From the middle of June to the middle of September the germ cells of Mytilus edulis L. and Macoma baltica L. in Passamaquoddy bay mature during the new-moon spring tides and are spawned throughout the remainder of the lunar tidal cycle. The initiation of spawning may be due to increased temperature since the low water of the neaps occurs about midday when the mud flats are heated by the rays of the sun. Mya arenaria L. shows the same phenomenon to a lesser degree from mid-June through August. Yoldia sapotilla (Gould), a deep-water form, has a bimonthly spawning on the spring tides.