Estuaries 31 



There are a few animals which can survive direct transfer from 

 ocean to fresh water, and there are many marine animals which 

 hve at times in fresh water or in diluted sea water (Pearse, 1929; 

 Schlieper, 1929a; Sumner, 1906; Vaughan, 1919). As would be 

 expected, small marine animals usually have less ability to live in 

 diluted sea water than large animals. Young animals are gen- 

 erally less resistant but often possess "greater capacity to acclimate" 



(Andrews, 1925). Invertebrates with hard exoskeletons or slimy 

 coverings are more resistant to changes in salinity than those with- 

 out such protection. Brues (1927) kept a portunid crab, Callinectes 

 ornatus Ordway, alive in fresh water for three months. Along the 

 coast of Holland shrimps (CragO' vulgaris Fabricius) migrate from 

 salt to brackish water in spring, and immature individuals move 

 further inland than adults (Havinga, 1930) . According to Harms 



(1929), 22 species of selachians are established in fresh water, and 

 tides have been an important factor in facilitating their passage 

 through estuaries. The clams belonging to the family Dreisensidae 

 have apparently spread from the Black Sea and established typical 

 species after entering rivers (Andrusov, 1897) . The Black Sea 

 was probably more salty and characteristically marine in the past 

 than it is now. 



In estuaries not all types of spreading animals are moving up- 

 stream. Some fresh-water species are making progress toward the 

 ocean. For example, various lines of evidence indicate that insects 

 had their origin on land. Many of them have since invaded fresh 

 water and a few have become established in or on the sea. Several 

 species of water striders skim over the surface of the open ocean. 

 The more specialized insects appear to be able to endure high 

 salinity better than more primitive types (Buxton, 1926; Thorpe, 

 1927) . Chironomid larvae endure variations in salinity better than 

 corixids. Among the insects as a whole, the flies (Diptera) have 

 been the most successful colonizers of the sea. Some of them live in 



