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Starts With A Bang

This One Species Of Animal Proves That The Moon Can Affect Biological Evolution

On human-length timescales, there are only two effects that the Moon has on our world: the light emitted from it at various times of day/night throughout its phases, and the gravitational pull it exerts on Earth, causing our tides. (KASABUBU OF PIXABAY / PUBLIC DOMAIN)

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.

The Earth-Moon distances as shown, to scale, relative to the sizes of the Earth and Moon. This is what it looks like to have the Moon be approximately 60 Earth radii away: the first ‘astronomical’ distance ever determined, more than 2000 years ago. Note the magnitude of the Earth-Moon distance compared to simply the diameter of Earth.(NICKSHANKS OF WIKIMEDIA COMMONS)

Orbiting Earth with a revolutionary period of just under one month, its physical effects are limited.

Tidal rhythmites, such as the Touchet formation shown here, can allow us to determine what the rate of Earth’s rotation was in the past. During the time of the dinosaurs, our day was only 22.5 hours, not 24. Back billions of years ago, shortly after the formation of the Moon, a day was closer to a mere 8 hours, rather than 24.(WIKIMEDIA COMMONS USER WILLIAMBORG)

On long timescales, it slows our planetary rotation while migrating away from us.

The cycle from new Moon to full Moon to new Moon again coincides with increases and decreases in apparent size as the Moon moves along its elliptical orbit. Because it moves faster at perigee and slower at apogee, but has a constant rate of rotation, we see slightly more than 50% of the Moon over the course of a lunar month: this is the phenomenon of lunar libration. (WIKIMEDIA COMMONS USER TOMRUEN)

On human timescales, we only notice its monthly phases and our terrestrial tides.

The Moon exerts a tidal force on the Earth, which not only causes our tides, but causes braking of the Earth’s rotation, and a subsequent lengthening of the day. As the Moon creates two tidal bulges on Earth, which itself spins once per day, we experience two low tides and two high tides daily. (WIKIMEDIA COMMONS USER WIKIKLAAS AND E. SIEGEL)

The combined effects of the Moon and Sun create two tidal bulges around Earth, yielding high tides and low tides twice daily.

When the Moon is very close to either the new or full phase, its tidal forces add with the Sun’s tidal forces (which are about 1/3 the Moon’s magnitude), creating the highest high tides and lowest low tides: spring tides. When the Moon is in its first quarter or last quarter phase, it interferes destructively with the Sun, creating the smallest tides: neap tides. (KEITH COOLEY, VIA HOME.HIWAAY.NET/~KRCOOL)

When the Sun, Earth, and Moon all align, we get spring tides: the highest high tides possible.

Tide height at Bridgeport, CT, over a 30 day timespan. Note that there are two high tides and two low tides per day, and that the greatest amplitude tides occur in a periodic, 14 day fashion, coinciding with the new and full phases of the Moon. (ARTHUR THOMAS DODSON / NICKYMCLEAN OF BRIDGEPORT, CONNECTICUT)

Tidal extremes occur during new and full Moons, with twice the magnitude of intermediate-phase neap tides.

Gorey Harbour at low tide, illustrating the extreme difference between high and low tide found in bays, inlets and other shallow, coastal regions here on Earth. Jagged, uneven coastal regions are where the greatest difference between high and low tide, as well as spring and neap tides, can be seen. (FOXYORANGE / WIKIMEDIA COMMONS)

One terrestrial animal, the grunion, has uniquely adapted to take advantage of this lunar-induced phenomenon.

A grunion is a small, 5–6″ (12–15 cm) long fish with uniquely-adapted fins and a tail that allow the females to burrow into the sand and maneuver themselves into an upright position, where the males can wrap around them and mate. Found on the Pacific and Baja California coasts, ‘grunion runs’ are popular ways to catch these fish by hand. (ERIC WITTMAN / FLICKR / CC-BY-2.0)

During the highest spring tides, the females come onto sandy beaches, dig with their tails, and lay eggs.

Shown here, a lone grunion female has buried her lower half into the sand beneath her, where she will lay her eggs. She will only do this during a high spring tide, as her eggs must remain dry for the eggs to properly incubate. A male has not yet mated with her. (ERIC WITTMAN / FLICKR / CC-BY-2.0)

The males intertwine with the females, depositing sperm before departing.

From her position, upright and half-buried in the sand, the female grunion lays eggs with just her head sticking out, while the male grunion gyrates around her attempting to fertilize her eggs. There are only two known species of grunion that exhibit this reproductive behavior. (Bob Chamberlin/Los Angeles Times via Getty Images)

As the tides regress, the grunion eggs incubate on shore.

These are just some of the thousands of grunion eggs which are laid every spring tide across the beaches of southern/western North America. The eggs will remain beneath the sandy surface once the spring tides retreat; their incubation period aligns perfectly with the lunar period that signals the return of another spring tide. (Bob Chamberlin/Los Angeles Times via Getty Images)

The eggs only hatch after 10–11 days: when the next spring tides arrive, washing them out to sea.

Many coastal regions, such as the Pacific and Baja California coasts where the grunions spawn, exhibit big differences between high tide and low tide. During a high spring tide, regions of land will be exposed to ocean/seawater for a short period of time: sporadically across 3 to 4 days only. They will then become dry for 10 to 11 days, until the next spring tide comes in. (POINTILLIST / ENGLISH WIKIPEDIA / CCA-SA-3.0)

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.

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