At the end of each spring, some of the strangest creatures in the world come ashore along the Delaware Bay. The horseshoe crab migration is something I heard about only a few years before visiting, despite it occurring just a few hours from my hometown. Horseshoe crabs can be found elsewhere along the Atlantic coast, but Delaware Bay is famous for the enormous numbers that arrive here to spawn. In the photo above, you can see one of the first horseshoe crabs I spotted coming out of the water.
Being that this was my first time trying to watch the annual migration, I didn’t really know what I was doing. The one thing I knew was that late May and early June, especially around the full moon and high tide, was supposed to be a good time to visit. My friend and I first showed up at Prime Hook Beach around 5pm and were disappointed to find only a dead horseshoe crab. Maybe we had missed the migration, or maybe we were simply in the wrong place. We decided to get dinner and do some research before giving up. As it turned out, we were just too early. We headed to Slaughter Beach as the sun was going down and immediately began seeing horseshoe crabs along the shore.
As it got darker, more and more of them began arriving. Despite their name, horseshoe crabs aren’t actually crabs or even crustaceans. They’re marine arthropods and are more closely related to arachnids such as spiders and scorpions. They’re truly unusual animals, with multiple pairs of eyes, ten legs, the ability to regenerate lost limbs, and blue blood that has played an important role in medical testing. Even more incredible is their age: horseshoe crab ancestors appeared hundreds of millions of years ago, long before the dinosaurs.
Once the horseshoe crabs began crowding the beach, they would stack on top of each other and occasionally get rolled over by the waves. Every so often I’d see one of their long tails sticking out of the water like in the photo above. The tail, known as a telson, helps a horseshoe crab right itself when it gets flipped upside down.
Every few minutes I’d see one of the horseshoe crabs completely upside down, giving me a good look at just how strange they are underneath. Their eyes are mostly located on the upper surface of their shell, while underneath you can see their legs, mouth, and gills. Horseshoe crab blood is blue because it uses copper-based hemocyanin to transport oxygen. Their blood also contains cells that react strongly to bacterial toxins, which led to its longtime use for testing the safety of medicines and medical equipment.
This particular horseshoe crab wasn’t having much luck turning itself over. I thought about helping, but eventually it grabbed onto another horseshoe crab passing by and managed to get upright on its own. In the photo you can also get a good look at the underside of the animal, including the book gills it uses for breathing.
Lastly, here’s what the entire migration is about: spawning. Females come ashore and bury clusters of eggs in the sand while males gather around them and fertilize the eggs. The enormous number of horseshoe crabs arriving at the same time turns Delaware Bay into one of the world’s great wildlife spectacles. What makes it even more amazing is just how ancient these animals are. Their ancestors were already living in Earth’s oceans hundreds of millions of years before dinosaurs appeared, and every spring they’re still returning to the shoreline to continue the same cycle.
