Oysters Open and Close Their Shells as the Moon Wanes and Waxes

A new study suggests the mollusks may widen and narrow their shells depending on movement of plankton, which shifts with the lunar cycle



Shucked oyster shells lay beneath the moonlight at Fanny Bay Oyster Company on Vancouver Island in British Columbia, Canada. (Michael Wheatley / Getty Images)

By Jason Daley
smithsonian.com
January 10, 2019 5:08PM

Plants and animals follow all sorts of biological clocks—seeds sprout when soil temperatures and day length are just right, internal circadian rhythms regulate the release of hormones in humans and other mammals, the lives of some sea creatures are ruled by the ebb and flow of tides and even moonlight is an important cue for some species to mate or hunt. Now, a new study published in the journal Biology Letters suggests oysters are one of the creatures that keep tabs on the moon, and that the lunar cycle influences how widely they open their shells.

Nicola Davis at The Guardian reports that researchers discovered the oysters’ lunar love affair after tracking 12 Pacific oysters, Crassostrea gigas, that they submerged along the French coast. They then watched them carefully through three lunar cycles, each of which lasts 29.5 days. Using electrodes, they measured how widely the oysters opened their shells every 1.6 seconds, then compared that data with data about the moon’s cycle.

They found the oysters paid attention to the phases of the moon: as the moon was waxing, or growing fuller, the oysters narrowed their shells—never closing them completely. And when the moon started waning, or receding to the new moon phase, they widened their shells back up.

What that suggests is the oysters may rely on a internal lunar clock rather than direct cues, like the intensity of the moonlight. If that was the case, they would open their shells equally during the first quarter moon and the last quarter moon since the intensity of the light would be similar. But the oysters reacted differently to those phases suggesting they are following an internal calendar rather than reacting to the moonlight itself.

Read more: https://www.smithsonianmag.com/smart-news/oysters-sync-lunar-cycle-180971230/#tVHAmzBefzk1ZWYm.99