54 THE BIOLOGY OF MARINE ANIMALS 



ally made up in the food. In addition, as shown by Krogh, the gills of 

 several species of freshwater fish are able to absorb chloride actively against 

 a concentration gradient (Salmo, Gasterosteus, etc.). 



It is therefore concluded that the ability of teleost fishes to regulate the 

 osmotic concentration of their blood and thereby maintain a steady water 

 content is due to two complementary mechanisms. In a hyperosmotic 

 environment, sea water is absorbed through the alimentary canal and the 

 excess salt is eliminated through the gills, while the kidneys produce 

 minimal quantities of urine. In hypo-osmotic media, however, the gills 

 resist the penetration of water, which is eliminated through the kidneys, 

 and the necessary salts are obtained partly in the food but to a large 

 extent in many species by absorption through mucous membranes in the 

 buccal and branchial regions (77, 126). 



The cells responsible for salt transfer have been identified as columnar 

 acidophilic elements located in the gill filaments and elsewhere in the oral 

 and pharyngeal region (Conger, Fundulus, Pleuronectes, etc.). In the eury- 

 haline species Fundulus heteroc/itus, cytological changes appear in these 

 cells when the fish are transferred from salt to fresh water, and fresh to 

 salt, and it is probable that salt transfer — absorption and excretion — is 

 performed by the same cell under altered conditions (19, 23, 37). 



A number of marine fish have become morphologically adapted to a 

 regime of diminished urine production by reduction or complete atrophy 

 of kidney glomeruli, which are essentially filtration devices. Aglomerular 

 kidneys occur in toadfishes (Haplodoci), anglers (Pediculati) and pipe- 

 fishes (Lophobranchii). 



Teleosts vary greatly in their ability to tolerate salinity fluctuations, and 

 the majority of strictly marine and freshwater species are stenohaline. 

 Tolerance of osmotic changes is important in estuarine teleosts and in 

 euryhaline species which migrate to and from the sea for spawning pur- 

 poses. Examples are estuarine flounder Platichthys flesus; anadromous 

 salmon; catadromous eels. The eel {Anguilla anguilla) tolerates an abrupt 

 change from fresh to salt water, and after an initial loss in weight due to 

 exosmosis, it re-establishes equilibrium in about 48 hours. Adjustment of 

 the killifish Fundulus heteroclitus to both fresh and sea water has been 

 investigated from several aspects. On transferring to fresh water from salt 

 there is a temporary increase in weight, which returns to normal after 18 

 hours, and a loss of chloride amounting to 60% in 4 days. Adaptation to 

 fresh water is complete after 24 hours. When returned to sea water they 

 regain their normal chloride content and density within 6 hours. The 

 presence of calcium in fresh water reduces chloride loss and water uptake 

 under experimental conditions and is probably a factor influencing the 

 distribution of euryhaline species, and permitting the colonization of fresh 

 waters by marine species (11, 12, 46). 



In the littoral zone some species must be able to resist desiccation when 

 the tide is out. For example, the mud skipper Periophthalmus hops about 

 actively on the mud in the heat of the sun during tidal ebb. Waters where 



