the day and night. 

 During the first and last quarters of the moon neap tides occur, i.e., 

 tides of low amplitude, but during periods of new moon and full moon 

 the tides rise higher and fall lower and are known as spring tides. In 

 some localities, such as the west coast of the United States, one set of 

 high and low tides each day has a much greater amplitude than the 

 other set. The height of the tide differs greatly from locality to lo- 

 cality, and the duration and depth of inundation varies according to 

 the level within the tidal zone at which a given organism exists. For 

 example, an animal or a plant attached near the upper extreme limit of 

 the tide will be covered by sea water for only a brief period about 

 every two weeks, whereas an individual living near the lower extreme 

 tidal limit will receive only a correspondingly short exposure to the air. 

 The tidal zone comprises the upper portion of the littoral zone, as 

 indicated in Fig. 2.4. 



Despite these severe conditions a dense population of a very wide 

 variety of plants and animals characteristically inhabits the tidal 

 zone. The faima and flora consist chiefly of sessile forms, such as the 

 common seaweeds, barnacles, mussels, and clams. Those plants and 

 animals that cannot move out of the tidal area must endure the full 

 effect of the differences in the two media. In addition, certain active 

 forms forage here under favorable conditions and then retreat. At 

 low tide animals come from the land and feed on the material that 

 has been exposed. Shore birds are familiar in this area, but less 

 observed are rats, mice, mink, skunks, and other small mammals that 

 feed chiefly at night. During high tide the reverse situation obtains. 

 Fishes, crabs, and other active animals from deeper water move in for 

 a few hours and then retire. The alternating visitations of these forms 

 remind one of Box and Cox in the famous operetta. 



It is of interest to inquire why the tidal zone should be so densely 



