#25-15 Sex in the Surf Zone
In which the E@L experiences Life, the Universe, the Carbon Pump, Horseshoe Crabs Mating on a Moonless Night on a Delaware Beach, and Everything
As a crowd of onlookers gathered around me on the beach, I gently picked a horseshoe crab out of the surf and turned it over to show them.
“This crab is a male. You can tell by looking at the pedipalps, the first pair of appendages. His are shaped like boxing gloves, and he uses them to hold onto the female when mating”. I continued to show them the gills, and the “pushers”, the last pair of appendages with splayed out tips that help push them along the seafloor.
“Why do they have such a long spiky tail?” someone asked.
“Watch,” I said as I placed him back onto the sand upside down. In a few seconds, he flexed his abdomen, pointing his tail down into the sand, and used it to lever himself over onto his side, and then turn himself right-side up. When the next wave washed over him, he scooted back into the water.
Amore Under the Stars
As the sun set, we watched the waves lap the shore while more horseshoe crabs washed up into the surf zone. On this moonless night, I was leading a group of two dozen nature lovers sponsored by the Natural History Society of Maryland (NHSM). We were observing the annual ritual of mating by thousands of horseshoe crabs along the beaches of Delaware Bay. At high tide after dark, they began coming ashore, mating, and depositing their eggs in the sand.
Earlier in the day, we had scanned the beaches with binoculars and counted thousands of shorebirds feeding on the eggs that had been deposited the night before. Red knots, having flown thousands of miles from the Caribbean, were fattening up on the lipid-rich eggs and doubling their weight before flying off to the Arctic for egg-laying. Ruddy Turnstones, Dunlin, and short-billed Dowitchers congregated like gangs in various parts of the beach. A few laughing gulls and herring gulls wandered around, harassing the smaller shorebirds and stealing bits of larger food items from them.
Horseshoe crabs have been around for over 200 million years, and for much of that time they have been returning to these beaches to mate and spawn. I previously wrote two articles on Substack about the relationship between horseshoe crabs and red knots, and the horseshoe crab fishery that catches them as bait for conchs, and use of their blood to produce Limulus Amoebocyte Lysate (LAL), a pharmaceutical product used to detect the presence of bacterial endotoxins. So, I won’t repeat all of that here.
As an update, suffice to say that harvesting of female horseshoe crabs has been prohibited (for now) which should help conserve their populations and support bird populations. And the number of red knots passing through Delaware in 2024 were estimated at 46,123, indicating that their populations are at least stable, and not declining1.
Sex in the Surf
The previous week, I had given a zoom talk to the NHSM and 70 participants about horseshoe crabs, shorebirds, and the bait/LAL fishery (Here is a link to the presentation on YouTube). Reservations for my beach walk and crab-viewing excursion had filled up and been wait-listed. Today, two dozen participants showed up from various locations, many of them opting to spend the night in one of the local beach towns, where their dollars will flow into the coffers of hotels and restaurants, proving that, after ecological services, ecotourism is probably the most valuable use of horseshoe crabs.
As it grew dark, more crabs rolled out of the surf. The larger females buried into the sand, with males clinging onto their tails. Groups of two or three males hung onto some females. At one point there was a pair (or more) of mating crabs every yard along the beach, just at the water’s edge.
A few feet up the beach, I walked along the high tide line, inspecting the wrack of flotsam with a UV light that had been brought along by Jayne Ash, my NHSM contact and the tour coordinator. As I shone the light on the sand, little round horseshoe crab eggs fluoresced, emitting a dull blue-green light in response to the UV stimulation. In small pockets of sand, hundreds of eggs lay, washed ashore by the waves, or dug up and discarded by the shorebirds earlier in the day.
Occasionally, pale white ghost crabs ran across the sand, stopping to vet whether we were predator or prey, and quickly running back to their hidey-holes at the base of the dunes. Mixed in with the beach wrack were skeins of egg cases produced by knobbed whelks and channeled whelks, known to beachcombers as conchs.
A Fishy Discovery
One crab-viewer came up to me with her phone in hand: “Can you look at this photo and tell me what it is?” I hate these kinds of questions because the photos are usually too blurry to make out, or it was taken from an angle that obscures the relevant parts of the organism I need to identify it. But what I saw was surprising and distinctive.
“That looks like a sturgeon,” I said, “Can you show me where you found it?”. Sturgeon were once common in these parts but are now rare. I’m not a fish biologist, but sturgeon have distinctive plates on their bodies called scutes, that are unlike any other fish. Nonetheless, I was not highly confident of my identification, especially since there was no head or tail. The woman’s ten-year old son gleefully led me down the beach, along with a bevy of curious looky-loos, and into the dunes where he pointed out a decaying mass of flesh, about three feet long, and covered with large scutes and smaller bony scales.
“Yep,” I said, “I’m pretty sure that’s a sturgeon. It must have been run over by a boat and washed up here. Look at the scutes!” They were shaped like diamonds, about three inches across. “But I’m not certain, so get out your phones and prove me wrong!” I said.
Within seconds, six people had googled STURGEON, and come up with pictures that verified my conclusion.
The Carbon Pipeline
Watching horseshoe crabs mate in the surf zone is an amazing experience. The crabs are part of a complex ecological system that supports the shorebirds during their journey over thousands of miles from the Caribbean to the Arctic. Yet this ancient ritual goes unnoticed by most of humanity. A few people walked up to our group totally unaware of the events happening at their feet but happily joined us as we watched the ongoing orgies. Above us, bright stars twinkled in the night, and someone pointed out the International Space Station drifting through them.
But something else was going on here besides the connection between crabs and birds, that is both physical and spiritual.
Human civilization is producing carbon dioxide at rates higher than ever before. Much of that CO2 is absorbed by the ocean, then taken up by phytoplankton, where it is sequestered into long-chain omega-3 fatty acids (marine lipids). These are consumed by zooplankton and eventually passed to seafloor benthic ecosystems as marine snow, where it is consumed by snails, clams, crustaceans and worms. This process is known as the marine carbon pump.
The seafloor biota, in turn, are eaten by horseshoe crabs that transfer the lipids to their eggs, bringing the carbon up onto the shore. There, birds consume the crab eggs and use the lipid to power their flight to the Arctic, where they transfer some of it to their own eggs. During their spring stopover, the birds also consume large quantities of clams and amphipods, absorbing calcium carbonate into their bones. Both the lipid and calcium carbonate are then incorporated into their eggs, laid in Arctic nests. When their offspring hatch, the eggshell carbonate is left behind, but the birds eventually migrate back to the Caribbean, taking some of the carbon with them. I don’t think this process has a name, so I’ll call it the “Ocean-Arctic-Caribbean Crab-Bird Carbon Pipeline” or OACCBCP. You’re welcome.
As I stood on the shoreline in the deepening dark, I explained this process to my coterie of crab afficionados in the simplest terms possible. To my surprise, my ten-year old protégé then turned to me and said, in words I did not expect from someone his age, “I am enlightened”. That made my day.
The Pipeline of Experience
One of my favorite writers on Substack is Jason Anthony, who writes Field Guide to the Anthropocene. In his recent essay, The Extinction of Experience, Jason lamented the loss of memory of ecological abundance. As humans erode and erase ecosystems, and species decline and disappear, we don’t remember how abundant they once were. These shifting baselines cause us to care less about small losses because we no longer have any experience with prior abundance. And because most of us live our lives indoors, in cities, with little access to nature, we are no longer aware of natural events and how they are changing. The extinction of our experience leads to extinction of species and ecosystems.
By the simple act of going to the beach on a dark and moonless night to watch crabs mating in the surf, I and a handful of dedicated learners refamiliarized ourselves with a natural phenomenon. We immersed ourselves in the natural world to acquire the experience that so many of us have lost. We will remember it for them and share it with everyone we know so that they can know it as well. This is another kind of pipeline, the pipeline of experience, handed down from learner to learner.
We can’t let that experience become extinct, because doing so would contribute to the crabs becoming extinct. If we want to conserve nature, we have to become familiar with it and learn to love it. This concept was stated most succinctly by Baba Dioum, a Senegalese forestry engineer:
"In the end we will conserve only what we love, we will love only what we understand, and we will understand only what we are taught."
Go outdoors. Experience Nature. Create a memory. Take it with you. Share it. Become part of the pipeline.
This issue of Ecologist at Large is available to all readers. However, if you would like to support my work with a one-off contribution, click “Buy me a coffee” below.
J. E. Lyons. 2024. Stopover Population Estimate and Migration Ecology of Red Knots C. c. rufa at Delaware Bay, USA, 2024. U.S. Geological Survey, Eastern Ecological Science Center at the Patuxent Research Refuge, Laurel, MD, 20708, USA. 16 pp.
I too am enlightened. Thanks, Brad.