6 THE BIOLOGY OF MARINE ANIMALS 



are exposed on the shore this mechanism would tend to reduce the liability 

 to heating in sunshine. 



Salinity. Large variations in salinity occur in the inter-tidal region and 

 in estuaries. During low tide the shore may be washed by rain water and 

 those animals that are active and remain in the open may be surrounded 

 by water which is almost fresh until the return of the sea. Freshwater 

 seepage on the shore forms areas of reduced salinity, and these are favoured 

 by certain species. Tidal pools are subject to considerable fluctuations in 



Distance from the Mouth (kilometres) 



Bristol Channel Severn Estuary 



60 40 20 20 40 60 SO 



Fig. 1 .2. Salinity Changes in an Estuary (Severn), and the Decrease in the 

 Number of Marine Inter-tidal Animals away from the Mouth 



(Above) tidal and seasonal changes in salinity in the Bristol Channel and Severn 

 Estuary (1940). (Below) number of species of marine inter-tidal animals recorded at 

 various stations. (After Bassindale (4).) 



salinity, particularly those pools high on the shore which are cut off from 

 the sea for some days during the period of neap tides. Wide environmental 

 variations in salinity demand tolerance or functional methods of regula- 

 tion against osmotic stress, and these will be considered in the next chapter. 

 In estuaries the salinity conditions are peculiar owing to the effects of 

 tidal oscillations and river discharge, leading to variable admixture of 

 fresh and salt waters. Only a restricted number of marine animals can 

 exist under such conditions of reduced and variable salinity, and it is 

 found that the number of marine species decreases as the estuary is 

 ascended (Fig. 1.2). The distance which any given species can penetrate 

 up the estuary depends on the lowest salinity it can tolerate for a given 



