106 



Water 



On muddy and sandy shores zonation of plants and animals be- 

 tween tide marks similarly occurs, but the observer will have to dig 

 to ascertain the full ramifications of the tidal relationships since many 

 of the inhabitants are burro wers. In addition, microscopic examina- 

 tion of samples of bottom material reveal a zonation of the microor- 

 ganisms in the tidal zone. The distribution of copepods dwelling in a 

 sandy beach of Cape Cod will serve as an illustration ( Fig. 4.6 ) . The 



-1.5 



Low 

 tide 



1.5 3.0 4.5 6.0 7.5 



Horizontal distance above low-tide mark, m 



High 

 tide 



Pennak, 1939 

 Fig. 4.6. Abundance and distribution of sand-dwelling copepods across the tidal 

 zone at Nobska Beach, Woods Hole. 



different species present were so nearly alike in general appearance 

 that they could be distinguished only by careful study. Neverthe- 

 less, one species was found to live only below low tide where it is never 

 subjected to the drying action of air. Another form was most abun- 

 dant midway between the tide marks. A third species was most 

 numerous just below the high-tide line. Furthermore, when samples 

 of sand from different levels beneath the surface were studied, the 

 numerical abundance of the copepods was found to vary with depth 

 in the sand as well as with position across the tidal zone. At the low- 

 tide mark the largest number of animals was found in the upper 4 cm, 

 whereas at the high-tide mark the largest population occurred at a 

 depth of 12 to 16 cm. In the tidal zone, as in other amphibious situa- 



