PHOTOPERIODISM IN FISHES IN RELATION 

 TO THE ANNUAL SEXUAL CYCLE ' 



ROBERT WHITING HARRINGTON, JR. 



Florida State Board of Health, Vero Beach, Florida 



A brief, comprehensive review of experiments on photoperiodism in 

 fishes is precluded by incoherent results requiring lengthy qualifica- 

 tions. In the present state of knowledge, meaningful comparisons be- 

 tween experiments, moreover, can be made only in the context of the 

 annual reproductive cycle as a whole. A dearth of experiments long 

 enough or of data sufficient to cover all periods of this cycle is noted 

 by Atz (1957) in his inclusive review of the literature to the middle of 

 1956, with analyses of then unpublished material and tabular sum- 

 maries of all experiments. Other pertinent reviews are those of Hoar 

 (1951, 1955). Atz ascribes much of the difficulty in comparing ex- 

 periments and some discrepant results with the same species to prob- 

 able differences in response to day length by fishes at various periods 

 of their cycles. With these cues, attention here will focus on crucial 

 details of the few experiments in which the response mechanism has 

 been related to the annual cycle. This narrows consideration mostly to 

 the cyprinid cycle as studied chiefly in the bridled shiner, Notropis 

 bifrenatiis, in the European minnow, Phoxinus phoxinus, and to some 

 extent in the specialized bitterling, Rhodeiis amarus, and to the cycle 

 of the three-spined stickleback, Gasterosteus aculeatus, concerning 

 which additional information has appeared since the review of Atz. 



THE CYPRINID SEXUAL CYCLE 



Results of exposing fish to a long photoperiod at different seasons 

 of the year (Harrington, 1947, 1950, 1956, 1957) are shown in Fig. 

 1. The time axis, with monthly intervals above and annual cycle of day 



^ Contribution No. 67, Entomological Research Center, Florida State Board 

 of Health, Vero Beach, Florida. 



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