PHOTOPERIODISM IN FISHES 657 



individuals of Rhodeus to react at a later season to the same set of 

 stimuli that induced them to mature at an earher season is no indica- 

 tion of the phase of responsiveness of individuals in the wild at the 

 latter season. Therefore, the postulation of a different reaction to the 

 same stimuli by Rhodeus at different seasons (Verhoeven and van 

 Oordt, 1955) seems premature, and must be countered with the pre- 

 sumption that postspawning refractoriness had set in. This possibility 

 is made the more likely by the fact that the ovipositor hardly reacts be- 

 fore the end of October (Duyvene de Wit, 1939), which taken in con- 

 junction with the situation in Notropis suggests a postspawning refrac- 

 tory period in Rhodeus comparable to that in Notropis. 



Fisure 3 sums up the experimental results and defines a provisional 

 terminology for the annual cyprinid sexual cycle. Superficially, an inac- 

 tive or vegetative period alternates each year with an active or repro- 

 ductive period. Underlying these is an intrinsic sexual rhythm in which 

 a long responsive period (mid-November to mid- July) alternates with 

 a shorter, refractory period (mid-July to mid-November). Long days 

 imposed within the responsive period promptly start the prespawning 

 period, with its prespawning mechanism governing occurrence of 

 nuptial color, sexual behavior, and completion of gametogenesis. Its 

 progress is measured by egg-diameter increments exceeding a critical 

 diameter (an early stage of the secondary growth phase), and it ends 

 with the first spawning. Thus begins the spawning period, which is 

 consummatory and ends long before the environmental stimuli have 

 declined to the threshold at which they initiated the prespawning 

 period. A refractory period follows, for which the name postspawning 

 period is appropriate. Long days imposed before the postspawning 

 period ends, i.e., before mid-November, fail to activate the prespawn- 

 ing mechanism but accelerate egg-diameter increase up to, but not be- 

 yond, the critical diameter. The prespawning period appears to be the 

 phase of the annual sexual cycle set right each year by the celestial 

 clock. However, an internal reproductive rhythm capable of acting in 

 the absence of the appropriate extrinsic stimuli, which normally syn- 

 chronize it, has been inferred from the inability of four months of 

 drastically reduced day length to prevent final maturity in Phoxinus 

 although spermatogenesis was slightly, and oogenesis markedly, de- 

 layed (Bullough, r940). 



