INTRODUCTORY 3 



the shore. Conditions of life in the littoral zone are quite dissimilar from 

 those occurring elsewhere. When the tide is in the inhabitants are bathed 

 by sea water, a relatively constant medium, but during tidal ebb they are 

 periodically uncovered and exposed to the rigours of aerial climate. 

 Quantitatively the fauna of the shore is very rich, but the vicissitudes of 

 existence associated with this environment have led to a high degree of 

 specialization. Consequently the population of the shore is peculiar in 

 many respects and contains a high proportion of animals not found else- 

 where. Because the littoral zone is more readily accessible to the zoologist 

 than the waters offshore, and the animals living there can be observed 

 directly, this region has received much attention. 



In the tidal zone several environmental levels can be distinguished, 

 determined by the degree of atmospheric exposure to which they are sub- 

 jected. On the lower shore, lying below low-water neaps, there are long 

 periods during neap tides when the shore is not exposed. Here live many 

 sublittoral animals which can tolerate only limited exposure to the air. 

 Other sublittoral animals invade the inter-tidal region during tidal flow 

 only, or come inshore on occasion to spawn. Lying between the limits of 

 low- and high-water neaps is a region which is covered twice daily by the 

 sea, and which contains a rich fauna of typical inter-tidal species. Above 

 high-water neaps there are long periods when the shore is exposed for 

 days on end. The fauna of this region contains fewer species than lower 

 levels, and many of these are restricted to higher regions of the shore. 

 Consequently, zonation of animals is a conspicuous feature of the shore, 

 and is illustrated in Fig. 1.1 depicting conditions on a rocky shore in 

 New Zealand (30, 36, 43, 44). 



The environmental variables which adult animals encounter in the inter- 

 tidal zone, and which they endeavour to counter by morphological, 

 physiological and behavioural means, are manifold and complex. No other 

 region in the ocean presents such diversity of habitats and range of physical 

 conditions as the inter-tidal zone. The variables with which we are con- 

 cerned may be considered as follows. 



Water Movements. In the littoral region water movements result from 

 waves, swell and tidal action. In sheltered bays and estuaries such move- 

 ments may be slight and gentle, but on rocky coast lines facing the open 

 ocean the mechanical force of the waves is tremendous. 



Animals living on wave-swept shores resist the destructive effects of 

 wave action by suitable structural devices, and by modifications of form, 

 or they actively seek shelter and cover. Acorn barnacles live cemented to 

 rocks, chitons and limpets adhere firmly with their broad feet, and mussels 

 attach themselves by strong byssus threads. Certain shore-fish, for example 

 Lepadogaster, have their pelvic fins modified into strong suckers. Finally, 

 the depressed or conical shape of littoral chitons and limpets offers minimal 

 resistance to water movements. 



Emergence. As the result of tidal movements animals on the shore are 

 periodically exposed to air, either daily, or for longer intervals if they live 



