68 FROM FISH TO PHILOSOPHER 



evitable increase in absorption of water through the gills, 

 but species habituated to salt water have not been suc- 

 cessfully established in fresh water under aquarium con- 

 ditions, possibly because the transition must be effected 

 gradually. However, many of the naturally occurring 

 fresh-water forms are identical with marine species and 

 can probably migrate freely back and forth. This is un- 

 questionably true of the fresh-water sawfish Pristis 

 microdon, which belongs technically among the sharks. 

 In 1930 the writer was able to study this sawfish in some 

 detail at Teluk Anson, Perak, in the Federated Malay 

 States. 



Teluk Anson is about forty miles inland from the 

 mouth of the Perak River where it flows into the Straits 

 of Malacca. Survey of the salinity of bottom and surface 

 water showed that the lighter fresh water tends to float 

 out on the surface of the Straits, while at high tide the 

 salt water shelves inland on the river bottom. At the 

 time of the survey, however, the bottom water showed 

 no significant salt some thirty miles below our Teluk 

 Anson station. Pristis occurs in considerable numbers at 

 Teluk Anson and for fifty miles up-river, frequenting the 

 shallower water and growing to a size of sixty pounds, 

 and there is every reason to believe that it is thoroughly 

 habituated to life in fresh water and can reproduce there. 



Small sawfish (three to ten poimds) were caught by 

 hand nets in shallow water and transported to the station 

 in spindle-shaped bamboo baskets, which the Malays 

 use to transport hve fish from the fishing grounds to the 

 village, towing them behind the sampan, and which 

 they also use to store the fish for periods of a week or 

 more. At least foiu: species of rays and three of sharks 

 at one time or another invade the Perak River to or above 

 Teluk Anson, but the only ones other than Pristis taken 

 at this time were the ray Dasyatis uarrwk and the shark 

 Carcharhinus melanopterus, neither in suflBcient num- 

 bers to permit experimental studies. 



In fresh water, the urine flow in Pristis is large, aver- 



