The Biological Moon

by Bruce Scofield

(originally published in Llewellyn’s 2002 Moon Sign Book)

One of the most incredible sights I’ve ever seen occurred on a New Jersey beach when the Moon was full. Here’s what happened. My band and I had played a graduation party at Glassboro State College on a Saturday night in May, finishing up around 2 in the morning. After breaking down the equipment and loading our cars, we went to a late night bar for a drink, and after that a diner for breakfast. Before we knew it, it was 5 in the morning. One of us suggested we next drive to Delaware Bay to watch the sunrise and take a swim. So we did, arriving there just before dawn and just as the Moon was setting in the west over the water. In a state of tired excitement we bolted from the cars and ran toward the water, only to be jolted by an amazing sight. The beach was covered with crawling horseshoe crabs, tens of thousands of them, in every direction. I’d never seen anything like it. It was the most primal scene I’d ever witnessed, and to top it off, the backdrop was a giant full Moon setting over the bay. We chose not to go swimming that morning and stood there stunned by the spectacle.

What we had witnessed that morning on Delaware Bay was a massive sexual event. Horseshoe crabs, which are not really crabs but more closely related to insects, mate at the full Moon and have been doing this for 400 million years. They are ancient survivors, animals that lived long before the reign of the dinosaurs. But here they were, doing what they’ve been doing for longer than we can imagine. They were moving to the Moon. It’s called lunar periodicity.

The realization that the Moon is a regulator of the cyclic functions of marine organisms goes back to the ancient Greeks and Romans. In the 4th century BC, Aristotle noted that the gonads of sea urchins are largest at the full Moon. This was good to know as those gonads were the part of the urchin that was eaten. Cicero and Pliny also noted that oysters and shellfish grow according the cycle of the Moon, becoming fullest at the full Moon. They spawn at both full and new Moon. Many other marine organisms reach a sexual peak at the full Moon because that’s when their mating activities occur. When sexual fertilization in a species takes place outside of the body, as it does with these marine organisms, survival is enhanced if the population comes together at a specific time. And the timer for this group mating ritual is, of course, the Moon.

All sorts of things under the sea happen according the cycle of the Moon. Even today fish vendors in Mediterranean markets will know that crabs are biggest when caught at the full Moon. Crabs molt at the full Moon because that’s when they’ve grown so big that they can no longer fit into their shell. Eels being their long migration from the rivers of Europe to their home in the Saragaso Sea right after a full Moon. In California, a small fish called the grunion has a bizarre mating ritual that occurs two to four days after full and new Moon. The females come ashore and dig themselves into the sand, where they lay eggs. Males land and cover the females with sperm, which drips down to the eggs. All of these are examples of monthy rhythms, lunar cycles used to time biological events.

Perhaps one of the strangest examples of lunar periodicity in marine organisms is that of the Palolo worm that lives in the coral reefs near Fiji and Samoa. These worms,which live in cracks and holes in the coral, grow segments on the end of their bodies that store, depending on their sex, either sperm or eggs. At dawn on the first day of the last quarter of the Moon in November, the ends of the worms break off and swim to the surface. There they encounter all the other worm-endings and together they create a bath of sperm and eggs. But, as it turns out, the worms are considered a delicacy and the natives long ago figured out how to time this event by the Moon. The islanders board canoes at dawn on the critical day, paddle out to the reefs, and scoop up the delicious worms.

The first day of the last quarter of the Moon in November may be the trigger for the Palolo worm orgy, but its not what gets the coral going in the Gulf of Mexico. There, on the ocean floor, at the Flower Garden Banks National Marine Sanctuary, several species of brain and star corals release eggs and sperm in August – on the eighth night after the full Moon of that month. These huge 15-foot corals know just when their annual reproductive fest must occur, and like the Palolo worms, the timing is absolutely critical. The only way these organisms can insure their sperm and eggs meet and perpetuate their species is to simultaneously let go of millions of eggs and sperms. A lack of synchonization would mean that these reproductive cells would probably never join because they would be widely dispersed by the ocean currents.

Mass coral spawning was only recently recognized by science. It was first observed on the Great Barrier Reef in Australia during the 1980’s. Then, in 1990, some recreational divers witnessed a mass spawning on a raised section of the ocean floor some 123 miles south of the Louisiana-Texas border. This led to the area becoming a designated marine sancturary, the one that’s hooked on the 8th night after the full Moon. These pristine Flower Garden Banks are believed to be an example of what coral reefs were like before modern times – massive colonies of organisms all swooning to the Moon. Although marine biologists study other coral reefs throughout the world, many suffer from pollution and human disturbances and are no longer able to spawn on such a massive scale.

The 29.53-day cycle of the Moon, from new to full and back, is but one lunar cycle that marine organisms have attuned themselves to. Every day the Moon rises and sets. If one counts the time from one moonrise to another, it amounts to rougly 25 hours, just over a solar day. Richard Alden Knight, who wrote books on fishing, found that fish bite more frequently during the times that the Moon rises and sets. He even created what are called “solunar tables” that fishermen have used for years to time their fishing trips. In his book “Moon Up – Moon Down” he tells of his informal studies of the behavior of fish and the Moon’s rising and settings. His conclusion is that it’s more than just the fish that get active as the Moon rises or sets, it’s the entire aquatic environment – and especially the worms and other smaller creatures that the larger fish feed on. His insight is probably right. Scientific studies have shown that crayfish and crab feeding activity increases according to the lunar day.

Every one knows that the Moon causes the tides. One question that arises is whether the organisms are responding to the Moon itself, or simply to the tide. In the 1950’s this question was addressed by Frank A. Brown, Jr. In a famous experiment, Brown took oysters from Long Island Sound near Connecticut and transported them to Evanston, Illinois. He had already established that oysters actively feed twice a day at 12.5 hour intervals, half of the lunar day. In Illinois he wired the oysters shells in such a way that each time they opened their shells it would be recorded. He kept them in a salt water tray and under low lighting, the best that could be done to simulate their natural environment. For two weeks the oysters opened up and fed as they had done in Long Island Sound, and at the same time of day. But then they shifted their schedule. They had tuned themselves into the cycle of the tides – in Illinois. It seemed clear to Frank Brown that they oysters were responding somehow to the Moon itself.

It’s more than the marine world that moves with the Moon. I learned this one evening camping on the Kittatinny Ridge in New Jersey, again in May. After years of coaxing, I had finally convinced my wife to go backpacking with me. One late afternoon we, and another couple who had also never backpacked, hiked south on the Appalachian Trail to a special campsite that I had been to before. It was grassy, almost perfect, and there was a 180 degree view to the east. We put up our tents and, as the day was ending, I made a small campfire. We sat there by the fire and were all quite amazed when a gigantic full Moon rose up in the east before use. Just then, the other couple noticed a few 4- inch long millipedes climbing up the side of their tent. A few minutes later there were a dozen slowly making their way up the ripstop nylon. Within fifteen minutes our tents were covered with large millipedes and a panic broke out among my companions. My wife and the other couple dove into their tents, zipped them up, and refused to come out. I found the situation hilarious and walked around the campsite noting that the throngs of millipedes were climbing trees and rocks as well. While I had seen these insects before, I had never seen them in such large numbers. When morning came, they were gone. Completely, not a one in sight. Needless to say, the perfect camping trip was ruined.

Like marine organisms, the millipedes were probably engaged in some kind of mating ritual timed by the full Moon. Many other land animals do the same. It’s been shown that lemurs, distant cousins of humans, come into heat about 5 to 6 days before the full Moon. One study reported on lemur orgies occuring every other full Moon. Studies of Indian buffalo cows noted that the females come into heat more often at full Moon and new Moon than at any other time. African rats tend to mate on the midnight before the full Moon, but this is less true for domesticated rats. And there’s also the case of the human female whose natural menstrual cycle (menstrual actually means month or moon) is not 28 days, it’s been shown to be 29.5 days, exactly the lunar month.

For three years my son had a gerbil named Boffer who would go bonkers for about 3 to 4 days at the full Moon. During those periods she’d run on her wheel and scurry around her cage, waking us up in the middle of the night. Frank Brown did some studies with hampsters and found that their activity periods, which were timed by a gage on their running wheels, were active according to the lunar day of 25 hours, as well as at full and new Moon.

There are also lunar cycles longer than the 25-hour lunar day and the 29.5-day lunar month. European grouse, Norwegian lemmings, and the Canadian varying hare all reach population peaks about every 4 years. The timing seems to be based on the date when the full Moon, or some other phase, occurs on or very near the same day it did several years ago. This is a phase-relationship kind of linkage, one of which occurs precisely every 19 years. For example, a full Moon today will be repeated on this date 19 years from now. Divide 19 by 4 and you get 4.25.

The existence of lunar rhythms in organisms has long been known, but explanations for it are a different matter. Several have been proposed though none appear to explain all the phenomena observed. One explanation for lunar periodicity is that moonlight, the actual sensing of light, triggers a biological rhythm. It’s been shown that human females with inconsistent menstrual cycles could regularize their periods to a 29-day rhythm by sleeping with a light shined on the walls and ceiling of the bedroom for several days in the middle of their cycle. Lemurs also seem to be affected by light. When there is none, they don’t have orgies. Other studies on worms have shown that exposure to light can cause them to mature sexually well before they should. Even the Palolo worms fail to swarm without lights to que them, and if they are exposed to lights too early, they mate prematurely.

In their exploration of how light can possible trigger sexual responses, scientists have examined the pineal gland, a small gland located deep in the brain. This gland appears to have a kind of control over other glands which, in turn, produce hormones that affect the body. It appears that the pineal gland is light sensitive and that it has a direct effect on the pituitary gland which makes sex hormones. Although the pineal gland is deep inside the brain, there are direct links with the eyes, which, like fiber optics, bring light to it.

Another explanation for lunar periodicity has to do with what are called biological tides. We know the Moon moves the ocean, but does it also move the water in our bodies? There is indeed plenty of water in us; in our cells, between our cells, and in our blood. It’s been suggested that, as the water in the body of an organism is pulled by the gravity of the Moon, however slight, the change upsets the electrolyte balance and consequently causes hormonal shifts and triggers nervous irritability. It’s well known that manic- depressive persons have wide mood swings and may go “over the top” during a full Moon.

The ionization of the Earth’s atmosphere is yet another possible explanation for the lunar effect. Ions are molecules of gas that have been altered by either gaining or losing an electron. If a molecule gains an electron, which has a negative charge, it becomes a negative ion. Negative ions, which are found at the seashore or near a waterfall, are said to bring calm and tranquility. If a molecule loses an electron, it become positively charged, a condition that is not so good for the body and mind. There is also the daily fluctuations of the geomagnetic index. This index peaks when the Moon is directly overhead and directly below us, which are near the points at which the ocean tides peak. Why should organisms be affected by magnetic changes? Why not, nervous systems are all basicially electrical in nature.

The study of lunar rhythms in organisms might reasonably be considered a branch of astrology. The greatest astrologer of the ancient world, Ptolemy, wrote that astrology has two main branches, natal astrology and universal astrology, the later dealing with the effects of the planets on nature. During the Renaissance, this latter branch came to be known as natural astrology. Because most modern astrologers have concerned themselves with natal astrology, which they must do to make a living, work in the field of natural astrology has been carried on by biologists.

Today, a large body of scientific data has been gathered that suggests many organisms, particularly those of a marine nature, time their reproductive or feeding cycles with the movements of the Moon. Still, in spite of meticulous record-keeping and scientific analysis, the influence of the Moon on life on earth remains controversial. For example, scientists argue over whether the Moon is the external trigger, which Frank Brown showed to be the case, or whether organisms make their own time internally. If a biological clock appears to be driven from the outside, eg. the Moon, it is called exogenous. If it is driven by an internal circuit of some sort, it is called endogenous. With scientists, who lack any real knowledge of astrology, conducting nearly all the research on this subject, we shouldn’t expect them to validate the astrological hypothesis as we understand it. What we have seen and will continue to see is essentially an appropriation of the territory that was once astrology’s.


Brown, Frank A., J. Woodland Hastings, and John D. Palmer. The Biological Clock: Two Views. New York: Academic Press. 1970.

Moore-Ede, Martin, Frank M. Sulzman, and Charles A. Fuller. The Clocks That Time Us. Cambridge, MA: Harvard University Press. 1982.

Katzeff, Paul. Moon Madness. Secaucus, NJ: Citadel Press. 1981.

Luce, Gay Gaer. Biological Rhythms in Human and Animal Physiology. New York: Dover Publications. 1971.

Palmer, John D. and Judith E. Goodenough. Mysterious Monthly Rhythms, in Natural History. Vol. LXXXVII, No. 10. December 1978.

Playfair, Guy L. and Scott Hill. The Cycles of Heaven. New York: Avon Books. 1978.

Taylor, Pete. Spectacular Conception, in National Wildlife: October/November 2000. Vienna, VA: National Wildlife Federation.