It’s a warm June night and a full moon is painting a silvery path across the gentle ripples on Delaware Bay.
I’m on the beach, southeast of Dover. And from the darkness of the bay, I watch what looks like an invading force of army helmets with eyes on them emerge from the murk to crowd, clatter and scrape against each other along the shoreline.
These are horseshoe crabs – prehistoric creatures that have been summoned by full moons and high tides like this for hundreds of millions of years to perform this springtime mass mating ritual on the beach.
“So we have literally thousands of horseshoe crabs, or Limulus polyphemus, on the shore, and they are spawning right now in clumps,” said Paul Leingang, a graduate student at the University of Delaware who is part of a team studying the crabs. “So there will be one female surrounded by dozens of males. But one male will be like the central male that is attached onto the edge of the female, and the rest we call satellite males that are trying to come in and get some fertilization.”
Near Leingang’s right foot, a female crab the size of a car’s steering wheel – monstrously old, and crusted with barnacles and seaweed – erupts from the wet sand after laying its eggs.
“Imagine a zombie coming out from the ground,” said Leingang, watching the animal’s emergence. “I mean, if you had looked at this before, you would not even know that there was a horseshoe crab underneath. And then all of a sudden, the ground starts to lift up, and we can see the shell of a horseshow crab start to appear. They really dig deep.”
The female struggles to emerge from the sand, because a mob of smaller males piles on top of her. But finally she rises – triumphant, her life’s mission of reproduction accomplished. Then she motors, like a tank, back into the ocean.
Two decades and more ago, horseshoe crab populations were in sharp decline. This was mostly because fishermen would harvest millions of them as sell as bait for catching eel and whelks. The medical industry also catches them to draw their blue-tinted, cooper-rich blood for use in blood tests.
Over the last decade, the horseshoe crab populations have stabilized, as catch limits were put in place. But now Leingang and his fellow researchers at the University of Delaware are examining an additional potential threat: Sunscreen.
Vacationers flock to the same Delaware beaches where the horseshoe crabs lay their eggs in the sand, and the people spray and slather large amounts of sunscreen.
Preliminary lab research by University of Delaware Assistant Professor Professor Danielle Dixson last year found that too much sunscreen prevents the hatching of horseshoe crab eggs.
“All of our horseshoe crabs that were exposed to the sunscreen didn’t develop,” Dixson said. “They never hatched their eggs. So this year we are doing a follow up study with lower concentrations that are ecologically relevant. We are adding some environmental engineering components and some social science surveys and things like that to get at a better understanding of how horseshoe crabs are affected by sunscreen.”
Over the next several weeks, Dixson and her graduate students, including Leingang, will be taking samples of eggs they collect from the beach and exposing them to varying concentrations of three different varieties of sunscreen in Dixson’s laboratory.
The theory is that certain kinds of sunscreen – made with a chemical called oxybenzone, which has been shown to be toxic to corals and other marine life – could potentially be more deadly to crabs than other sunscreens. If the lab prove that oxybenzone is harmful at low doses, it and related chemicals could be avoided, in favor of safer sunscreens.
A change in sunscreen ingredients might be a small price to pay to save a living fossil.