The Nature of the Intertidal Zonation of Plants and Animals

Abstract

1. Church Reef, in Wembury Bay, near Plymouth, was selected for a survey of common intertidal plants and animals.2. Four traverses were accurately levelled relative to Ordnance Datum, and their floras and faunas examined.3. The relation between predicted tide levels and probable actual levels is discussed, and the importance of the “splash zone” is stressed.4. As an outcome of 2 and 3, it appears that on Church Reef the most critical levels on the shore, for the species examined, are: (a) between Mean and Extreme Low Water Springs; (b) between Mean Low Water Neaps and Springs; (c) at Extreme (Lowest) High Water Neaps. The least critical level is Mean Low Water Neaps.5. Two special problems are presented by the upper limits of Littorina saxatilis and L. neritoides, and by the frontiers between Ascophyllum and Fucus spiralis and between F. spiralis and Pelvetia, where the algæ do not overlap.6. The work of previous authors is discussed for comparison with the present paper.7. An important difference is suggested between the nature of the zonation of sedentary animals and plants on the one hand, and that of animals capable of locomotion on the other hand.