A similar late spring or summer spawning period has also been reported 

 for many other Pacific coast bivalves including Ostrea lupida, an Olympia 

 oyster, (Coe, 1932), Donax gouldi (Coe, 1953), Pododesmus oepio (Leonard, 

 1969), Siliqua patula, a razor clam, (Quayle and Bourne, 1972), and 

 Clinoaardium nuttalli , a Nuttall's cockle, (Quayle and Bourne, 1972). 

 A rarer midwinter spawning period has been reported for Mytilus edulis , 

 a common mussel, (Moore and Reish, 1969) and Mytilus calif ovniaus , a 

 California mussel, (Young, 1942). 



The common occurrence of late spring or summer spawning when the water 

 temperature is near its maximum, suggests that temperature may be an 

 important factor in governing the reproductive cycle. Orton (1920) and 

 Nelson (1928) first suggested that each species had a critical minimum 

 temperature requirement and would not spawn below it. The importance of 

 water temperature during spawning was further indicated by Loosanoff and 

 Davis (1963) who conditioned many bivalve species for out-of-season 

 spawning at various times of the year. 



Sufficient food was also found to be a necessary requirement for 

 gametogenesis and spawning (Sastry, 1966, 1968). Food for the Pismo 

 clam was studied by Coe (1947) who found the clams digesting naked 

 dinoflagellates, small diatoms, and detritus. He suggested that most of 

 the phytoplankton ingested was not fully digested, and that the principal 

 source of nutrition was detritus. The latter suggestion seems consistent 

 with the food habits of other sand beach organisms, i.e., Emerita analoga, 

 a mole crab, and Blepharipoda oaaidentalis , a spring sand crab, and 

 suggests the reason for the concentration of clams in the surf zone with 

 the greatest detritus concentration. 



To correlate between the gonadal cycle of the Pismo clam and both 

 water temperature and food data, temperature, chlorophyll a, phaeo- 

 pigment and stage of gonadal maturity data were compiled on a monthly 

 basis and are given in Table 7. Chlorophyll a was used as an indicator 

 of phytoplankton standing crop; phaeo-pigment may be an indicator of 

 phytoplankton detritus (Lorenzen, 1967) although its real significance 

 is uncertain. Temperature, chlorophyll a, and phaeo-pigment increased 

 during the months from February to June (Table 7) . If an increase in 

 phytoplankton pigment indicates an increase of utilizable food for Pismo 

 clams, the clams may use this increase for development of their gonads. 

 Clams require a substantial amount of energy since the gonad increases 

 from 12 percent of the body tissue weight in January to 28 percent in 

 September (Giese, et al., 1967). Other sources of food, such as 

 macro-algae detritus, that are nonplanktonic in origin may be important 

 in supplying the necessary energy for gonad development but estimates of 

 their abundance or seasonal variations are not available. 



Most of the gametes are shed in September and October when the water 

 temperature and values of chlorophyll a and phaeo-pigment have reached a 

 fall maximum. Spawning may be triggered by the increase in food or 

 temperature in the fall; however, other factors may also be important. 



41 



