FISHERY BULLETIN: VOL. 76. NO. 1 



Table l.— Oocyte diameter (microns) of "ripe" female Mya 

 arenaria from each of the spawning periods of the annual repro- 

 ductive cycles of 1973-75. Values represent the means of mea- 

 surements on 20 individual oocytes per clam. The number of 

 individuals sampled (n) and the overall means (x) are given. 



The limited data available for 1975 were not 

 analyzed. In addition, fecundity differed between 

 spawning seasons within a single year. Oocyte 

 production was larger during the summer spawn- 

 ing cycle than during the spring one in 1973 and 

 1974. Females <40 mm in length were never 

 gravid. 



Recruitment 



In sedentary bivalves, such as M. arenaria, set- 

 tlement of recently metamorphosed larvae from 

 the plankton is the only significant source of re- 

 cruitment. Spat were most abundant in May and 

 September of 1974 (Figure 6). Allowing approxi- 

 mately 5 wk for planktonic life, metamorphosis, 

 and growth to 2 mm, these peaks correlate with 

 and probably result from the spawnings described 

 above. This timetable for metamorphosis corre- 

 sponds with similar estimates made for M. 

 arenaria under natural (Kellogg 1905) and 

 laboratory (Stickney 1964) conditions. 



Large fluctuations in yearly recruitment are 

 characteristic of many bivalve populations 

 (Hughes 1970). Therefore, recruitment of young, 

 when it occurs, may represent a large proportion of 

 the population. A comparison of the size-frequency 

 distributions in samples taken in October 1973 

 and October 1974 reveals that this is also true for 

 M. arenaria. A substantial settlement occurred in 

 May of 1974, but by the fall, this cohort had nearly 



disappeared. Similarly, the fall set (Figure 6, IB) 

 quickly vanished. In contrast, survival of indi- 

 viduals from the spring and summer sets in the 

 previous year was good, as evidenced by the num- 

 bers of these size-classes persisting in the October 

 samples. The spring and fall sets of 1975 were 

 extremely poor (Robert Knowles pers. commun.) 

 and nearly 100% juvenile mortality had occurred 

 by November of that year (Figure 6). 



DISCUSSION 



The results of the gonad examinations indicate 

 that M. arenaria from Gloucester, Mass., spawn 

 twice each year (Figure 3). This spawning pattern 

 is similar to that reported for populations south of 

 Cape Cod (Mead and Barnes 1903; Landers 1954; 

 Pfitzenmeyer 1962), although isolated instances of 

 single spawnings have been documented (Shaw 

 1962). Previous investigators have reported that 

 clams from northern Massachusetts began spawn- 

 ing in July and completed spawning by late Sep- 

 tember. There is strong evidence to indicate that 

 populations from Plum Island Sound studied by 

 Belding (1930) and Ropes and Stickney (1965), 

 spawn only once annually. Proof of annual spawn- 

 ing in populations south of Plum Island Sound to 

 Cape Cod, however, is less convincing. Based on 

 the presence of larval M. arenaria in plankton 

 samples, Stevenson (1907) reported that clams 

 from Ipswich and Plymouth spawn in the late 

 summer. It is not clear from his report whether 

 sampling was conducted year-round or only dur- 

 ing the summer months. If the latter is true, then 

 early spring spawning activity may have been 

 overlooked and the later summer peak (Figure 3) 

 incorrectly interpreted as evidence for a single 

 late spawning season for clams of this area. It is 

 possible that the biannual spawning observed in 

 Gloucester is a local phenomenon. Nevertheless, 

 more work is needed before any generalizations 

 concerning the frequency of spawning of clams in 

 northern Massachusetts can be made. 



Indirect evidence also gives clear indication of a 

 biannual spawning cycle. First, recruitment pat- 

 terns both corroborated the evidence from gonad 

 examinations and indicated other populations in 

 the area spawned at the same times. Owing to 

 pelagic larval dispersal in M. arenaria, bursts of 

 recruitment (Figure 6) would be obscured if 

 spawning were not synchronous in nearby popula- 

 tions. Secondly, sea-surface temperatures for 1973 



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