INTRODUCTORY 5 



above the level of high-water neaps (Fig. 1.1). Such animals may possess 

 thick integuments which reduce the water lost through evaporation when 

 the animal is in air (e.g. shore crabs) or hard shells which can be clamped 

 firmly together during the period of exposure (e.g. shore barnacles and 

 mussels). Still other forms obtain protection by burrowing into the sub- 

 stratum, creeping under rocks or into crevices, or among the fronds of 

 shore-wrack, e.g. inter-tidal polychaetes and teleosts. The amount of 

 exposure to the atmosphere which an animal can tolerate, and the pattern 

 of its behaviour when the tide ebbs, are important factors governing its 

 distribution on the shore. 



Temperature. The inter-tidal zone experiences to a great degree the 

 vicissitudes of terrestrial climate, and at low water animals are subject to 

 the full range of aerial temperatures. Shore temperatures show great 

 variation with latitude and season. On the shores of temperate regions 

 ground temperatures may fall below 0°C in winter and reach 40C in the 

 heat of the summer sun, and higher temperatures will be encountered on 

 tropical shores. Rock pools vary greatly in temperature, depending on 

 their volume, on the air temperature and the length of time they are 

 separated from the sea. 



Littoral animals must of necessity be eurythermal in order to survive, 

 and this is illustrated in investigations dealing with the heat tolerance of 

 littoral gastropods. Broekhuysen (5) determined the lethal temperatures 

 and the survival times at high temperatures for a series of shore gastropods 

 at False Bay (South Africa), and he discovered that these factors were 

 graded according to the zonal sequence of the species on the shore. Evans 

 (20) has investigated the thermal death-points of littoral gastropods in 

 Cardigan Bay. Mean lethal temperatures varied from 46-3°C for Littorina 

 neritoides which lives above mean high-water neaps, to 36-2°C for Gibbula 

 cineraria which occurs in damp shaded positions below mean low-water 

 neaps. The highest temperatures recorded on the shore were 40-5°C for sun- 

 baked rocks and 30°C in tidal pools. Both workers concluded that the degree 

 of heat tolerance shown by these various snails is related to the temper- 

 ature range which they encounter in nature, and the safety factor is 

 sufficiently high so that they rarely, if ever, are exposed on the shore to 

 temperatures that are lethal. 



Abnormal temperatures may occasionally exceed the extremes tolerated 

 by the species and result in considerable mortality. Few cold-blooded 

 invertebrates have any means of opposing thermal fluctuations apart from 

 seeking shelter. An interesting exception is the littoral isopod Ligia which 

 has a temperature significantly below that of the environment in full sun- 

 light, owing to the effect of evaporation of water vapour from the surface 

 of the body. It has been suggested that such animals may migrate from 

 enclosed humid niches in which the air is still, upwards into the open, 

 where convection and evaporation can reduce the body temperature 

 (17, 18). Also, there are some shore crabs which blanch when illuminated, 

 and thereby reflect more of the incident light and heat. When these crabs 



