Makin’ Whoopee On The Beach: Blame It On The Moon

Full moon beach sq

Since Greek and Roman times a full moon has been blamed as the trigger for all sorts of weird goings-on. An article in Scientific American said:

“Even today many people think the mystical powers of the full moon induce erratic behaviors, psychiatric hospital admissions, suicides, homicides, emergency room calls, traffic accidents, fights at professional hockey games, dog bites and all manner of strange events.”

There’s some scientific debate as to whether this cause and effect relationship actually exists, but on a recent early-morning beach walk, we saw first hand at least one behavior that confirmed it – horseshoe crabs makin’ whoopee on the day of the full moon.

Horseshoe Crabs sq

Horseshoe crabs aren’t the most attractive sea creature, but the species has been essentially the same for the past 230 million years, making it a “living fossil.” One requirement for their longevity is the ability to mate successfully, and this is where the moon comes in.

Digging In

In the spring and summer, for the female, the full moon is a neon light saying – dig a nest and lay eggs … lots of them.

Pair of Horshoe Crabs

And for the male the sign says, hook up (literally) with a female, and fertilize the 2000 to 20,000 eggs she will lay.

If you’re wondering why the ocean isn’t overrun by horseshoe crabs, for these sanderlings and other shore birds, the moon says … fooooood!


We were lucky to meet a knowledgable local guy who gave us an impromptu and hands-on discourse on the specifics of the mating process.

After a successfull date, the female makes her way back into the ocean. And if these curly tracks are any indicator, the whole experience is a bit disorienting.


The beach is a great place to swim and catch some rays, but with some luck and an observant eye, it can be a personal bio-lab. And we love it for this.

Happy Trails,
Horseshoe CrabsPhoto Credit: 1. By veggiegretz


We're Terri and James Vance - high school sweethearts who went on to international careers and became world nomads. Today, 65 countries later, we're still traveling ... and still in love. Check out Our Story for more of the backstory at

41 thoughts

  1. As a college student, I definitely find myself in the full moon causing craziness camp, if nothing else than because you can see where you are walking if you are drunk, thus are more likely to go out and do something awesome/ stupid. I’ve only seen one horseshoe crab, kind of thought it was an alien, and never expected to see a picture of them mating. Nice pictures and awesome post!

    1. Thanks George, for dropping by the blog and for the comment. I’m glad to have another X in the full-moon column. Honestly, I’d never thought much about the effects before, but I will certainly pay more attention in the future. And I always thought of horseshoe crabs were in armadillo category. That is, a little weird, kinda dumb, but basically harmless. After my research for this post, I have a whole new image of them. Best of luck on the next full moon. ~James

    1. You’re right Dallas. This is one of those incidences that gets chalked up in the “there’s something to this” column. At least, the horseshoe crabs think so. ~James

  2. this is a fabulous post!! i love horseshoe crabs. always have. gotta love full moon lovin’. 🙂

    1. Thanks Liz. We were lucky to be at the beach at exactly the right time and get some good photos. BTW, good news on the move to Asheville. We lived there for a while and really loved it. It’ a very interesting and fun city, and having the mountains all around is wonderful. ~James

    1. Thanks Rusha. Pre-historic is exactly the right word for horseshoe crabs. They are quite an evolutionary success story. Frequently, there’s some cool nature stuff going on at the beach, and I’m always on the lookout. And as a blogger, I’m sure that you know that anything can be fodder for the ol’ blog mill. ~James

  3. I love this post. We’ve got horseshoe crabs galore here at WoodNeck Beach, Falmouth, MA. My kids always loved them and treated them like historic treasures. Many thanks for giving me more beach bio info!

    1. I’m glad to hear that your kids liked them. Horseshoe crabs actually look a bit creepy, and most kids I know would give them a big ewwwww! We don’t see lots of them on St. Simons Island, GA, so seeing the mating ritual was pretty special. ~James

  4. Imagine the responsibility of fertilizing 20,000 eggs. 🙂 Living out in the sticks, we often have the opportunity of watching natures wonderful, and often strange mating rituals. Peggy called my attention to a pair of grey squirrels last week. Slam, bam, thank you Ma’am is still arrive and well in nature!

    1. Curt, this reminds me of one of my favorite lines from the movie “Christmas Story”, when Ralphie says his dad can “change a fuse quicker than a jackrabbit on a date.” These horseshoe crabs were slo mo, compared to that. ~James

  5. I remember them in the waters off Long Island Sound – at full moon. Yes! Your post also reminds me of one of my all-time favorite Bob Seeger tunes: “Oh, blame it on midnight, oh, shame on the moon….” —Jadi

    1. Thanks Jadi. Apparently, from my research, horeshoe crabs are more numerous in the NE than the SE. And it was pretty funny, because we didn’t know that this whole process was related to the full moon until we got home and did some research. It certainly was fun to witness. ~James

      1. My friends and I would go swimming after dark and we’d literally be treading water over a swarm (pod? school?) of horse shoe crabs. In those days their populations were healthy – and, from what your post informs me – very busy multiplying! —Jadi

  6. Great post! I’ve only seen the shells of horseshoe crabs, and mostly on an island in Wassau Sound that you can only get to by boat…. Makes me happy to know there are some that are alive and well ( and having fun)!

    1. Thanks for the comment. Up to this point, I had only seen empty shells and what I thought were dead HS crabs. But this encounter solved that problem. It was great fun to see. ~James

  7. I was just wondering if the posting of this story within a couple days of the full moon (May 25th) was a coincidence, or if you were influenced by the full moon to get this story posted. Regardless, I found the story entertaining and informative. 🙂 – Mike

    1. Interesting question Mike. As a scientist, I’ve never put much stock in the full moon effect, but if it can impact other animals, why not humans. In fact, I’ve put the date of the next full moon (June 23) on my calendar, to see if I feel any changes coming on. And of course, anything worth reporting will be covered in our blog. BTW, let me know if anything happens on your end. ~James

  8. Great post. We were at Slaughter Beach in Delaware over the weekend during full moon and there were thousands of them mating on the beach. It was really an amazing experience thing to watch and capture. Although it was a little discomforting for me to snap away at these animals mating with my flash and all :-).

      1. James, thanks for the plug, appreciate it. I definitely recommend Slaughter Beach in Delaware if you can make it out that way, there was a sign on the beach designating that area is a horseshoe crab sanctuary.

  9. While it may or may not exist, I’ve definitely seen the full moon effect on drivers – I will do everything I can to avoid driving on full moon nights because everyone drives even crazier than they usually do!

    And the description of the horseshoe crabs is a blast from the past – I remember learning about them in elementary school and thinking they were so ugly, but hey at least they’ve been around for 230 million years!

    1. Thanks Jen. You make an interesting point about bad driving on full-moon nights. As a scientist, I’ve never put much stock in the full moon effect, but if it can affect crabs, why not people. I’ve put the next full moon on my calendar (June 23), and will be on the look-out for interesting stuff happening. ~James

  10. We have never seen or heard of these crabs. No change in 230 million years? Well when you are that good looking why change anything?
    As for the full moon effect my years of nursing, especially at night, I am a believer. As a young nurse I had a patient try to stab me with his table knife under the light of the full moon. In the morning , with a security guard sitting beside him, he had no recollection of the event and asked how my night had been.

    1. Yowza Sue, a stabbing would make me a believer as well. I can’t say that I’ve noticed any effects – good or bad – well except that yesterday I seemed to have a insatiable hunger for chips and salsa. The gravity of the moon causes tides, and since humans are approx. 60% water, who knows? And as a student of evolution, I find these living fossils incredibly interesting. There are a few other animals in this category, sharks and cockroaches for instance. Most animals have gone through so many changes in their development it’s amazing to me that some species get it right so quickly, and stay the course. ~James

      1. James I had not thought about why the full moon effect happens but definitely in the health care field it is a common term or question ” Is there a full moon?” When all heck is breaking loose on a ward.
        As far as whether it causes chips and also cravings, well that may require some research. 🙂

  11. Be glad the horseshoe crabs were the only exhibitionists you came across on the beach during the full moon. Its amazing that any of the eggs reach maturity with all of the birds feeding on them. It would be neat to go back and watch them hatch.

    1. I don’t know Laura, but I suspect that horseshoe crabs are like sea turtles; their survival rate is incredibly small (hence the need for huge numbers of eggs). These birds were having a feeding frenzy, running from nest to nest, and being very thorough about it. I don’t know how deep the eggs were buried, but I can’t believe that many survived either. Check out Chau’s post for more photos in Maryland ~James

  12. My goodness James, with this post, I just realized a year has gone by already since we witnessed this spectacle, it was such a special thing to observe and an experience to remember. Thanks for the trip down memory lane and great photos.

    1. Thanks Chau. I was surprised at the time as well, but when I saw it was the May full moon, I thought I’d try again. On our beach, when I was there, it was a very high tide and there were only a few crabs. And they were about 100 yards off the beach on a small sandbar, so I couldn’t get to them for photos. I put a link to your post in one of my comments. ~James

  13. I’ve never seen a horseshoe crab but I do think the full moon affects people. If it can change the tides it’s hardly surprising it makes us a little crazy.

    1. I agree Marie. As I said to another commenter, the human body is about 60% water. So if the moon can create tides, I wonder what effect that has in our bodies (and brains)? ~James

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