356 POPULATIONS 



of crowding." It is to provide an illustra- 

 tion of this phenomenon that the work of 

 Allee on goldfishes is reviewed (1938). 



It is relatively easy to show that mixed- 

 species populations of many animals may 

 grow faster than do populations of single 

 species. The common experience of aqua- 

 rists that the presence of the snails in 

 aquaria increases the rate of growth and 

 well-being of their fishes is a case in point. 

 Their experience has been somewhat veri- 

 fied by laboratory experimentation. A more 

 crucial test involves individuals of the same 

 species as, for example, all snails, or all 

 goldfishes. This leads to a brief formaliza- 

 tion of the problem : Is there some optimum 

 size of the population at which individuals 

 grow most rapidly? 



In the analysis of this question a synthetic 

 pond water is made up by dissolving in 

 distilled water selected salts of high chemi- 

 cal purity. Into such water three-inch long 

 goldfishes are placed so as to provide a 

 "conditioning coefficient" of about twenty- 

 five. This coeflBcient is obtained by multi- 

 plying the number of fishes by their mean 

 length in millimeters and then dividing by 

 the number of liters of water in the con- 

 tainer. Living in this water, the fishes con- 

 dition it by liberating organic matter and 

 carbon dioxide. They are left in the water 

 for twenty-one hours, while a similar 

 amount of the same water stands under 

 the same conditions except for the absence 

 of fishes. 



At the end of this period the clean con- 

 trol water is siphoned into a number of 

 clean jars, and a small photographically 

 measured goldfish is placed in each. At the 

 same time the conditioned water is si- 

 phoned into clean jars, either with or with- 

 out removing such particles as excrement 

 that may be present. A group of small, 

 measured goldfishes such as those used in 

 the control jars is transferred into the con- 

 ditioned water. These small, "assay" fishes 

 have been feeding for about two hours be- 

 fore their transfer. The larger conditioning 

 fishes are allowed to feed for a somewhat 

 longer time before being washed to re- 



' These reports are not limited to growth 

 per se, but concern also such related phe- 

 nomena as graft transplants (Price, 1941; 

 Rawles, 1936), regeneration (Wilson, 1910; 

 GaltsofiF, 1925; Chu and Pai, 1944), hatching 

 enzymes (Merwin, 1945), and egg cleavage 

 ( Allee and Evans, 1937). 



move food residues and replaced in an- 

 other lot of water to condition that. 



After some twenty to tliirty days of such 

 care, each fish is again photographed to 

 scale, the photographs are measured, and 

 the relative growth is determined for the 

 forms that have daily been placed into per- 

 fectly clean, synthetic pond water, as com- 

 pared with those daily put into conditioned 

 water— water in which other goldfishes have 

 hved for a day. Allee found that the fishes 

 in the conditioned, i.e., sUghtly contam- 

 inated, water grew significantly more than 

 did those in clean water. "Hence we have 

 demonstrated that under the conditions of 

 our experiments the goldfish grow better in 

 water in which other similar goldfish have 

 Uved than they do when they are daily 

 transferred to perfectly clean water" (1938, 

 p. 94). 



The question now arises: What are the 

 factors involved that make conditioned 

 water a more suitable medium for the 

 growth of goldfishes than an unconditioned 

 medium? Although this matter has not yet 

 been completely analyzed, Allee has un- 

 earthed some important leads. 



The conditioning fishes, it will be remem- 

 bered, are fed for two or more hours daily 

 and are then washed and placed in a 

 fresh batch of water. Although the fishes 

 are never fed in the water they are condi- 

 tioning, within a few hours after their 

 transfer into it from the feeding aquariiun 

 the water becomes more or less cloudy with 

 regurgitated food particles. These bits of 

 food are large enough so that the growth- 

 assay fishes can strain them out of the 

 water. When such particles are removed by 

 filtering, the growth-promoting power of the 

 conditioned water is greatly lessened, but 

 not completely lost. In one group of experi- 

 ments Allee showed that suspended food 

 particles accounted for about 80 per cent 

 of that increased growth displayed by the 

 experimental forms. This suggests the pres- 

 ence of some factor, apart from food in- 

 take, which is also operating to produce the 

 total observed efiFect. 



Certain statements about the possible 

 existence of a "growth-promoting" substance 

 can be made. For example, it is known that 

 the skin glands of goldfishes secrete slime. 

 When a chemical extract of this substance 

 is prepared, a growth-promoting factor can 

 be recovered. This seems to be a protein, 

 and it is efi^ective as a growth stimulant 



