Horseshoe crabs coming ashore in Delaware Bay © Gregory Breese/USFWS

We’ve posted about them before, but did you know that horseshoe crab blood is not only a powder blue color, but also is used in the medical industry to detect any trace of bacterial contamination in humans, even if that infection is only one part per trillion?  And that they are more closely related to scorpions than true crabs? Or that they’ve been having the longest-running mass-breeding efforts on the planet, given that they haven’t changed much in hundreds of millions of years? Marah Hardt writes for The Nature Conservancy on the importance of these crawling Chelicerates to their ecosystem:

The lapping waves and silent dunes of the Delaware Bay shoreline create a perfect backdrop for a moonlit summer stroll. But a few weeks ago, this beach was not nearly so quiet. Instead, the silver light of the full moon shone upon jostling crowds of horseshoe crabs.

“If the crabs were rocks,” says Moses Katkowski, marine conservation coordinator with The Nature Conservancy, “you could walk on their backs the entire stretch of beach and never touch the sand.”

Every year, they emerge from the depths for one reason and one reason only: sex.

Lots and lots of sex.

Long before Roman emperors threw outlandish orgies, horseshoe crabs — contemporaries of the first sharks, wooly mammoths, and now us — have been making annual migrations to the shore to forge their next generation.

Truth is, horseshoe crabs haven’t changed much over their time on Earth. The four living species today look much the same as their ancestors did nearly 400 million years ago. It’s likely they reproduce just like them, too. Which means horseshoe crabs may hold the record for longest-running sex parties on the planet.

But They’re Not Really Crabs at All

Distant relatives of true crabs, horseshoe crabs aren’t really crabs at all.

Flip over a horseshoe crab and the undercarriage-view of its six paired appendages reveals the resemblance of horseshoe crabs to their closest cousins: scorpions and extinct trilobites.

The first pair of appendages is used for eating; the next four pairs are used for walking and each appendage ends in a small pincher-like claw that doesn’t pinch.

In males, the first pair of walking appendages is modified to aid in grasping the edges of a female’s shell. (More on that in a minute.) The last pair are “pushers,” which aid the crab in locomotion.

Male horseshoe crabs arrive just ahead of the females, forming a shifting boulder field that females must plow their way through to reach suitable nesting sites along the high-tide line. As they make their way, the smaller but determined males swarm the females, shoving and sliding, until one male can successfully grab onto her shell with his front legs and pull himself aboard.

A small notch at the front of the male’s helmet-like head can then latch onto the ridges on the female’s back. In this way, the two shells sort of fit together, like a tongue-and-groove joint.

Read the rest of the article at Cool Green Science.