630 PHOTOPERIODISM IN INVERTEBRATES 



E. analoga, and the seabat P. m'lniata show a spring to summer breed- 

 ing season. Some, lilce the crabs P. cinctipes and P. product a, appear 

 to breed all year round. Time does not permit examination of all the 

 species cited, and therefore, only two are singled out to illustrate the 

 approach, namely, the sea urchin S. purpuratus and the ochre star 

 P. ochraceus. 



The Purple Sea Urchin, Strongylocentrotus purpuratus 



It is easiest to get eggs for experimental work from this species of 

 sea urchin during the winter months; however, some usable eggs are 

 obtainable during most of the year. To find one female with mature 

 eggs in an unfavorable month, however, may require the sacrifice of 

 a hundred or more animals, e.g., in July. This probably means that 

 some animals may develop and mature eggs in almost any month of 

 the year, but only during the winter season are all of the animals 

 developing gametes. At the height of the gonad index all the females 

 carry numerous eggs, and the eggs fertilize and develop. As the gonad 

 index declines from its winter peak all the females usually have normal- 

 sized eggs which fertilize and develop nomially but the number of eggs 

 per shrunken gonad is small. On the other hand, months later when 

 the gonads have again grown in size and the average gonad index is 

 large, the eggs are generally small and unfertilizable, many of them 

 appearing as germinal vesicles. It is only as the gonad size becomes 

 very large again with the approach of another breeding season that 

 some of the females and later, all of the females, again have normal- 

 sized fertilizable eggs. 



The males are capable of producing motile sperm in any part of the 

 year. When the gonads are large, spawning is easily induced by electric 

 shock or by shaking. This does not occur when the gonad has shrunk, 

 and only small quantities of sperm can be seen when a piece of the 

 gonad is teased in sea water. 



A number of pertinent facts have been discovered by statistical 

 analysis of data gathered on the sea urchin. ( 1 ) The gonadal cycles of 

 the males and females for any year are the same as shown by the data 

 for the years 1952-53 and 1953-54 (Bennett and Giese, 1955). (2) 

 The average size of gonads is similar in males and females at all 

 times during the cycle. (3) During all months of the year gonad size 



