210 ADVENTURES IN RADIOISOTOPE RESEARCH 



The fact that no significant difference was found in tlie uptakes of 

 32p from waters rich and poor in oxygen does not prove conclusively 

 that the main uptake of ^^P does not take place through the gills, as 

 it is possible that the organism reacts to oxygen shortage in the water 

 not by an increase in the water circulation through the gills, but by an 

 enhanced oxygen extraction from the water. However, the above 

 result induces us to draw our attention to the other probable way of 

 entrance, the digestive tract, which, as marine fish drink large quanti- 

 ties of the water in which they swim, is the most probable path of ionic 

 uptake by such fish. 



Homer Smith (1930, 1931) added phenol red to aquarium water and 

 found that the dye became concentrated in the intestine and also that 

 it could not be absorbed through the gills and the skin. By measuring 

 the concentration of the dye he was able to calculate the extent of water 

 absorption taking place^. An eel weighing 143.5 gm, was found to have 

 Swallowed in the course of 20 hours 12.3 ml of sea-water. A number of 

 experiments on eels and sculpins show that per kgm of weight those 

 fish Swallow from 40 to 225 ml of sea-water per day. The minimum 

 swallowing observed is thus 0.04 mlpergm per day. The amount of labelled 

 phosphorus present in 4.04 ml of water is that found by us in a stickle- 

 back weighing 1 gm after the lapse of 16 days. K the amount of water 

 swallowed by the stickleback is not still larger than the largest amount 

 observed in eels and sculpins, then we have to conclude that only a 

 part of phosphate present in the swallowed water is absorbed by 

 the sticklebacks. This result is by no means improbable. Though 

 water is very easily absorbed, Homer Smith found that only 81 per 

 cent of the water swallowed by the eel had been absorbed and, further- 

 more, that, while monovalent ions such as Na+, K+, and Cl~ were 

 absorbed to a very large extent, divalent ions such as Ca"*""*", Mg"^"*", 

 and SO^" were not, these ions being concentrated in the rectal fluid. 

 In relation to chlorine, sulphate was, for example, concentrated 24 

 times in the rectal fluid, (ef. A. Krogh, 1939). The result obtained by 

 us suggests a similar fate of the phosphate ions. 



A fish weighing 0.92 gm. took up in the course of 8 days 4 • 10"^ per 

 cent of the activity of the labelled sea-water and, thus, as the phosphorus 

 content of the sea- water was 880 y (730 added -f 150 present beforehand), 

 3.5 7 of phosphorus were taken up mainly as sodium phosphate by the 

 fish. The amount of phosphate taken up by the fish during the same 

 time varies appreciably from fish to fish. However, these variations 

 do not influence the results obtained for the rate of renewal since, when 

 calculating this magnitude, the skeleton activity and the liver activity 

 of the same fish are considered. 



1 A dofailod account of the work of Homer Smith is given by Krogh (1939) 



