102 Water 



populated since the habitat presents such great difficulties for exist- 

 ence. In the first place all species tend to spread; marine animals 

 and plants are pressing on their boundaries, and terrestrial forms are 

 likewise attempting to invade regions at the margin of their habitats. 

 These species extend their ranges just as far as they can endure the 

 conditions. Furthermore, the tidal zone has certain advantages over 

 conditions in deep water. Shallow water has more light, oxygen, and 

 circulation; and certain predators are excluded. A great amount of or- 

 ganic matter occurs in this zone, and much of this forms a valuable 

 food for the inhabitants. A large part of the organic material exists 

 as detritus resulting from the breakdown of seaweeds and the dis- 

 integration products of dead animals. In addition many kinds of 

 smaller planktonic organisms abound in the shallow water and add 

 to the food supply. The adaptations of the intertidal organisms and 

 the control of their activities by the environment will be discussed 

 briefly here; more comprehensive treatment of this special habitat will 

 be found in such books as Yonge ( 1949 ) and Wilson ( 1951 ) . 



Practically all the permanent inhabitants of the tidal zone are 

 aquatic organisms, and accordingly their chief problem is dealing with 

 the adverse conditions of the air medium when the tide is out. Many 

 physiological and morphological adaptations are displayed for with- 

 standing the recurring periods of water shortage. Many of the 

 simpler organisms can endure a considerable drying of their tissues 

 without permanent injury. Sea anemones, for example, have been 

 kept out of water 18 days; at the end of this period the animals looked 

 like dried raisins but became active again when replaced in sea water. 

 Attached forms with shells like the barnacles, mussels, and oysters 

 simply close up shop during the period of low tide, and many can 

 remain sealed up in this way for unbelievably long periods without 

 injury. If kept at a low temperature oysters may be stored out of 

 water for 3 months or more and will resume normal activity when 

 replaced in the sea. Snails and starfish move into sheltered crevices 

 when the tide is out; worms, clams, and other burrowing forms dig 

 deeper in the mud and wait for the tide to return. 



Certain intertidal forms seem to benefit by periods out of water. A 

 practical application of this fact is made use of by many oyster 

 growers who prefer to raise oysters in situations where they are ex- 

 posed to the air for at least a portion of each day. The oystermen 

 believe that as a result the muscles which close the shells are strength- 

 ened. When the oysters are shipped to market, the shells are held 

 more tightly together thus retaining the fluids within the mantle 



