Current Nature: Spawning Season
By Kristin Bullett, Community Engagement Coordinator, Linda Loring Nature Foundation •
Horseshoe Crabs are one of the many natural treasures found on Nantucket. They are commonly seen this time of year scooting about in shallow water, or along the shoreline. May and June is their breeding season, when hundreds of Horseshoe Crabs come ashore during full and new moons to breed. Did you know that these creatures are “living fossils”?
The Atlantic Horseshoe Crab has been on Earth for more than 400 million years- well before the dinosaurs! In that massive time span, the Horseshoe Crab hasn’t changed much. Turns out their anatomy was an ecological success and there wasn’t much need to change and adapt to survive. There are three main sections to their body with a hinge between the two shells and a tail, or telson. Though they look a little sharp and pointy, Horseshoe Crabs are harmless and a great ambassador- people of all ages admire these creatures, inspiring them to learn about the ocean and ecology.
Their hard outer shell is called an exoskeleton, which needs to be shed as the Horseshoe Crab grows. Beaches along Nantucket and Madaket Harbors are littered with these Horseshoe Crab molts, which make excellent beachcombing keepsakes.
You might think that these creatures are crabs just like Blue Crabs or Green Crabs, but Horseshoe Crabs are technically not crabs at all. They are grouped with spiders and ticks, since they share more characteristics with arachnids than crustaceans.
The underside of a Horseshoe Crab is where you’ll find their mouth, between their ten legs that they use to move around, and the two legs they for passing food into their mouths. Horseshoe Crabs eat mostly worms and shellfish, crushing the shells with their legs and eating the meal on the go. Behind a Horseshoe Crab’s legs are their gills. This structure looks a bit like the pages of a book and are called book gills. This is how they are able to breathe underwater.
You might not think Horseshoe Crabs have any eyes, but they actually have ten! If you look closely at the bumps along a Horseshoe Crab’s shell, you will see that some of these bumps are basic eyes, which help them sense light and darkness. This adaptation is important for the Horseshoe Crab to know when it’s time to join the party during spawning season, when they congregate in huge numbers based on the moon and tide cycle.
If you spend time along Nantucket’s shores, you will almost certainly come across a Horseshoe Crab. Like any animal, it is best to observe from a distance and let them do their thing. Sometimes a Horseshoe Crab will be upside down, out of the water. It often looks like this is an emergency, but this is exactly what their telson is for! Horseshoe Crabs stick the telson in the sand to help themselves get back right-side up, so they don’t need human intervention. It’s okay to gently pick them up and send them on their way right-side up in the water, but you should never pick up a Horseshoe Crab that has buried itself in the sand. That means the crab is actively laying eggs and should not be disturbed. They can survive outside of water for quite some time, so it’s okay to leave them be- after all, they made it 400 million years without our “help”!
Horseshoe Crab eggs are a major food source for many migrating shorebirds, so many of those bid species rely on the health of the Horseshoe Crab population for survival. Both the state and conservation groups monitor Horseshoe Crabs in Massachusetts, and recent data suggests that their populations are recovering after decades of pressure from commercial harvesting. Horseshoe Crabs are used as bait for eel and snail fishing but are mainly harvested as a biomedical tool. Their blood is very special and contains a substance useful in testing medical equipment for sterility. While this is an important need, regulators have been working to balance the need for Horseshoe Crab blood without decimating the crab population.
Who knew that going to the beach could introduce you to a prehistoric creature! Horseshoe Crabs teach us important lessons about ecology and the important connections between species. This spawning season is a great opportunity to marvel at these living fossils!
Stay tuned for more editions of Current Nature, a biweekly column from the Linda Loring Nature Foundation featuring seasonal topics, natural history information, and advice on the outdoors!