PHOTOPERIODISM IN FISHES 653 



lengths below, records the fluctuations in the daily photoperiod at the 

 latitude concerned, between about 9 hr at the winter, and 15 at the 

 summer solstice. Diagrams below record events of a normal breeding 

 cycle and of three cycles induced experimentally. 



In bridled shiners in nature, the earliest sign of sexuality was nuptial 

 color (c) first apparent April 22, when the daily photoperiod was 

 13V2 hr, the earliest breeding behavior (b) was observed May 22, the 

 earliest spawning (S) June 7, the last spawning (S') June 30, the last 

 breeding behavior (b') July 13, and the last color (c') August 5. It is 

 noteworthy that the breeding season ends long before the photoperiod 

 has declined to the day length of April 22. The interval between the 

 first outward sign of sexuality and the onset of spawning will be called 

 the prespawning period. In nature, this numbered 46 days — spanned 

 by the arrow — and in view of later experimental results was started by 

 the advent of day lengths above a certain threshold. 



In the preliminary experiment on the bridled shiner, the prespawn- 

 ing period began with the imposition of long days (L) January 1, end- 

 ing 44 days later, at the first spawning (S), February 14. In an experi- 

 ment on a centrarchid fish, the banded sunfish, Enneacanthus obesus, 

 the prespawning period began with the imposition of long days (L) 

 October 4, ending 45 days later, at the first spawning (S) November 

 17. In the final experiment on the bridled shiner, the long photoperiod 

 (L) was begun September 26, but spawning did not occur 44 to 46 

 days later. On the contrary, the internal mechanism governing attain- 

 ment of functional maturity was refractory to the stimulus of the long 

 photoperiod for about the next 52 days — as symbolized by the broken 

 line — first responding to this stimulus about November 17. The 

 gonadal histology confirmed that the mechanism was refractory as late 

 as October 26, the testes of a male killed November 16 showed only 

 the start of spermiogenesis. the first outward sign of sexuality was 

 breeding behavior (b) observed November 17, and the first spawning 

 occurred 45 days after November 17 — a time interval in conformity 

 with the prespawning periods of the other experiments and with that 

 surmised from field observations. Thus, a postspawning refractory 

 period, from about mid- July to mid-November, appears to alternate 

 with a responsive period, from about mid-November to mid-July. 

 Several short-day fish and two males hatched from eggs spawned 



