48 



Comparative Animal Physiology 



Marine Fish, In the ocean the problem is to conserve water and exclude 

 salts, and the water-excreting glomerular kidney is a liability. The blood of 

 marine teleosts is not much more concentrated than that of fresh-water fish. 

 In other words, a mechanism for maintaining a high degree of hypotonicity 

 is necessary. The urine is scanty, 2.5 to 4 cc./kg./24 hours in sculpin and 

 toadfish. ^^ The urine is always hypotonic to the blood, and there is no rela- 

 tion between urine flow and CI" content, fience there must be some other 

 route for getting rid of salt which enters osmotically. Smith"-^ found, by the 

 use of dyes and a divided chamber, that marine fish, unlike fresh-water ones, 

 swallow large quantities of water and that both water and salt are absorbed 

 from the intestine. The intestinal concentration decreases down the gut and 

 approaches that of the blood, hence there is no significant separation of salt 



ELASMO- 

 CYCLOSTOME BRANCH 



MAMMAL 



00 



Q RENAL CORPUSCLE 



)l NECK SEGMENT 



\M PROXIMAL CONVOLUTED SEGMENT 

 I INTERMEDIATE SEGMENT 

 I THIN SEGMENT OF HENLE'S LOOP 



m DISTAL CONVOLUTED SEGMENT 



m INITIAL COLLECTING TUBULE 



lo 



Fig. 20. Schematic representation of the kidney unit (nephron) of different vertebrates. 

 (Elasmob ranch modified from Kempton,^"" others from Marshall."'^) 



from water and elimination of salt in the digestive tract. Nitrogenous wastes 

 are excreted by way of the gills. When the pyloric end of the stomach is tied, 

 urine formation falls off, body weight declines, and death results. *^ The 

 urine salts are largely Mg+ + , Ca++, S04=, and phosphate, and most of the 

 Na+, K+, and Cl~ absorbed must be excreted extrarenally. Also only part of 

 the water drunk is excreted in urine. An eel in sea water excretes only 20 per 

 cent of its fluid by the kidney. The sculpin, daddy sculpin, goosefish, haddock, 

 and others all show zero or low Cl~ in their urine, but with handling the fish 

 become diuretic and Cl~, Mg++, S04=, and phosphate in the urine are 

 increased. -°- 



In view of the reduced urine output it is not surprising to find less well 

 developed kidneys in marine teleosts than in fresh-water fish. As shown in 

 Figure 20, some of them lack glomeruli and the tubules are relatively short. ^'^^ 

 In Opsanns tau, the toadfish, and Lophiiis, the goosefish, for example, glomeru- 



