62 ANIMAL AGGREGATIONS 



Brown and Hatch, in their report, do not discuss the importance of 

 the presence of other individual gyrinids in the immediate neighbor- 

 hood in connection with the pattern complex. Rather, they give the 

 impression that each beetle is reacting as an individual to a general 

 environmental pattern, which traps it somewhat as Jennings regards 

 individual Paramecia as trapped by a drop of acid, until a collection 

 is formed. 



CATFISH AGGREGATIONS 



The young of the silurid fishes, the catfishes and the bullheads, 

 exhibit a striking type of aggregation, which has been analyzed by 

 Bowen (1929). In the species Ameiurus melas used in this work the 

 young may be observed in the summer months swimming in close 

 bunches near the surface of ditches or small ponds, packed together 

 in a more or less spherical mass. A single fortunate dip has yielded 

 over 500 of these minnows. 



If such a group is scattered, within a few minutes 2 or 3 individuals 

 appear singly and come together somewhere near the original loca- 

 tion of the entire group. Gradually they are joined by single fishes 

 or by small groups which come into the same locality, apparently 

 swimming at random. These show no reaction to the larger group 

 until they are within 2 or 3 feet of it, when they swim directly toward 

 the larger aggregation and join it. Within 30 minutes to i hour the 

 original aggregation will have re-formed. 



Appropriate tests showed that the individual fish were not react- 

 ing to a gradient of chemical emanations from the group, for they 

 do not respond to fish-conditioned water (i.e., water in which fish 

 have stood until it exhibits various chemical and perhaps physical 

 changes), even when the conditioning is greater than could be the 

 case with an aggregation in nature. Cutting the olfactory nerves had 

 no effect either on normal or on blinded fish. 



With these bullheads, as with the gyrinids, vision is the important 

 factor in the formation of the aggregation and in its subsequent inte- 

 gration. Neither blinded fish nor normal fish in the dark ever ag- 

 gregate, and normal fish will follow a moving fish model in a way 

 similar to that which results in aggregation when in the company of 

 other normal fish. 



