FORMATION OF ANIMAL AGGREGATIONS 63 



Ameiurus melas can sense through the skin the presence of another 

 fish in motion, probably by detecting the vibrations set up by the 

 tail of the nearby fish. Shght positive responses to others of a 

 group are shown by blinded fish; this reaction is not afifected by the 

 destruction of the lateral-Hne organs but is almost ehminated when 

 the skin is anesthetized with magnesium sulphate. 



When the bullheads come into actual contact with another object, 

 a positive thigmotropic response is given. The barblets are dragged 

 over the object; and by means of the sensations received, apparently 

 chemical in nature, the fish is able to discriminate between paraffin 

 models and live fishes, but it is apparently unable to distinguish be- 

 tween fish of the same or of different species. 



Bowen sums up her observations in practically these words: "In 

 the evening, as soon as it begins to grow dark, the aggregated young 

 catfish separate and swim about, sweeping through the water or 

 along the bottom with the barblets, giving a feeding reaction similar 

 to that given by blinded fish at any time of the day. As soon as it 

 begins to grow light the young fish come together into aggregations 

 in which they remain for the entire day, re-forming in a short time if 

 scattered by a disturbance. Some feeding may occur while the fish 

 are aggregating, but it is doubtful if this occurs to any extent. Usu- 

 ally the fish are in a close bunch actively pushing against each other, 

 or resting at the surface in contact or close proximity. A thigmo- 

 tactic reaction seems to be at the base of this behavior. Unless dis- 

 turbed, older fish in the aquarium rest during the day in contact with 

 the substratum, or more often in contact with one another. By 

 means of aggregations the young fish can satisfy their positive thig- 

 motaxis even while in motion. The pushing in a group suggests the 

 importance of this. Catfish will also push against other species of 

 fish, which, however, do not reciprocate. This contact reaction is 

 largely one of pressure, but gustatory response apparently plays 

 some part, as shown by the different responses to paraffin models and 

 to fish. Whether this factor is instinctive or is influenced to any ex- 

 tent by conditioning is yet to be determined. The reaction is ap- 

 parently not species specific, since there is no evidence that young 

 catfish show different behavior toward members of their own species 



