28 Routes from the Sea 



merely of organisms that have proved more capable of establishing 

 themselves in abnormal circumstances and, therefore, more success- 

 ful in the peculiar line of life adapted to them." Annandale be- 

 lieved that a slight change would enable other species to become 

 established in the river above the delta. He cites a crab, Varuna 

 litter at a (Fabricius) , as one species which migrates overland in vast 

 numbers during the rainy season each year, but does not become 

 established because crabs of the family Potanionidae already occupy 

 favorable situations and can not be dislodged. A hydroid, Cam- 

 panulina ceylonensis (Browne) , also spreads continually but is elimi- 

 nated in water which is so fresh that its specific gravity falls below 

 1.006. 



As a result of his studies of the Thames River, Robson (1925) 

 states that there are usually three elements in an estuarine fauna: 

 (1) permanent indigenous species, (2) seasonal migrants, and (3) 

 stragglers. He found that a holothurian, a chaetognath, four mol- 

 lusks, a few species of copepods, and a shrimp, Palaemonetes 

 varians Leach, were restricted to brackish water. He agrees with 

 Annandale that few marine species became established in fresh 

 water by migrating through estuaries. Thorpe (1927) studied estu- 

 aries in Sussex, England, and reached similar conclusions. 



Various species are doubtless kept from spreading from the ocean 

 through estuaries into fresh water because salinity falls below their 

 limits of toleration. In the estuaries of the Rivers Tamar and Lyner 

 in Great Britain, Percival (1929) found that the number of marine 

 species decreased greatly when salinity fell below 3%, but some 

 persisted in salinity as low as 2.1%. Littoral species showed greater 

 toleration for low salinities than others. In Holland, Redeke 

 (1922) studied the distribution of brackish-water animals in rela- 

 tion to salinity and found that they were divisible into three groups 

 as shown in Table I. 



