EXCRETION 301 



In certain marine teleosts glomeruli are degenerate or absent, and in such 

 kidneys filtration no longer occurs, the urine being formed by tubular 

 secretion. A large proportion of nitrogenous waste is excreted by extrarenal 

 routes, across the gills. The volume of urine formed by marine teleosts is 

 relatively small, around 2-4 c.c. /kg/day, as compared with 300 c.c. in 

 freshwater species. In addition to dealing with a fraction of nitrogenous 

 excretion, the kidney of marine fish also plays an important role in ionic 

 regulation by excreting magnesium, calcium and sulphate. In elasmo- 

 branchs, which possess a high osmotic pressure by virtue of uraemia, the 

 glomeruli are large and the kidney conserves urea. The urine flow of the 

 dogfish {Mustelus cam's) amounts to 15 c.c. /kg day. 



The subject of kidney excretion has been repeatedly reviewed and we 

 shall confine ourselves to certain problems posed by marine fish (44, 45). 

 Kidney function can be partitioned into two aspects, glomerular filtration 

 and tubular activity. Some substances, such as glucose, which are carried 

 out into the glomerular filtrate, are reabsorbed in the proximal convoluted 

 tubule and appear in the urine only when the concentration in the blood 

 exceeds a certain value, i.e. when the rate at which they enter the tubule 

 exceeds the rate at which they can be absorbed. The glomerular filtration 

 rate is measured by using some substance such as xylose or inulin which is 

 not absorbed or excreted by the tubule. The glomerular filtration rate 

 (xylose) in marine teleosts is rather low, 14 c.c./ kg day in the sculpin 

 Myoxocephalus. Some water and salts are resorbed by the tubules since 

 only 3 c.c. /kg day are evacuated. 



Tubular function is measured by comparing the rate of excretion of 

 inulin or xylose with other substances. The rate of excretion of a substance 

 is expressed as clearance, which refers to the volume of blood cleared of a 

 particular substance in a given time. Since a small amount of xylose is 

 resorbed, inulin clearance rates slightly exceed those of xylose. In dogfish 

 {Mustelus, Squalus), for example, the clearance of magnesium, phosphate 

 and creatinine exceeds that of xylose or inulin, while clearance of water, 

 urea and glucose is less. Experiments of this nature show that magnesium, 

 phosphate and creatinine are actively secreted by the tubules, whereas 

 water, glucose and urea are resorbed. The proportion of filtered urea which 

 is taken up by the tubules varies with the concentration in the blood plasma 

 and ranges from 70-99-5%. TMO is also conserved in elasmobranchs by 

 tubular resorption. 



The kidney of aglomerular fish has proved a very useful organ for in- 

 vestigating tubular excretion. When aglomerular and glomerular fish are 

 compared, the tubule of the aglomerular kidney shows striking capacity 

 for excretion of certain substances by active secretory processes. There is 

 little difference in the rate of urine formation in the two types of kidneys 

 and under normal conditions marine teleosts appear to be functionally 

 aglomerular. The aglomerular kidney is capable of developing a secretion 

 pressure greater than that existing in the aorta, and of excreting and con- 

 centrating a wide variety of plasma constituents. There are, however, 



