636 PHOTOPERIODISM IN INVERTEBRATES 



Crisp (1957) tested effect of a rising and falling temperature on the 

 breeding of the cold-water barnacles (B alarms balanoides and Balanus 

 balanus) which normally breed once a year between November and 

 February. When the animals were placed in tanks kept between 14° 

 and 18°C, no copulatory activity or fertilization was observed. When 

 taken to between 3° and 10°C, breeding commenced after one or two 

 months, the interval depending upon the previous temperature and 

 history. Both barnacles therefore require a period of cold for induction 

 of breeding conditions. They may even be starved without altering the 

 course of events (for a more detailed account see Giese, 1959). 



The importance of temperature to breeding of some vertebrates that 

 do not seem to be affected by photoperiod is reviewed by Bullough 

 (1951). The data correlating temperature changes and induction of 

 breeding are particularly good for the killifish, and indicative for the 

 three-spined stickleback and the fresh-water perch, as well as for the 

 edible frog and the American newt {Triton viridescens) . 



When the breeding of the sea urchin and the ochre starfish are com- 

 pared with the photoperiod, distinct correlation is seen for the latter 

 species, but no clear-cut relation in the former (Figs. 1 and 2). How- 

 ever, examination of the more extensive data in Table I shows that the 

 breeding peak is different in successive years. Since photoperiod is 

 invariant, even the slight differences in breeding cycles seen in these 

 two species raise some doubt as to the correlation, and experiments are 

 called for. It is quite likely that the spawning stimulus is different from 

 the stimulus which leads to the development of the gonads. If so, then 

 spawning might occur at almost any time during the period in which 

 the gonads are enlarged, thus making the cycle appear quite different 

 in two successive years. Early rains, for example, usually lead to 

 spawning-out in the sea urchins; no data are available for the starfish. 

 Experiments are clearly needed to establish whether the build-up of the 

 gonads can be affected by lengthening the days when in nature they 

 are decreasing in length, or suppressed by shortening the days when 

 they are lengthening. Such experiments are in progress for the starfish. 



That photoperiod governs the breeding cycle of many animals is too 

 well known to require documentation. Photoperiodism in animals was 

 discovered by Rowan (1925) for the Canadian junco, in which the 

 breeding condition of the gonad can be induced in the dead of winter 



