Oysters gape a little wider in the dark of the moon

I have unusually strong feelings about daylight savings time. Without fail, every spring I am ecstatic to gain an extra hour of daylight. And every fall … well, I don’t like to talk about that hour of lost daylight.

While animals couldn’t care less about daylight saving, they are actually responsive to lunar cycles. Moonlight plays a crucial role for some species’ feeding habits.

Lunar cycles, or lunar phases, refer to the percentage of moon we observe every night. A lunar cycle lasts 29.5 days it takes the moon to orbit the earth (which orbits the sun). We see the sunlit portion of the moon every night — sometimes this is a slice of the moon, other times it’s a full moon. The moon “waxes” as the light increases and it seemingly grows in size. It “wanes” after a full moon, becoming darker until we have a new moon.

For some creatures, the most important part of this cycle is either the full or new moon. Forget about the myth of wolves howling to the moon — oysters actually open their shells wider during a darkened new moon!

The most common oyster species on our beaches — and dinner plates — are Pacific oysters (Crassostrea gigas). And while they seem unassuming and like an easy way to scrape exposed toes on the shore, new research has shown these bivalves may enjoy a midnight snack under new moons. Researchers submerged and monitored 12 Pacific oysters on the French coast over three and a half lunar cycles. Using electrodes, they discovered that the oysters opened their shells wider as the moon’s brightness decreased. As the moon waxed, they opened their shells more narrowly.

Oysters got eyes

How can oysters tell what stage the moon is in? Oysters have “eyes,” usually around the outer rim of their shells, that sense light and shadows. These eyes are particularly useful when predators are overhead — they can snap shut to avoid becoming a midnight snack themselves!

Researchers think this lunar sensitivity might be linked to food sources. Oysters open their valves when there is food available. Research has shown that plankton, which oysters filter out of saltwater, also react to moonlight. In oceans across the world, zooplankton rise to the surface at night to feed on algae (phytoplankton) and avoid predation by much larger animals that hunt during the day.

Plankton also happen to be attracted to light. Humans long ago figured this out and have been using it to their advantage ever since, particularly when fishing. While shining lights into the water at night seems to attract fish and larger predators, the light is actually attracting plankton, which is then gobbled up by those larger predators. A single light in Puget Sound at night can attract a frenzy of activity.

Does this reaction relate back to the moon’s cycle? Are humans confusing creatures when they shine artificial light from flashlights or shore-side homes? Researchers still aren’t entirely sure.

One thing we can be sure of: the Puget Sound and greater Salish Sea often resemble the weirdness and wonder of a good sci-fi novel. We can continue to observe the wild marine wonders around us from piers, boats, or the beach — of which Gig Harbor has plenty!

Carly Vester is a staff member of Harbor WildWatch. She writes for the Gateway on environment.

This story was originally published March 11, 2020 at 12:00 AM.