BRUNSWICK — Several years ago one warm spring evening, I witnessed one of the most bizarre and magical natural occurrences on the shore. There were horseshoe crabs mating in twos, threes, fours and fives on mud that was sparkling with phosphorescence. As they moved across the mud, it lit up under the moonlight looking like stars right beneath our feet.

I could hardly move, I was so transfixed by contrast between these far-from-beautiful, prehistoric creatures and the sparkling world upon which they crawled.

I’ve seen this happen many years since, but somehow never taken the time to learn exactly what was going on. This is a bit unforgivable since I studied lobsters, one of the horseshoe crab’s decapod (10-legged) relatives, in graduate school and could tell you all about its mating habits! As often is the case, it wasn’t until I was with a group of curious kids that I took the time to research these strange creatures.

I got to join some fifth graders from Harriet Beecher Stowe School on a field trip to Thomas Point Beach, yet another great place to see horseshoe crabs. The students had to fill out worksheets with questions about how many legs they have, what they eat, and what kinds of animals they are related to. But, then, the question came up about how the crabs were attached to each other and whether they could really be mating when there were often three or four males attached to a single female.

Part of the mystery was solved during the scavenger hunt, another component of the day, when we discovered two horseshoe crabs nestled in a depression in the sand together. We determined that this must be their nest and where the female lays her eggs. So, were they mating in the water or on the sand?

Lucky for us, near the end of our day, there was a biologist from the Maine Department of Natural Resources who was collecting some horseshoe crabs for research and education and she filled us in that the males were simply staking their claim to the females by grabbing on to them with their strong first set of pincers. These are often called boxing gloves because of how thick they look as compared to the females and serve as a good way to tell the sexes apart.

They can stay attached like this for weeks as the male waits for the critical moment when the female goes up onto the sand, makes her nest, and then lays her eggs. This is his chance to fertilize them and so it is worthwhile to latch on to her so as not to miss it.

While this behavior might seem modern, horseshoe crabs are one of the oldest animals on earth. It’s not that each horseshoe crab is ancient, but that they have hardly changed in over 450 million years, meaning that they were around when dinosaurs were alive. Anything that hasn’t changed in that long must be pretty fantastically designed for survival. This is definitely true for horseshoe crabs. Of course, their giant shell helps protect them, but they have other superpowers.

A horseshoe crab’s blood has rare bacterial-fighting ability. If it gets wounded, components in its blood form sealed-off capsules around the bacteria to protect it from getting infected. We use horseshoe crab blood to test vaccines and intravenous medications for bacteria. If there is clotting between the crab blood and the vaccine, then there are bacteria present. So, chances are good you have benefited from horseshoe crab blood without even knowing it. And, did you know that their blood is blue? It has copper in it (iron makes ours red), which gives it special properties.

Also, what sea creature do you know that can live out of water for four days? As long as their gills are moist, horseshoe crabs can. They can even fold themselves in half to stay moist! They also have unheard-of vision. In addition to their two compound eyes, which you can see on top, they have four other sets of eyes – some on top and some underneath, and over 1,000 light receptors on their tail so that they can see at night. This enables them to see in virtually all directions at any time of day in or out of water!

If you’re interested in seeing them, the full and new moons in May and June are the best times, as that is when they are mating. Shallow intertidal areas are best, especially near sandy or soft sediment shores where they can nest.

As for the phosphorescence, that’s a tiny plankton that blooms in giant numbers in the spring and early summer and lights up when disturbed. But, I’ll save that topic for another day!