314 MARINE ECOLOGY 



where the annual exposure is less than 5 %. This marks the lower 

 limit of several intertidal species. 



(b) Between the mean low-water marks of neap and spring tides 

 where the exposure is about 20 %. This marks the upper limit of 

 several submarine species. 



(c) At the extreme high-water mark of neap tides where there is 

 about 60 % exposure. This marks the upper limit of several inter- 

 tidal species. 



The least critical level appears to be mean low-water mark of 

 neap tides where there is about 40 % exposure because the maxi- 

 mum number of species occurs at this level. 



It can be seen therefore that the zonation depends very largely 

 upon 



(a) Extent of tidal rise. 



(b) Degree of exposure. 



To these two factors may be added yet a third : 



(c) Angle of slope. 



This latter feature is very well illustrated in the accounts of the 

 algal vegetation at Clare Island and Lough Ine in Ireland. Table 

 XIV sets out the differences to be seen at Clare Island between a 

 sloping and a flat shore. 



A rather more detailed analysis of the same problem has been 

 presented by Rees (1935) for Lough Ine. 



Apart from the actual control of the zones themselves it has 

 become increasingly evident that the position of a zone on the 

 shore is to some extent determined by the temperatures of the 

 different seasons. Attention was first drawn to this aspect of the 

 problem by Knight and Parke (1931) in their work on the algal 



