Most of the Moon’s effects on Earth are small and imperceptible. But for the grunion, it’s absolutely essential.
Although it’s the closest astronomical body to Earth, the Moon is still a whopping 380,000 kilometers away.
Orbiting Earth with a revolutionary period of just under one month, its physical effects are limited.
On long timescales, it slows our planetary rotation while migrating away from us.
On human timescales, we only notice its monthly phases and our terrestrial tides.
The combined effects of the Moon and Sun create two tidal bulges around Earth, yielding high tides and low tides twice daily.
When the Sun, Earth, and Moon all align, we get spring tides: the highest high tides possible.
Tidal extremes occur during new and full Moons, with twice the magnitude of intermediate-phase neap tides.
One terrestrial animal, the grunion, has uniquely adapted to take advantage of this lunar-induced phenomenon.
During the highest spring tides, the females come onto sandy beaches, dig with their tails, and lay eggs.
The males intertwine with the females, depositing sperm before departing.
As the tides regress, the grunion eggs incubate on shore.
The eggs only hatch after 10–11 days: when the next spring tides arrive, washing them out to sea.
Without our Moon’s effects, the grunion’s reproductive cycle would be impossible.
Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words. Talk less; smile more.Ethan Siegel is the author of Beyond the Galaxy and Treknology. You can pre-order his third book, currently in development: the Encyclopaedia Cosmologica.