632 PHOTOPERIODISM IN INVERTEBRATES 



index did not decline at the same time. During 1955-56 and 1956-57 

 the spawning, as determined in our work by the fall in gonad size, 

 occurred during April and April-May, respectively. 



Again, statistical analysis shows that the cycle is the same for both 

 males and females (Bennett and Giese, unpublished). It is also inter- 

 esting that the gonads of the females always reach a relatively larger 

 size than those of the males, by as much as a third, during the peak of 

 the gonadal cycle. 



It is not possible to test the fertilizability of the eggs, even at the 

 peak of the gonadal cycle, unless they are shed naturally. In one case 

 where eggs were spawned in the aquaria, sperm were also shed and the 

 eggs were fertilized and developed. In the ochre starfish the males can- 

 not be distinguished from the females during the months when the 

 gonads are shrunken. 



DISCUSSION 



The ocean is such a highly constant environment that the small 

 seasonal changes in temperature and chemical nature may not be 

 sufficient to set off cyclic changes such as occur after the much more 

 drastic temperature variations affecting terrestrial and fresh-water or- 

 ganisms. Photoperiod, however, may well play a role, at least for ani- 

 mals in the lighted zone of the ocean. It is of interest to determine 

 whether the breeding cycles of the animals discussed in this report are 

 correlated with any of the environmental variables, such as food, 

 temperature, and light (for literature review, see Giese, 1959). 



That food plays an important part in the reproductive cycles is sug- 

 gested by the fact that a spectacular growth of some 20- to 25-fold in 

 mass occurs during the period when the immature shrunken gonad 

 grows to the size of the turgid gonad ready to spawn. Such expansion 

 must require adequate food supply. Particular chemicals such as 

 desoxyribonucleic acid in the male and ribonucleic acid in the female 

 gonad may show an increase of about thirty-fold (Giese et al., 1959). 

 It is, of course, conceivable that the food for such increases in gonad 

 size might come from the use of food in storage organs. 



If the ochre star, P. ochraceiis, is starved for a prolonged time, it 

 fails to develop its gonads and sheds no eggs when its fellows, given 



