Horseshoe crabs spawn in Great Bay, off the coast of NH, Maine

I was visiting my aunt in Manhattan last weekend. Watching the news, nestled among all the typical big city tales, was one about the return of horseshoe crabs to the coast.

On beaches like ours, horseshoe crabs were shown scurrying about in the sand. From now through June, you can see horseshoe crabs gathering at high tide throughout the Great Bay (and as far south as Florida) as they congregate to spawn. Here in New England we are in the northern part of their range

I’d say they’re everyone’s favorite crab, but they’re not crabs. They are instead more closely related to the spider clan. Most people consider them cute (even those who find spiders scary) with their round, horseshoe-shaped shells, prominent eyes and spiky tail.

Volunteer Bob Fox teaches the public about horseshoe crabs in Great Bay

Horseshoe crabs are what we often call living fossils – not because they are living fossils, but because they have remained relatively unchanged in their form for a very long time, over 450 million years, which is 200 million years before the dinosaurs!

The reason they have been successful for so long is that they have undergone some interesting adaptations that have enabled them to survive our constantly changing planet. The first is that spiky tail, it’s called a telson and isn’t a weapon, although I think if you stepped on it wrong it would hurt. The horseshoe crab uses it to aid in steering and to straighten itself when turned over – which happens more often than you might think, as it is ravaged by waves in the intertidal zone.

Horseshoe Crabs in Great Bay

I think my favorite features are his eyes. Those two prominent large eyes on top of the shell (the prosoma) are compound eyes that use horseshoe crabs for finding mates and perceiving light. Each of these has about 1,000 receptors, rods and cones, which are similar to ours, except they are 100 times larger. They can change their sensitivity to light so that they become much more sensitive to light at night to help recognize other horseshoe crabs. They have eight other eyes that have different known and unknown functions, one of which is on the tip of the telson! The tail eye is thought to help synchronize their brains with light and dark cycles, according to the Maryland Department of Natural Resources.

Horseshoe crabs spawn in Great Bay.

So this is the start of the breeding season here in the Seacoast area. The work of the labs of Win Watson (UNH) and Chris Chabot (Plymouth State University) suggests that our horseshoe crabs infer ocean temperature to signal the start of the spawning season. “They come to the water’s edge day and night at high tide to breed. The myth was that they only bred at full moon, but we proved that,” said Win Watson. To mate, the smaller male crabs hook themselves on top of the female’s shell using specialized front claws (an easy way to determine sex) and hang on as she crawls to the beach. The female digs small nests in the sand and deposits her eggs while the male fertilizes them. Other males (called satellite males) hang out nearby and often manage to fertilize some eggs as well. The larvae hatch within a few weeks and spend most of their long lives in the tidal sand flats feeding on a variety of invertebrates.

A female horseshoe crab with two males.

Horseshoe crabs are important to humans. We use their copper-based blue blood in medical tests and as bait, but more importantly, they are an essential part of coastal food chains. Their eggs are an essential food for a large number of migratory and coastal birds, as well as some reptiles and fish. They are currently listed as a vulnerable species due to habitat loss, beach development, and overfishing due to the aforementioned human use. Therefore, there are a number of active monitoring programs along the Atlantic coast to track populations.

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