

STUDIES IN MARINE ECOLOGY. 219 



present in small clusters, and Styela. A more complete list is 

 given in List 3, which while not exhaustive, does represent the 

 association in an adequate manner. The species starred were 

 taken above low spring tide in collecting on the rocks at Kettle 

 Cove, August 4, 192 1. 



The chance of differentiation in the vertical distribution is 

 not so great here as on the wharf pilings and, in addition, the 

 spaces between the rocks and under them hold moisture better 

 than in the case of the pilings so that the break at the tide level 

 is less marked. Nevertheless the animals to be found on the 

 rocks vary greatly with the height above mean low tide. Dif- 

 ferences found in animal life and in water analyses at different 

 tide levels are given in List 4 and Table VI. The high tide 

 levels were studied on the Northwest Gutter rocks. The tidepool 

 and region below low tide are from data obtained at Gansett. 

 There is no reason to suppose that high tide conditions are 

 different in the two places. 



The tidepool was well separated from the water of the Bay. 

 It contained about 6-12 inches of water and was one foot wide 

 by three feet long. Some rockweeds were present but not in 

 the numbers common in a typical rockweed association. 



It is possible that the greater acidity combined with the 

 decreased oxygen content of the water at the margin at high 

 tide might cause sensitive larvae to turn back. However when 

 the waves are running well they would have no power in the 

 matter. It is also possible that the sensitive animals of the 

 exposed rock association are kept out of the tide pools by the 

 great range in acidity which was over twice that found on the 

 nearby rocks below low tide and, as seen in List 4, the animals 

 present are fewer in numbers and in species. 



(2) Protected Rocks or Rockweed Association. — The best example 

 of the rockweed association to be studied was found in the creek 

 between Southwest Gutter and Hadley Harbor. In this place 

 the rockweeds, Ascophyllum and Fucus, form a patch about two 

 rods square which reaches half way across the creek. The water 

 ranges from six inches to four feet in depth. Between the rocks 

 on the bottom are small patches of gravelly sand mixed with 

 humus. Large boulders, which extend well out of the water at 



