REPRODUCTIVE CYCLES 627 



(the purple sea urchin), S. franciscauus (the large red sea urchin), 

 and S. fragilis- (the fragile sea urchin from deep water). The species 

 of mollusks under investigation are the chitons, Kather'ma tunicata 

 and MopaUa hindsii; the giant chiton, Cryptochiton stelleri; and red 

 and black abalones, Haliotis rufescens and H. crackerodii, respectively. 

 The crustaceans under study include the isopod, Ligia occidentalis, 

 and the crabs, Pachygrapsus crassipes (the green shore crab), Hemi- 

 grapsus niidus (the purple shore crab), Petrolistes cinctipes (the 

 porcelain crab), Piigettia product a (the kelp crab), and Emerita 

 analoga (the sand crab). These specimens are usually collected at 

 low tide once a month. 



Many observations of the spawning of marine animals of the West 

 Coast have been made, especially by the MacGinities (1949), both in 

 the field and in the laboratory. While such information is interesting, 

 it probably gives only the peak of the activity in the reproductive cycle. 

 We have seen instances where the gonads of sea urchins were large 

 and the animals ready to spawn, but they did not do so until some- 

 thing triggered them. Spawning in various animals may be triggered 

 by any one of a number of factors — chemicals in the sea (including 

 chemical secreted by one of the sexes on spawning), light or phases of 

 the moon (Korringa, 1947), temperature change (Crisp, 1957), tidal 

 rhythm and pressure (Korringa, 1947, etc., while the annual reproduc- 

 tive cycle probably depends upon some other factors. A method used 

 in the past for determining an annual reproductive cycle is the size 

 of the gonad at different times in the year (Bullough, 1951). This 

 method is used here at least when the gonads are clearly separable 

 from other tissues in the body, e.g., in the echinoderms. In that case 

 the volume of the gonad, determined by adding the gonadal tissue to 

 a graduate filled with sea water, divided by the weight of the animals 

 was designated as the gonad index (Lasker and Giese, 1954). In 

 later studies this value was multiplied by 100 to give a whole number. 

 In recent work bits of gonads were examined microscopically, and 

 tests for fertilizability of the eggs were made when possible. Eggs of 

 starfishes and mollusks do not fertilize unless spawned naturally. 



While in the chitons the gonad is clearly separable and can be 



2 This species perhaps should be called Allocentrotes fragilis according to 

 Swann (1953). It occurs in water below 48 fathoms 



