ROSKNBLUM and NIESEN: SPAWNINC CVCl.K OF An'/l /IftAW/IWM 



and San Francisco Bay populations cannot be as- 

 sumed, the age classes can be used as a first estimate 

 of approximate age with size. 



Studies of the spawning cycle of soft-shell clams 

 from the east and west coasts of the United States 

 reveal both similarities and differences in spawning 

 pattern. Spawning on both coasts begins in early 

 spring as the water warms from the lower winter 

 temperatures. The majority of east coast populations 

 studied show two separate spawnings each year, 

 while populations studied on the west coast show a 

 single more protracted spawning. Differences in the 

 length of spawning and the number of separate 

 spawning episodes are probably partially related to 

 the phenology of water temperature change and the 

 difference in the range of water temperatures that 

 occur on either coast. 



Mya arenaria in San Francisco Bay, studied dur- 

 ing the 1979-80 season, began ripening earlier than 

 M. arenaria of the New England region studied 

 previously. Ropes and Stickney (1965) encountered 

 active clams in eastern Maine by late January; 

 however, ripe clams were not discovered until mid- 

 May, and at that time none appeared to have spawn- 

 ed. Clams from their Booth Bay Harbor samples 

 showed the earliest ripening, which was in April and 

 May; by September spawning was over. 



Brousseau (1978) reported a biannual cycle of 

 spawning for M. arenaria from Cape Ann, MA. The 

 first spawning at Cape Ann occurred as early as that 

 in San Francisco Bay (March); however, it was of 

 short duration, being over by April. A separate sec- 

 ond spawning took place during June through July. 

 Brousseau's figures indicate water temperatures 

 began to rise from a low of 1°C around Cape Ann as 

 early as mid-February, but did not rise above 10° C 

 before May. It is possible that the increase in 

 temperature triggered an early spawning, but the 

 continuing, relatively cold temperature prevented an 

 adequate build-up of mature gametes to sustain a 

 prolonged spawning. Once spawning had taken 

 place, the clams may have had to undergo another 

 period of gametogenesis prior to a second spawning. 

 Brousseau (1978, page 159) stated, "The presence of 

 cytolyzed unspawned gametes in the summer 

 samples suggested that the same individuals had also 

 been ripe earlier in the year. Thus the observed 

 spawning pattern was due to repeated spawning by 

 the same individuals rather than asynchronous 

 spawning of individuals within the population." 



Pfitzenmeyer (1962) also reported two annual 

 periods of spawning in M. arenaria at Solomons, 

 MD. He noted that "The first umbone larvae of the 



year usually were found in May after the surface 

 waters rise above 15°C or a mean temperature of 

 16.7°C." Salinities which remained constant 

 throughout the spring remained near 10 ppt. This 

 corresponds to the March temperature-salinity pat- 

 terns in San Francisco Bay. During March, San 

 Francisco clams were just beginning to spawn. Pfit- 

 zenmeyer also noted the disappearance of larvae 

 from the Maryland waters as the temperature rose 

 above a mean of 21.4° C. The larvae did not reappear 

 until temperatures had fallen below this point. He 

 concluded an optimal temperature range exists dur- 

 ing which spawning may occur. In San Francisco 

 Bay, temperatures rose to or above 21 °C only in the 

 following instances: Foster City- 11 April - 23° C, 2 

 July - 21°C, 25 August - 21°C; Candlestick Point- 

 9 April - 23°C, 30 June - 23°C; Point Isabel- 10 

 April - 23°C (Fig. 4). 



Porter (1974) noted a single yearly spawning from 

 late May to early September among M. arenaria 

 from Skagit Bay, WA. This is a shorter spawning 

 season than seen among San Francisco Bay M. 

 arenaria and may be a result of lower temperatures 

 (4.8°-15.7°C) encountered in Washington. 



Simel^ reported a single spawning from late March 

 through April for soft-shell clams from Humboldt 

 Bay, CA. Generally, this more northerly part of 

 California has a cooler climate than the San Fran- 

 cisco Bay area. Simel indicated that the later stages 

 of gametogenesis corresponded with a peak in the 

 phytoplankton abundance. 



Studies of the spawning cycles of M. arenaria 

 from the east and west coasts of the United States 

 suggests a pattern of spawning behavior. Spawning 

 begins as the water temperature rises in the spring. 

 Pfitzenmeyer's (1962) work suggests M. arenaria's 

 optimal spawning range falls between 15° and 21 °C. 

 Differences between the spawning cycle of M. 

 arenaria from San Francisco Bay and that of M. 

 arenaria from New England, Canada, Washington, 

 and northern California may be explained as the 

 logical result of the different seasonal warming pat- 

 terns and extremes encountered in the different 

 areas. San Francisco Bay does not cool to the same 

 temperatures as the other areas and has a much 

 longer period of moderate water temperatures, ex- 

 tending from late March through summer and into 

 September and October. Consequently, M. 

 arenaria's spawning season is equally protracted in 

 San Francisco Bay. 



^N. Simel, Humboldt State University, Areata, CA 94542, pers. 

 commun. 1982. 



411 



