Horseshoe crabs have stood the test of time

Strange dark shapes were scuttling around on the sandy bottom. I watched them from the wharf at Chebeague Island with my little 8-year-old hands gripping the splintery planking and my face stuck out over the edge, looking down into the water. They glided around in gangs.

Horseshoe crabs, they were called, which puzzled me, the way you’re not sure what to think when someone tells you the stray bright star at Orion’s foot is a dog’s shoulder. It does not look like a dog. In the beginning, anyway. Horseshoe crabs were shaped only vaguely like horseshoes and definitely were not crabs, which run sideways on dangerous-looking legs and do not have tails like big stingers.

That was more than a half-century ago. Now I clearly see a dog bounding along behind Orion, and Limulus polyphemus’ rounded, double-ridged shell is close enough to horseshoe-shape that the common name seems right. But the deeper they get in time, the stranger they seem.

For one thing, they have a lot of eyes. Seven are on the shell – two on the back, similar in structure to human eyes; two “median” eyes toward the front, with another just behind them; and two on the sides that are used by the embryo inside its egg.

Underneath, near the mouth and legs at the middle of the body, are two more eyes. And simple photoreceptors are arranged along the top of the tails.

The three eyes toward the front of the shell can pick up ultraviolet light, which humans can’t see. And they have special sensitivity to moonlight, apparently in use during spawning times, which peak on the evening high tides of full and new moons in May and June.

A horseshoe crab’s nervous system includes millions of sensory receptors all over its body, especially in the eight legs and two chelicerae (the small crablike claws that place food in their mouths), which can detect the chemical composition of the water.

More strangely, horseshoe crabs have no immune system. In other words, they don’t produce antibodies to fight infections. Instead, they make chemicals that inactivate bacteria and viruses, and that create clots to seal out further intruders. Horseshoe crab blood is used in serums and vaccines. And studies of their eyes turned into a Nobel prize in medicine in 1967.

They’re so different from other creatures, they seem almost alien. There is a notion among some entomologists that spiders – who are cousins of horseshoe crabs and also have eight legs and chelicerae and multiple eyes – did not originate on Earth because they show few signs of having evolved from water-going beings, which practically everything else here does. The horseshoe crabs, being water-goers, may disprove the notion, but they still seem deep and remote.

Strangeness and age play tricks with your memory. It seems like, back there when I was 8 gazing down into the water, I already knew horseshoe crabs are among the most ancient of still-living beings. The oldest horseshoe crab fossil dates to about 360 million years ago, which means they scuttled across the floors of Earth’s silent seas 260 million years before any flower blossomed. Spider fossils date to 400 million years ago. Horseshoe crabs’ direct ancestors, the trilobites, appeared about 530 million years ago, and disappeared in a mass extinction 250 million years ago.

Horseshoe crabs survived the whole span. And the memory of them slipping across the floor of Chandler’s Cove is burned into my mind and seems to be evolving there like ancient starlight.

Dana Wilde may be reached at naturalist@dwildepress.net. Recent Naturalist columns can be seen at www.bangornews.com/topic/95/browse.html.