64 ILLINOIS BIOLOGICAL MONOGRAPHS [64 



for under certain conditions, and at certain times of the year the "intense 

 activity" of the species during the day is as naught compared to the 

 frenzied activity of the species at night. A small point, perhaps, but the 

 key to many of the problems requiring solution in a study of the life 

 history of the silversides. 



BREEDING HABITS 



The adult of Labidesthes sicculus is primarily a shallow water inhabitant. 

 After the fish has attained approximately two thirds of its mature size, the 

 species takes up permanently a shallow water habitat which is in direct 

 variance with the habits of the immature fish. Both young and adults 

 are surface species, living normally within less than a meter of the surface 

 of the water, and spending most of their time within ten or twelve centi- 

 meters of the surface. Indeed, the species is the most consistently "top- 

 water" fish in these fresh water lakes, distinctly more so even than the gar, 

 Lepisosteus osseus. The gar often seeks very deep water when it feeds, 

 grasping a minnow sidewise the fish descends at an angle of about 45° 

 to a depth of from ten to thirty feet, where the juggling which accompanies 

 the adjustment of the food into a position permitting its entrance into the 

 small mouth takes place. After feeding the gar may remain in the deep 

 water for some time, upward of fifteen minutes, before ascending slowly 

 to the surface. The silversides never under any conditions descends below 

 the upper meter of water, this being the maximum depth sought by the 

 adults, while nothing can drive the immature individuals more than a few 

 centimeters below the surface. Hence the statement that the silversides 

 is the most characteristic of our surface fishes. It is as typical of the upper 

 stratum of water as the Johnny darter (Boleosoma nigrum) or Miller's 

 Thumb (Coitus bairdii bairdii) is of the very bottom of the lakes and 

 rivers. 



The breeding habits occur during May and June under normal, season- 

 able conditions, the exact time being closely correlated with the tempera- 

 ture of the water. As the surface waters warm up under the influence of 

 the May sun, the silversides are to be seen swimming near the shores often 

 in pairs. As there is no sexual dimorphism it is impossible to distinguish 

 the sexes while the fish are in their element, but repeated captures and 

 laboratory examinations show that the two individuals associated at this 

 time are male and female. The fishes often swim in perfect alignment, 

 one above the other, a distance of approximately ten inches to a foot 

 intervening. This alignment is under the control of the upper fish which, 

 because of the position of the eyes, can keep the lower individual under 

 observation. This was repeatedly borne out by observations which showed 

 the fact that when the path of progression deviated from a straight line, 

 the first sign of the turning was made by the lower fish. The upper fish is 

 the male, the lower the female. Frequently two, three or more males may 



