BROUSSEAU: MYA REPRODUCTION AND RECRUITMENT 



about 75^ had completely spawned and returned 

 to the indifferent condition. Gametogenesis usu- 

 ally resumed after spawning, and by early June 

 about one-quarter of the clams were again ripe 

 and partially spawned. The presence of cytolyzed 

 unspawned gametes in the summer samples sug- 

 gested that the same individuals had also been 

 ripe earlier in the year. Thus the observed spawn- 

 ing pattern was due to repeated spawning by the 

 same individuals rather than asynchronous 

 spawning of individuals within the population. 



A similar spawning pattern was observed in 

 1973, except that gametogenesis did not begin 

 until April (Figure 4) and the summer spawning 

 peak occurred in July rather than late June-early 

 July. The data for both years indicate a more or 

 less consistent recovery period between repro- 

 ductive cycles. The data for the 1975 season indi- 

 cate that spring spawning occurred in March as it 

 did in 1974, but the summer sampling intervals 

 were too irregular to describe details of the sum- 

 mer spawning. Nevertheless, occurrence of a 

 summer spawning is confirmed by the gonad con- 

 dition of the clams in the August sample. 



Photomicrographs of representative female 

 stages in the spring and summer peaks of the 

 annual cycle are shown in Figure 5. The pattern of 

 development in the clams during the spring cycle 

 differs from that of the later summer one. In the 

 female, the spring cycle is characterized by rapid 

 gametogenesis, resulting in smaller oocyte size 

 and fewer numbers of oocytes produced per unit of 

 gonad tissue (Table 1), so different density values 

 were used for calculations of fecundity (gonad vol- 

 ume X density) in different seasons. A significant 

 seasonal difference in the diameter of ripe oocytes 

 of female M. arenaria was detected using one-way 

 analysis of variance (P^0.05). Similarly, male 

 clams appear to undergo rapid maturation and 

 produce fewer gametes than during the summer 

 spawning. Fully ripe males were not encountered 

 in any of the spring samples, however, spent males 

 were numerous, indicating that spawning had 

 taken place. Spring spawning may be a facultative 

 event, characterized by rapid maturation and the 

 subsequent utilization of the abundant food sup- 

 ply that is available during the major phyto- 

 plankton "bloom" that occurs nearshore during 

 this period. 



Temperature is an important factor influencing 

 the gonadal cycle in a variety of marine bivalves 

 (Loosanoff 1937a, b; Landers 1954; Giese 1959; 

 Carriker 1961; Ansell et al. 1964; Galtsoff 1964; 



Calabrese 1970). If temperature is indeed a factor 

 in the onset of reproduction in M. arenaria as 

 previously believed (Nelson 1928; Belding 1930), 

 short-term temperature patterns in winter and 

 early spring should correlate with the annual tim- 

 ing of gametogenesis (Figure 4). In fact, tempera- 

 tures during January-March 1974 averaged about 

 2° higher than during the same period of the pre- 

 vious year (Figure 2) and gametogenesis began a 

 month earlier than in 1973. 



The actual role of temperature in the timing of 

 gamete release remains unclear. Spring spawning 

 peaks occurred at surface water temperatures of 

 4°-6°C and summer spawnings at 15°-18°C. Al- 

 though the interstitial water of exposed tidal flats 

 warms up considerably during midday spring lows 

 (Johnson 1965), it is unlikely that interstitial 

 temperatures would be high enough to account for 

 these differences. If these is a critical minimum 

 temperature for spawning it is at or above 4°-6°C. 

 No maximum limit can be discerned from these 

 data. The role of rapid temperature change in 

 triggering spawning as suggested by other au- 

 thors (Battle 1932; Stickney 1963) has not been 

 assessed here. 



Sex Ratios and Fecundity 



The reproductive potential of a population de- 

 pends, in large part, on the number of fertile 

 females and the number of young produced per 

 female. The proportion of females in all size- 

 classes in three large samples from the Jones 

 River in 1973 (n = 1,266), 1974 (n = 859), and 

 1975 (n = 150) did not differ signiflcantly from 

 one-half. In size-classes <25 mm, male and female 

 gonads were indistinguishable. No evidence of 

 hermaphroditism or protandry was observed. 



The number oocytes produced was found to in- 

 crease exponentially with increasing female body 

 size. The regression equations for oocyte numbers 

 (O) versus female shell length (S) are: 



Spring 1973: log^^O = 



Summer 1973: logj^O^ 



Spring 1973: log^oO^ 



1.45 + 3.29 1og,.S 



-1.29 + 3.28 1ogj^S 

 -0.90 + 2.91 \og^^S 

 Summer 1974: log,oO= -1.42 + 3.32 log^^S 



Comparison of the regression lines by analysis of 

 covariance indicated that the lines were parallel 

 (P 3=0.05) but the elevations of the lines were sig- 

 nificantly different (P^0.05). Total oocyte produc- 

 tion during 1973 was greater than during 1974. 



159 



