Every year about this time, one of the planet’s most amazing migrations takes place on Christmas Island, a volcanic mountaintop in the Indian Ocean. Tens of millions of red crabs leave the island’s forests and head for the beach. The females lay their eggs in the water, where the eggs hatch and the larvae drift to sea.

The timing of this migration depends on a couple of things. One is the start of the rainy season, which can be anywhere from October to December. And the other is the Moon: It needs to be a waning crescent, as it is right now, because the tides are gentle then, making life easier on the baby crabs.

The migrations and spawnings of many other species also are tied to the phases of the Moon. A full Moon provides extra light, while full and new Moon bring higher tides.

Some sea turtles, for example, wait for high tides to return to shore to lay their eggs. Riding the higher water level makes it easier for them to lay eggs high up on the shore, where they’re less likely to be uncovered by waves. And Barau’s petrel, a tropical seabird, returns to a tiny island in the eastern Atlantic around the full Moon to mate.

One of the most incredible lunar cycles involves many species of coral on the Great Barrier Reef in Australia. They all spawn at the same time, around December’s full Moon, filling the water with many billions of tiny eggs — a natural cycle on Earth tied to a natural cycle in the sky.

More about the Moon tomorrow.

Script by Damond Benningfield