50 Comparative Animal Physiology 



density measurement is accomplished in 6 hours on going to sea water or in 

 24 hours on going to fresh water. The body density decHne in fresh water is 

 due partly to chloride loss and partly to increase in gas volume in the swim 

 bladder. Reverse changes occur on going to sea water. Thus marked changes 

 in density can occur with little change in osmotic concentration. "*" 



Both American and European eels breed in the Sargasso Sea of the mid- 

 Atlantic; then the young fish migrate to fresh water where they mature. 

 Osmotic adaptation in the European eel, Anguilla vulgaris, has been carefully 

 studied. Figure 21 shows that the blood is hypotonic in sea water and hyper- 

 tonic in fresh water. ^- It changes in concentration in going from the ocean 

 to fresh water by about the same amount as that of the king salmon. The 

 osmoconcentration of blood of the eel is as follows: in sea water Ai=.73, in 

 fresh water Ai=-6l (data from Keys^^^); in sea water Ai=.69, in fresh water 

 Ai=.62 (data from DuvaP^) (Fig. 21). 



A starving Anguilla in either fresh or sea water lost 0.2 to 0.5 per cent of 

 its weight per day. ^^^ When the esophagus was blocked with a balloon the 

 eel died in sea water after a 12 per cent loss of weight in 3 days, whereas in 

 fresh water it lived well but lost 0.7 per cent of its weight per day. Thus the 

 eel does not drink fresh water but does drink sea water. When transferred from 

 fresh to sea water there is a weight loss and a new equilibrium is reached in 

 about 48 hours if drinking is permitted. Conversely, if the eel is transferred 

 from sea to fresh water there is an initial weight increase. According to Duval 

 about 75 per cent of the osmotic pressure of the blood is due to NaCl, and the 

 NaCl concentration of the blood is higher in sea water than in fresh water; 

 the serum protein also is higher in sea water. ^^^ 



The kidney of Anguilla has glomeruli, also a distal convoluted segment, 

 and in fresh water its kidneys behave as do the kidneys in typical fresh -water 

 fish. In addition, Anguilla has a very low skin and gill permeability to both 

 chloride and water. Eels in distilled water lose chloride at a rate about one- 

 twentieth the rate of loss from Sahno irideus.''^^^^' ^'^^ Krogh was unable to 

 obtain evidence for chloride absorption from dilute solutions, and unfed eels 

 died even in dilute Ringer solution for lack of salt before they starved for 

 organic material. It may be that the secretory epithelium can concentrate 

 chloride outward against a gradient to sea water, but not inward. Duval and 

 others maintained that the mucus which surrounds an eel is a protective barrier 

 which decreases permeability to water and to salt, but Krogh believes that the 

 increased permeability after mucus removal is due to skin injury in the removal. 

 Anguilla, then, in the ocean is hypotonic, swallows sea water, and gets rid of 

 salt by the gills. In fresh water it excretes a dilute urine with its glomerular 

 kidneys, replaces salt by feeding, and shows very low skin permeability to 

 salt and water. 



Elasmobranchs and Cyclostomes. Elasmobranch fishes are today largely 

 marine, although palcontoiogical evidence indicates that they were once more 

 abundant in fresh water. 



Osmoregulation in elasmobranchs is reviewed by Smith. --^' As shown in 

 Table 2, the blood is al\va\'s lupertonic to the medium. 



The salt concentration (if the blood of both marine and fresh-water elasmo- 

 branchs is of the same order as that in fresh-water teleosts, but the osmotic 

 pressure of the blood due to NaCl is only 41 to 47 per cent of the total. ^'- 



