Tidal Zone 



103 



cavity. These oysters are said to be better able to "hold their 

 liquor!" 



Actively swimming or crawling animals may be able to go without 

 food for the period when the tide is out, but they cannot remain 

 active and do without a supply of oxygen for this length of time. 

 There is no lack of oxygen in the air, of course— the problem is to 

 get it without allowing the respiratory membranes to dry out. The 

 gills of many of these animals are placed in partially enclosed cavities 

 of the body where oxygen may reach them but the loss of water from 

 the respiratory surfaces is retarded. A sti^iking gradation in the 

 anatomical adaptations of crabs in the tidal zone has been described 

 by Pearse ( 1950 ) for the North Carolina shore. He called attention 

 to the fact that species living higher up on the shoreline show a re- 

 duction in the number of their gills and particularly in the ratio of 

 gill surface to the size of the whole animal (Fig. 4.3). 



Pearse, 1936 



Fig. 4.3. Reduction in number of gills and increase in ratio of body to gill 

 volume in species of crabs living at progressively higher levels in the littoral zone. 



Differences in ability to withstand the progressively more difficult 

 conditions from the low-tide mark to the high-tide mark have resulted 

 in a zonation of the plant and animal species. On rocky shores this 

 zonation is often so pronounced that it can be seen at a distance when 



