INTERMITTENT STREAMS 



91 



oxygen-consuming excreta and decaying plant materials. This would 

 cause death to less hardy fishes. Allee (53) found very little oxygen in 

 the waters of such pools. As it is, the pools often dry up, and the fish 

 die. The second fish to enter a small stream appears to have many of 

 the characters of the first. It is usually the red-bellied dace (Chrosomus 

 erythrogaster), which breeds on sandy or gravelly bottom (93) but toler- 

 ates standing water, being found also in some of the stagnant ponds at the 

 south end of Lake Michigan. In some streams, the black-nosed dace 

 (Rhinichthys atronasus) (Fig. 35) is second from the source. These fishes 

 go against the current, but avoid the places where it is most violent. 



Breeding Habits of a Pioneer Stream Fish 



Fig. 33. — Showing, in longitudinal section, the nest of a horned dace (Semotilus 

 atromaculatus) , with male and female fish in the nest. The stream flows in the direc- 

 tion indicated by the arrow at the upper left-hand corner of the picture; | natural 

 size (after Reighard) . 



Fig. 34. — Male and female horned dace during the spawning act. Each time 

 the male clasps the female she deposits 25 to 50 eggs in the nest. Note pearl organs on 

 the head of the male (after Reighard) . 



This one also breeds on gravel bottom, and can withstand the stagnant 

 conditions of the summer pools. 



As the stream lowers its bed, this type of formation passes gradually 

 into a later one. The beginning of the succeeding formation is heralded 

 by the coming of the Johnny darter (Boleosoma nigrum), the common 

 sucker (Catostomus commersonii) (Fig. 36), and the blunt-nosed minnow 

 (Pimephales notatus) (Fig. 37) (79). 



d) Characters of the communities. — The intermittent-stream com- 

 munities are made up of animals which are dependent upon water 

 during only a part of their lives and which possess a means of attach- 

 ment and move against current (94) (positive rheotaxis). The pool 

 communities are made up of animals tolerating great extremes of 



