Horseshoe Crabs mating off the coast of Cape Cod. Image from "Under Cape Cod Waters" by Ethan Daniels.

When I first began interning at Union Park two weeks ago, I didn’t expect that the time I had spent leisurely flipping through the pages of Ethan Daniels’ Under Cape Cod Waters would come in handy this past weekend. But when I found myself standing in awe on the pebbly shoreline of Barrington, Rhode Island, smack-dab in the middle of an arthropod love-fest, I could not have been happier for the breathtaking marine images that are already so firmly planted in my mind.

At first, I could not decipher what the dozens of strange creatures were. Looking like flattened brown army helmets with tiny, jagged spikes jutting from their sides and a sharp, pointy tail protruding from their backs, the beady-eyed creatures that writhed before me seemed like something straight from a science fiction novel. As a biology major and a nature lover, I thought I could readily identify most of the wildlife that I encountered. But for the first time in a while, I found myself completely stumped as to what these mysterious creatures were. Were they skates? Strange-looking stingrays? Mutant crabs? Just then, a vivid image from Under Cape Waters popped into my mind, and I yelled out to everyone on the beach, “horseshoe crabs! They’re horseshoe crabs!”

Horseshoe crab courtesy of Mary Frances Johnson

After re-reading the section about horseshoe crabs in Under Cape Cod Waters and doing some of my own research, I have since discovered that horseshoe crabs are not actually crabs at all, but rather, prehistoric arthropods that are more closely related to scorpions and spiders than they are to any living crustacean.

Dubbed “living fossils,” horseshoe crabs are estimated to be at least 300 million years old, and the earliest species actually predates the evolution of dinosaurs by more than 100 million years.  Since then, thousands of other species have come and gone, but horseshoe crabs have evolved little over time; the particular species that I saw—the Atlantic horseshoe crab (Limulus polyphemus)— for example, has not evolved at all in the past 20 million years! This evolutionary feat was possible partially because of the horseshoe crab’s remarkable ability to breath in and out of water and their ability to quickly fight off bacterial infections through the use of special clotting and bacterial deactivating agents found in their blue-colored blood—agents which, since their discovery in the 1960s, have been used extensively by pharmaceutical companies to ensure that their products are free of bacterial contamination.

The ancient reproductive spectacle that kept me captivated for hours usually takes place along the shoreline of the east coast in late May and early June, with mating activity peaking during the full and new moons.   Adult males usually arrive on the beaches a few weeks before the females, and must compete fiercely with one another in order to attract a mate. Males who are not lucky enough to find a mate must do the next best thing: attach themselves to mating males in hopes that their sperm may also somehow sneak by. This is why horseshoe crabs can often be found mating in groups of three or more, a sight that has astounded New Englanders  for hundreds of years.

For more information on horseshoe crabs and to see stunning pictures of this ancient mating ritual for yourself, be sure to pick up a copy of Under Cape Cod Waters—after all, you never know when you, too, may find yourself invited to one of nature’s most fascinating spring flings.