It's not just Midsummer, it's also full moon – and then the blue mussel along our coasts is expected to release a large quantity of sperm and eggs.
The blue mussel reproduces just at full moon, but it also requires light and warmth.
I guess the blue mussel in the Stockholm archipelago will reproduce on Midsummer evening, provided it's not windy, says Lena Kautsky, professor of marine plant ecology at Stockholm University's Baltic Sea Centre to Sveriges Radio's Science Radio.
However, how the blue mussel knows it's full moon remains a mystery.
It also appears that the blue mussel is releasing its eggs and sperm earlier in the year due to climate change.
