170 



ANALYSIS OF THE ENVIRONMENT 



vide an interesting check on the ways in 

 which fishes adjust to the salt concentration 

 of their environment. Anadromous fishes 

 breed in fresh water, and the young migrate 

 to the sea, where they Hve until sexual 

 maturity; catadromous fishes exhibit con- 

 verse migration. The anadromous chinook 

 salmon, Oncorhynchus, shows a somewhat 

 greater salt concentration of the blood 



abruptly from salt to fresh water, or vice 

 versa. The equally severe tests of knowledge 

 concerning osmoregulatory mechanisms fur- 

 nished by inhabitants of brine cannot be 

 met at present. Artemia, the brine shrimp 

 shows eury salinity in an extreme form; it 

 can hve in fresh water and in salt lakes 

 with a concentration of 222 per mille and 

 more. The permeability of the surface is 



Fig. 38. Posterior segments of Aedes larvae (lower row) and Culex larvae (upper row), 

 showing size of anal papillae, from media with different concentrations of sodium chloride. A, 

 Distilled water; B, tap water (0.006 per cent of sodium chloride); C, 0.075 per cent; D, 0.34 

 per cent; E, 0.65 per cent; F, 0.90 per cent. ( From Wigglesworth. ) 



when in the sea than after the spawning in- 

 vasion of fresh water. The measured de- 

 pression of freezing point of the blood of 

 marine specimens ranges from 0.7° to 0.8°, 

 that for spawning fish from 0.61° to 

 0.67° C. The indications are that these salm- 

 on swallow sea water while in the ocean as 

 do other marine teleosts, and that they de- 

 crease the amount swallowed while in their 

 long, nonfeeding existence in fresh water. 

 They apparently have the mechanisms of 

 marine animals for osmotic regulation while 

 in the sea, and those of fresh-water ani- 

 mals, including lowered permeability of gill 

 membranes, while in fresh water. 



The eel, a catadromous fish, shows a simi- 

 lar set of behavioral and physiological ad- 

 justments, including the remarkably low 

 permeability of the skin and gills (Krogh. 

 op. cit., p. 150). Known principles meet the 

 tests furnished by these fishes that can pass 



low, but it is permeable to water, and the 

 method of osmotic regulation is unknown 

 (Krogh, 1939). 



IONIC EXCHANGE 



The other side of the osmotic picture, the 

 diflFusion of ions, is more obscure than the 

 diffusion of water. Animal membranes, 

 weakly permeable to water, are also peiTne- 

 able to many ions. The precise mechanisms 

 whereby ionic transfers through a mem- 

 brane occur scarcely concern us here. 

 Marine invertebrates approach ionic balance 

 with sea water; hence their membranes 

 must be permeable to ions as well as to 

 water. With marine teleost fishes, the prob- 

 lem is to secrete ions engulfed when sea 

 water is swallowed, and there is evidence 

 that at least CI" is excreted actively through 

 the gills, and Krogh (1939, p. 145), 

 who has done much to formulate the prob- 



