PHYSICOCHEMICAL AND CHEMICAL PHASES 



171 



lem clearly, believes that marine fishes gen- 

 erally excrete chlorine and sodium inde- 

 pendently. The process is reversed in fresh- 

 water organisms. In them there is a steady 

 inflow of water and a steady loss of ions. 

 Both processes must be counteracted. As 

 we have already seen, the excess water 

 is characteristically controlled by secretion 

 of a dilute watery urine. The inner 

 concentration of electrolytes is main- 

 tained by an active ionic absorption. 

 This interpretation is supported by a 

 considerable body of evidence and in- 

 cludes that presented by Wigglesworth 

 (1938), summarized in Figure 38. As the 

 figures show, the size of anal papillae of 

 mosquito larvae is correlated with the con- 

 centration of chloride solutions in the media 

 in which they five, supposedly because 

 these act in absorbing ions from the sur- 

 rounding solution. Papillae that may func- 

 tion similarly are found in various parts of 

 many insect larvae that five in fresh water. 



One of the primary ecological divisions, 

 the separation of aquatic animals into the 

 communities associated with marine and 

 those associated wdth fresh waters, is not 

 only a matter of permeabifity to water and 

 of methods of supplying water or of efimi- 

 nating excess water intake by osmosis, there 

 is also the equally important matter of the 

 maintenance of an appropriate concentra- 

 tion of necessary ions. Both aspects of 

 osmoregulation are also highly important for 

 inhabitants of brackish water. The perme- 

 ability of skin, gill, and gut membranes, 

 the functioning and even the structure of 

 contractile vacuoles of the Protozoa, of 

 nephridia of invertebrates, and the kidneys 

 of vertebrates, and the structure and proc- 

 esses associated with extrarenal transport 

 of ions through membranes are among the 

 important morphological and physiological 

 adjustments associated with the distribution 

 of aquatic animals into waters of various 

 salinities. It is interesting to find the ne- 

 phridial system, usually located deep in the 

 internal anatomy of animals and, in many 

 ways, well insulated from the outside envi- 

 ronment, directly associated with animal 

 ecology. 



As for many other ecological processes, a 

 study of osmotic relations throughout the 

 Ufe history shows that habitats suitable 

 for reproductive processes are frequently 

 more restricted than those that can be tol- 

 erated by adult animals. Many aquatic 



animals shed eggs and spermatozoa into 

 the water, and fertilization takes place ex- 

 ternally. Sea water is a fairly favorable me- 

 dium for external fertiUzation; fresh wa- 

 ter is much less so. Fish spermatozoa 

 retain their activity for but a minute or two 

 in fresh water, and in this medium, artifi- 

 cial insemination at the hands of fish cul- 

 turists is often more efficient than is natural 

 fertiUzation, because the spermatozoa can 

 be so placed that they are exposed to water 

 for a shorter period before reaching the 

 eggs. About 10 per cent of British trout 

 eggs are fertihzed on the natural breeding 

 grounds; 90 per cent can be fertilized by 

 artificial methods (Gray, 1920). Spermato- 

 zoa of fresh-water trout remain active for 

 ten or twenty minutes in brackish water in 

 which the fish do not breed, although, so 

 far as longevity of spermatozoa is con- 

 cerned, brackish water is ten to twent)' 

 times more favorable than the fresh water 

 in which trout do breed. Other maladjust- 

 ments to sahnity are known; shad spawn in 

 fresh water, but the optimum for egg de- 

 velopment is about 1 per cent sahnity. 



Evidence from optimum sahnity, such as 

 has just been reviewed, points to the sea 

 as the ancestral home of fishes. This con- 

 clusion goes against much evidence from 

 the osmoregulatory processes and the kid- 

 ney structures of adult teleosts. Homer 

 Smith (1932) was much impressed by the 

 latter evidence and argues that it points to 

 a fresh-water origin for all fishes. Others 

 have been struck by the resemblance be- 

 tween the salt concentration and ionic bal- 

 ance in the blood and other body fluids of 

 many aquatic and even of land vertebrates 

 with that supposed to have existed in the 

 ocean at the time of the evolution of the 

 early representatives of present day classes 

 of animals. Rogers (1938), from the point 

 of view of comparative physiology, and 

 Pearse (1939), from that of ecology, re- 

 view these ideas sympathetically, and 

 Beadle (1943) also is friendly to them. 

 Available comparisons are stimulating to 

 the imagination without being definitely 

 convincing. The subject of osmotic regula- 

 tion in nature will be extensively developed 

 in future books on ecological physiology. 



IONS AS ENATRONMENTAL FACTORS 



The influence of ions in the environment 

 of animals is best shown in the ocean. Sea 

 water is nearly a physiologically balanced 



