the occurrence of a spring tide. In our study, spawn- 

 ing began around the time of spring tides, but in- 

 duction of spawning by hydrostatic pressure has not 

 been reported in mussels. Alternatively, spawning 

 may be induced by other factors associated with 

 spring tides, such as increased temperature fluctua- 

 tions, air exposure, and water movement. Temper- 

 ature fluctuations have been shown to induce labor- 

 atory spawning in Mytilus edulis (Bayne 1976). 



While a temperature threshold is suggested, time 

 of year may also be important as indicated by the 

 spawning periods in Table 2. Of the 10 studies ex- 

 amined, all but one reported the initiation of spawn- 

 ing from May to June. Aside from temperature, the 

 initiation of spawning may be influenced by another 

 cyclic phenomena such as photoperiod. Light and 

 photoperiod in particular have been shown to affect 

 the timing of reproduction in a number of marine 

 invertebrates (Segal 1970). While adult mussels are 

 sensitive to changes in light intensity (Bayne et al. 

 1976), the ability to detect changing photoperiod has 

 not been demonstrated. The results of this study 

 have been attributed to annual temperature cycles, 

 but until light response of mussels is more fully ex- 

 amined photoperiod cannot be ruled out. 



Variations in larval abundance from year to year 

 do not appear to be linked to temperature, nor to 

 availability of food energy. Kautsky (1982) reported 

 that Baltic Sea mussel populations were limited to 

 one major spawning by reduced food availability dur- 

 ing the remainder of the year. Similarly, Thompson 

 (1979) attributed annual variation in reproductive 

 condition and fecundity of mussels along the coast 

 of Nova Scotia to annual variations in food supply. 

 Bayne (1975) noted that while poor nutrition does 

 not significantly alter the timing of gametogenesis, 

 it can result in resorption of gametes prior to spawn- 

 ing. Newell et al. (1982) suggested that the cycle of 

 food availability could affect both the nutrient 

 storage cycle and the timing of gametogenic events, 

 including spawning. In every year of our study the 

 spring augmentation of phytoplankton was well 

 under way by March or April, with densities >10 5 

 cells 1 _1 . Significant numbers of mussel larvae 

 were first detected between late May and early 

 June. Thus, it appears that food is not limiting to 

 either adult or larval mussel populations in our area. 

 Our phytoplankton data, however, do not include the 

 smaller naked nanoplankton which, together with 

 particulate organic matter, could account for more 

 than half of the available energy in the Damariscotta 

 River (Incze 1979). This fraction would be a better 

 index of food available to mussel larvae and should 

 be included in studies attempting to link abundance 



or setting success of larvae to their food supply. 

 Onset of spawning in Damariscotta River mussel 

 populations is predictable from year to year. It oc- 

 curs when water temperature exceeds 10°-12.5°C, 

 and near the spring tide portion of the neap-spring 

 cycle. Food does not appear to be limiting to either 

 gametogenesis or the development of larvae. 



Acknowledgments 



We thank H. Hidu for stimulating discussions and 

 for criticizing an earlier draft of the manuscript. E. 

 S. Gardella and A. L. Heinig contributed greatly to 

 the sampling efforts. Greg Podniesinski was sup- 

 ported by UMO-UNH Sea Grant R/FD-99 awarded 

 to H. Hidu. 



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