The first 450 million years apparently were the easy ones for horseshoe crabs. They survived largely unchanged while dinosaurs, mega-mammals and countless other species went extinct.

They seemed to follow a meek-inherit-the-Earth strategy, safe in their armored shells on the ocean and bay floors, eating worms and mollusks. Harmless oversized bugs of the deep and without the claws of actual crabs, they have counted on would-be predators figuring their little flesh isn’t worth the effort of chewing through their shells.

Humans changed that. People couldn’t eat horseshoe crabs, but watermen could readily grab them and chop them up to bait pots to catch edible seafood such as eels and whelks — or just grind them up for fertilizer.

In the modern age, just about anything people can take for free and turn into money will be depleted. In the 1990s, that was happening to the largest concentration of Atlantic horseshoe crabs, in the MidAtlantic and Delaware Bay. Watermen were taking more than 2 million per year.

People might not have noticed or cared much, except the billions of tiny eggs left on bay beaches this time of year by the spawning horseshoe crabs feed massive concentrations of migrating shorebirds — an internationally famous phenomenon of nature. When bird numbers plunged, birders noticed, and conservationists and regulators followed.

One bird in particular seemed especially dependent on Delaware Bay horseshoe crab eggs: the red knot, which each spring flies from the tip of South America to the Canadian north to nest. The red knot must gorge on the eggs to successfully complete nature’s longest annual migration and reproduce. Since it’s a federally protected species, that meant the horseshoe crabs producing those eggs had to be protected too.

People came to appreciate this living fossil, so primitive-looking with fixed eyes staring from a hinged and horned half-round shell. The latest, last month in Cranston, Rhode Island, was an elementary school whose kids voted to drop the tiger as their mascot in favor of the horseshoe crab.

The annual harvest was cut by three-quarters at the start of this century. But the annual counts by wildlife officials have found that more than a decade later, the horseshoe crab population isn’t rebounding, just stabilized.

Now scientists suspect a much more valuable, modern use of the horseshoe crab might be a growing threat. An extract from the creature’s blue blood is the only substance that can detect certain infectious bacteria and endotoxins in vaccines, other medicines and implants such as artificial joints.

Medical labs catch horseshoe crabs, remove about a third of their blood to get the extract, then release them. Since the 1990s, the number caught and bled has doubled to more than 500,000 per year.

For years, few questioned the industry’s assertion that mortality of released horseshoe crabs was low. But scientists now think the many hours spent out of the water, their gills unable to breathe, often overheating in a truck or on a dock or deck, might be killing a quarter of them and leaving others impaired, according to an April report in Popular Mechanics.

Last year, the International Union for the Conservation of Nature determined the horseshoe crab is vulnerable to extinction. The Atlantic States Marine Fisheries Council will include biomedical use of horseshoe crabs in its next population assessment, due out next year.

But the council manages the taking of horseshoe crabs by watermen. If the growing use of them by the biomedical community is a threat, the ASMFC has no power to regulate medical company use and handling to protect the species.

Companies are trying to synthesize the blue-blood extract, but so far none has FDA approval.

For now, horseshoe crabs are the only source of a medically necessary test and a crucial support for a threatened bird species. State and federal authorities need to develop a new plan to keep shielding them from the old threat of cheap use and protect them from the new one while allowing their very profitable medical use.

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