Horseshoe Crab Solstice
Featuring: Ancient Aliens, Roll-n-Roaster, the Sun, the Moon
Hi everyone,
This week’s Landlubber is later than I had planned, but it has lots of cool photos and, if you read to the end, an eight second video that will blow your mind.
Some announcements up top: I’m leading a bird walk in Marine Park for NYC Bird Alliance (formerly NYC Audubon) on July 13th at 10:30. If you come, you’ll get to see saltmarsh/estuary habitat at its peak, when there’s life everywhere. It’s also free! I’m also going to be leading some very cool boat tours for Classic Harbor Line, filling in for Gabriel Willow. The tours go around lower-Manhattan and up the East River to visit some of NYC’s forgotten islands, including the entire area covered in last week’s letter about the wreck of the General Slocum. Here’s some of my previous writing in Urban Omnibus about NYC’s many overlooked but very-cool islands (the tour visits many of these!) Gabriel is doing these tours every Sunday and Monday evening, and I’m filling in on July 28th, July 29th, August 11th, August 12th, August 18th, and August 19th). You can book them here. Those tours cost money but that part of the city is really special and hard to see, and the ticket comes with a free drink.
Last Thursday was horseshoe crab night for me, an annual ritual that consists of traveling out to southern Brooklyn to watch hundreds of large, prehistoric arthropods crawl out of the bay. This year, by sheer coincidence, it fell on the longest day of the year, the summer solstice. I took the subway most of the way, ate a perfect roast beef sandwich, and then walked over to meet the six or so people who had driven and biked to a desolate parking lot on the side of the Belt Parkway.
On the east coast of the United States, May and June means breeding season for these weird animals. They do it at night, on high tides, with the number of horseshoe crabs peaking on the highest two tides of each month, which, for cosmic reasons related to gravity and the moon’s position in relation to the earth, are almost always the full moon and the new moon. I had picked the night before the full moon, figuring that there would still be a lot of crabs, but as we walked down the beach, stepping over bottle caps, and tampon applicators, and little pieces of styrofoam, I looked out at the choppy water, where kite surfers were doing all kinds of cool tricks, and began to worry that for some reason, this year, the horseshoe crabs wouldn’t show up. I get this feeling every year, and I’m always wrong. Moments later, as the horizon behind us exploded in an orange sunset, and pale blue light started to wash over the sky to the east, horseshoe crabs began to nose up out of the bay like they always do.
Horseshoe crabs are otherworldly lifeforms—creatures so bizarre that even describing their physical anatomy is challenging. From above they’re…helmet-esque? Teardrop shaped? It’s tricky to find a good analogy for something that doesn’t look like anything else. One way to describe them would be to say that they’re smooth and rounded at the front, and then, as you move towards the back, slender and pointy, with the slender/pointy trend finding its apotheosis in a single rigid spine that trails behind them like a tail. Another way would be to say that they have three interlocking sections: a large domed carapace, a squared-off abdomen, and the aforementioned tail spine, which is called a telson.
Their undersides are equally weird. Below a horseshoe crab’s carapace is eerie, prehistoric clockwork: rows of segmented appendages divided down the middle by a bristled maw of a mouth. They have eight legs for walking, a pair of small appendages at the front for directing food towards their mouth parts, and then another set at the back. The back ones are shaped like brooms and called “pusher legs” because horseshoe crabs use them to kick off the bottom when they want to propel themselves faster by flapping their gills. The animals are harmless and, once you’ve spent some time with them, even kind of cute, but at first glance they look like monsters out of an H.P. Lovecraft story. They’re freaky, nightmarish, creepy-crawly things that fall somewhere on a spectrum between insects and armored transportation. I’ve seen people of all ages shriek with fear at the sight of them.
Another imperfect analogy: in motion, hovering just above the sandy bottom, horseshoe crabs resemble Roombas, the disk shaped robotic vacuum cleaners, and just like every Roomba I’ve ever seen, they look pretty stupid when they encounter something that’s blocking their path. Confronted with a wall, or a bulkhead, or a rock, a horseshoe crab will run into the obstruction again and again, inching down each time until it can get around. On their backs, they become even more helpless, writhing around as they try to flip back over.
Watching them flail, or bump into the same wall eleven times, they can seem almost uniquely unsuited for survival, but the opposite is much closer to being true: horseshoe crabs have been around for nearly 450 million years. They predate not only human history, and primates, and mammals, but also insects, and seeds. They watched the dinosaurs come and go. They’re older than the continent you’re on right now.
Horseshoe crab spawning, like the animals themselves, is pretty weird. It’s basically a big orgy, or a bunch of simultaneous orgies, that take place in the shallow water and wet sand at the high tide line. Females, which are larger than males, release eggs into the water, as males cling to their backs or play bumper cars nearby, releasing sperm. Fertilization happens in the water around them. Females will often bury themselves, allowing the males crowd above them. Afterwards, when the tide goes back down, the fertilized eggs are left to incubate in warm sand until the next big high tide carries them out to hatch, which, again, because of the way the moon rotates around the earth, is almost exactly 14 days later (the minimum incubation period for horseshoe crab eggs). Sometimes, walking around a few days later, you’ll see females that accidentally stranded themselves at high tide digging their way out of the sand and crawling back into the bay.
Horseshoe crabs are often covered in epibiota—plants and animals that live on their bodies. Many are barnacle-encrusted, and caked in bryozoan colonies, and matted in algae and marine sponges, giving them a kind of Rip Van Winkle look that I guess is fitting for an animal as old as they are. Some have big bunches of mussels growing on their carapaces, or sea snails called slipper limpets grazing on the other epibionts. It’s basically a moving ecosystem. Sometimes, the limpets even have their own small habitats growing on their shells—little gardens of marine life that they carry around on their trips back and forth across the horseshoe crab’s backs. I’m sure if I brought out a good hand lens, I could observe a few layers of concentric worlds. Some hitchhikers will live their entire lives on the surface of a horseshoe crab, landing there as plankton and settling in for good.
For other animals with no need for real estate on the backs of horseshoe crabs, the eggs represent a huge windfall of protein. I’ve seen worms, snails, pigeons, grackles, sparrows and even raccoons gorging on the tiny turquoise globes. Earlier in the breeding season, during May, horseshoe crab beaches will often be teeming with frantic shorebirds jogging up and down the shoreline to hover them them up on their way north. The red knot, a handsome sandpiper that makes astonishing seasonal migrations from southern South America to the Arctic, often in excess of 9,000 miles, wouldn’t be able to survive without the eggs. Close to the entire population stops over on horseshoe crab beaches every year to eat them, with each bird nearly doubling its body weight before completing the long, energy intensive trip north.
So what makes horseshoe crabs so special? How did these weird, clumsy, bottom-dwellers stick it out through meteors, and ice ages, and the dissolution of Pangea? Why do they get to clunk around on the seafloor through mass-extinction after mass-extinction while the rest of us go the way of the dodo?
It’s a question I’ve asked a lot over the years. Sometimes I think it’s their helmet-like anatomy—the chitinous carapace that covers them—and that the secret to longevity on the species or genus level is just to carry an armored dome overhead at all times—to be an armored dome. It’s also plausible that it has to do with their milky blue blood, which coagulates when it comes into contact with bacterial endotoxins, and is vital to the biomedical industry, which uses it to make sure everything from vaccines to band-aids are safe for human beings (to ensure, basically, our collective survival.)
I like to think that it has to do with the mating ritual, though—the coordinated beachings that bring them all to the water’s edge at just the right moment. Those spawning sessions can result in huge windfalls for the population, with one successful cycle on a single beach resulting in thousands of new horseshoe crabs. Distributing them across four sessions over a two-month period reduces the chance that a single storm or a shorebird feeding frenzy will wipe out the entire season’s eggs at once.
The tricky part, as with so many things, is the timing. Because of the way the tides work, and because the eggs need at least two weeks of incubation on the sand, if the horseshoe crabs show up at the wrong time, on a half-moon let’s say, then subsequent high tides will wash the eggs away before they’re ready. I plan my horseshoe crab night by consulting a tide table, and looking up the full moons in May and June, but horseshoe crabs need to be able to predict those nights without looking it up, and it’s still not clear exactly how they do that. Their secret, in other words—and one of th reasons that they are so good at not going extinct—might have to do with how they keep track of time and tides.
Scientists are still trying to pin down the precise biophysical cue that gets horseshoe crabs trundling towards shore at the same time to spawn. In laboratory settings they exhibit dramatic behavioral changes when the lights are dimmed or changed, (although they also seem to get pretty worked up when the water levels are raised, too, implying that they might be responding directly to the tide change itself.) It’s plausible that something else, a still-unidentified signal in the environment triggers their impulse to head for shore, or that it’s a combination of several cues. This year, maybe because it was the summer solstice, or because two of the people who came out to the beach last week had also driven up with me to see the total solar eclipse in April, I couldn’t stop thinking about how these animals perceive the sun and the moon, which play such important roles in their lifecycles.
Horseshoe crab’s eyes (like everything else about their anatomy) are pretty strange, and simple questions such as “how many are there?” and “what are they all for?” have answers that are not so simple. There are certainly a lot of them. On the sides of their carapaces they have two “compound eyes,” which are structures made of around a thousand small, primitive light sensors called ommatidia. These are the ones they use to create an image (or, really, thousands of images) of the world immediately around them, and their most obviously eye-looking parts. Then, scattered around their carapaces, are clumps of simple photoreceptors. There are two of these clumps called lateral eyes just behind the compound eyes, and two more on the front that look like little nostrils; above the nostril eyes (which are actually called median eyes) they have an additional sensor called an endoparietal eye, which is structured to pick up ultraviolet light. Then, on the inside of their carapaces, looking down at the sand, are their ventral eyes. They also have a small array of photoreceptors on their telson. If we agree that the telson array and the compound eyes each count as one, that gets us to ten eyes. At least some of these light sensors begin to function inside the eggs, during that two-week incubation period. Shine a flashlight on a little egg that’s ready to hatch, and you’ll see the small larval horseshoe crab inside turn over, as in the video below. I’m not a scientist, but from an evolutionary perspective, to me, at least, all that hardware seems to imply that light is important to these animals in ways that probably aren’t totally legible or intuitive to us.
The explanation I like best for how these animals all get to the shore on time, and the one that seems to fit my own intuitive sense about horseshoe crabs, is that it’s the light, and that the secret to longevity on the population-level has to do with staying dialed in to the position of the sun and the phases of the moon. There is (to use a final imperfect analogy) something clock-like about these animals, to me. Maybe it’s the mechanical-looking appendages below the hood, or the telson that looks a little like a second-hand circling around, or their (okay this is actually the last analogy) vaguely pendulum-shaped bodies. Their domed carapaces sort of remind me, for reasons I can’t quite explain, of sundials. It seems plausible to me that part of the reason for the intricate, ancient network of photoreceptors is to help them log changes in the lengths of the days, and keep track of the moon as it cycles through phases, the way that neolithic people in Britain used Stonehenge to mark the solstice, and that all of those eyes that don’t seem to stop them from bumping into things are there, at least in part, to keep their internal rhythms set to the cosmic ones, so that they can make sure they don’t miss horseshoe crab night.
Fascinating creatures. You have given me a new appreciation for them.