By Susan and Biff Andrews

Last week we dealt with a good friend of beach-goers, the mole crab. This week it’s to be a good friend to anyone who gets a shot, the horseshoe crab. You can’t make this stuff up.

First, the basics. Horseshoe crabs are one of the oldest critters on the planet, going back to the trilobites of 450 million years ago. They are arthropods, they have no spine, yet oddly they are more closely related to spiders than crabs.

Everybody who goes to the beach has found them, from tiny shells (left over from a molt) to huge ones nearly two feet long. They look like big ticks — smooth brown on top, ten legs or claws underneath. Most of the legs have specific uses, but too much detail gets into overload.

The mouth is right at the center between the legs (as well as two eyes). Eyes underneath? Sure. They have seven more on their back, some compound, some simple.

The rods and cones in their eyes are 100 times larger than humans’ and their night vision is a million times more sensitive than by day.

Perhaps their most striking feature, visibly, is their tail, or telson. They use it for steering when swimming and to turn over if they get flipped. The females are about 25 percent larger than the males, and when they all come together for mating, four or more males may be seen riding on or clasping one female.

They usually crawl along shallow ocean and bay waters with sandy or muddy bottoms, sucking up worms and mollusks. They swim upside down, legs up, at about a 30 degree angle. And they can grow lost limbs, like a starfish.

All very interesting, but what about the shots?

Well, here’s where it gets really crazy. Scientists draw blood from about 500,000 horseshoe crabs a year. The crabs are then released with only a 15-percent mortality rate. Their blood is blue, as it is copper based, not red and iron-based like humans.

So why do scientists need blue blood from half a million horseshoe crabs?

It turns into a gel when it encounters endotoxin — bacteria byproducts. When scientists make a batch of flu vaccine or tetanus vaccine, they test it for bacteria by exposing it to some blue blood. It’s the most sensitive test there is.

They also, after “sterilizing” an implant to put into someone, check it for bacteria with blue blood.

But there’s a problem. Horseshoe crabs are used by commercial fishermen to attract conch and whelks. They’ve been using so many that horseshoe crabs are getting scarce. Limits have been placed on how many horseshoe crabs fisherman may take and use, and now they are increasing in numbers, but it stands to reason that blue blood bacteria gel is more important.

So if you get a flu shot this fall, thank a horseshoe crab that it’s not toxic. And pet the next live one you see at the water’s edge. He’ll be back again next year and the next (they live up to 40 years), a friend to man.

Isn’t science wonderful? You can’t make this stuff up!

Susan and Bradford “Biff” Andrews are retired teachers and master naturalists who have been outdoor people all their lives, exploring and enjoying the woods, swamps, rivers and beaches throughout the region for many years. Email them at b.andrews22@live.com.