They like to do it in the middle of the night, while the waves lap softly at the sandy beach, under the glow of a full moon.

They do it in the middle of the day, too, sun bright, right where you can see them.

Not spring breakers. We're talking about horseshoe crabs.

Spring is when these crusty critters — 445 million years old and vital to the ecosystem — emerge from the continental shelf and come ashore to mate.

Like sea turtles, horseshoe crabs find calm, sloped beaches to spawn. With the male on her back, the female crab will dig down into the sand where the tide is breaking to deposit her eggs.

This mating takes place along the beaches of all coastal counties in Florida, said researcher Tiffany H. Black with the Florida Fish and Wildlife Conservation Commission.

And if you see some copulating, officials want you to report it.

Not for indecent exposure.

For science.

Don't forget to report mating horseshoe #crabs on #FL beaches! https://t.co/kuhurQw1X7 pic.twitter.com/252BnMcMM9

FWC keeps a database of horseshoe crab sightings, primarily to keep in compliance with the Atlantic States Marine Fisheries Commission based in Washington, D.C., which developed a Horseshoe Crab Fishery Management Plan.

"The reason why we do this too is because there hasn't been a lot of work done on the populations of horseshoe crabs in Florida," Black said. "We think they're declining."

Sea level rise and the building of sea walls are two contributing factors. Both reduce the amount of valuable beach space for mating.

The crabs can also get caught up in power plant intake pipes.

Their decline could have far-reaching consequences, Black said.

Many shorebird species feed on the crabs' eggs. One in particular, the red knot, stocks up on eggs during its long migration north to the Arctic Circle. A reduction in eggs hits the red knot hard.

For humans, the horseshoe crab serves an even greater purpose.

Their unique, copper-based blue blood is used to test for the sterility of medical equipment and all intravenous drugs. The blood contains a substance called Limulus amebocyte lysate (LAL), which coagulates when even the smallest amounts of bacterial toxins are present, Black said.

At least five major pharmaceutical companies do the LAL tests.

The biomedical bleeding, it's called, usually doesn't harm the crabs, which are released back into the wild once the procedure is done.

"Horseshoe crabs are incredibly important to human health," Black said.

Beyond reporting any sightings, Black said humans can also help save thousands of future crabs by doing one thing: turn them right side up if you see one has flipped onto its hard shell back.

Sometimes the waves knock them over, Black said, and if they can't right themselves they could die.

"You could save a lot of horseshoe crab lives," she said.

Contact Katie Mettler at kmettler@tampabay.com or (813) 226-3446. Follow @kemettler.