114 FROM FISH TO PHILOSOPHER 



food. The sturgeon {Acipenser sp.) and the salmon 

 (Salmo solar) spawn in fresh water, but grow to ma- 

 turity in the sea; while the fresh-water trout, which are 

 small members of the genus Salmo, frequently descend 

 from the rivers into the coastal waters. 



Both the eel and salmon make their migration from 

 fresh to salt water with but little change in the osmotic 

 pressure of the blood. That a reduction in filtration rate 

 plays an important part is certain, but no data are avail- 

 able on any of these fishes during migration. Nor is 

 anything known about how the marine teleost goes about 

 reducing the filtration rate to levels that will enable it 

 to remain in water balance. One bit of evidence indicates 

 that this reduction is attributable to active constriction 

 of the glomerular arterioles: namely, when marine fishes 

 are kept in captivity under not too favorable conditions 

 they ultimately sicken and die, and in this physiological 

 debacle the filtration rate and urine flow increase to as- 

 tonishingly high values as though some renal inhibitory 

 mechanism had broken down and the animal was drink- 

 ing itself to death in an effort to compensate for water 

 loss. In the longhom sculpin, for example, during this 

 laboratory diuresis' (which is of unknown origin) the 

 filtration rate may increase from 12 to nearly 200 cc, 

 the urine flow from 5 to 100 cc. per kg. per day. It has 

 been suggested that loss of slime increases the osmotic 

 theft of water through the skin, but the point is far from 

 proved. Forster has shown that this 'laboratory diuresis' 

 also occurs in the essentially aglomerular daddy sculpin, 

 M. scorpius, in which the urine flow may increase from 

 2 to 40 cc. per kg. per day, as well as in the aglomerular 

 goosefish, Lophius. This is not surprising because the 

 basic fact is that if the fish drinks more sea water for 

 any reason, it has more magnesium and sulfate to be 

 excreted by the tubules, and increased excretion of these 

 solutes will lead to an increase in urine flow. In any case, 

 a salt-water fish is one of the most delicate experimental 

 animals with which the renal physiologist has to deal, 



