I don't know if this is true throughout the world of sea worms and shellfish but according to the April 2011 issue of Wadden magazine it goes for those in the Waddenzee (Wadden Sea , The Netherlands, Germany and Denmark).
When there is a full moon in April all the shellfish and worms on the bottom of the Waddenzee simultaneously mate.
The Sea becomes full of eggs, far too many for the fish and birds to consume. So, there is a greater chance that more eggs go on to hatch and the offspring have a greater chance of survival than in normal circumstances
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