On May 4th I joined natural history photographer Sandy Tilton to videotape one of the most spectacular scenes in nature that you can see in New England -- the annual spring spawning of horseshoe crabs.

But we were sadly disappointed. Every 50 feet there was another female horseshoe crab dead in the shallows. That represented almost half a million eggs of this uniquely beneficial animal that has saved millions of human lives and whose processed blood called limulus amoebocyte lysate, or LAL, is worth over $30,000 a quart. If all those eggs had survived they could have produced several million dollars worth of the life saving pharmaceutical.

But the dead crabs had all been females. Did that mean that the population was so small that not enough males had been around to fertilize the eggs? Did that mean that the Plum Island population was primarily females and that a significant number of them had died? Neither of those two scenarios meant that the struggling population was very healthy.

We have known since the 1950s that Plum Island Sound has the smallest crabs of any estuary on the East Coast because of its cool water temperatures and the influx of large quantities of fresh water from the Merrimack and Parker Rivers.

Finally it hit me. If a horseshoe crab gets caught in fresh water its gills swell and rupture. And if gram-negative bacteria invade the crab through its exposed gills the crab will sludge up and die. In fact that was the observation that led the discoverer of LAL, Frederick Bang, to realize the copper-based blood of the crab had such unique lifesaving properties. 

So what had happened to these crabs? The female horseshoe crabs had probably cambered up the beach during one of the preceding night’s high tides and dug in to lay their eggs. They would then stay buried until released by the next high tide 12 hours later. But the males would have only stayed around long enough to fertilize any eggs and then returned to the safety of the estuary.

What had happened during those 12 hours? We had experienced another one of our seemingly never-ending rainstorms that have a habit of overwhelming the Merrimack River’s wastewater facilities.

The combination of too much fresh water and too many gram-negative bacteria had overwhelmed the crabs’ already stressed immune systems and they had died during the night.

Then the incoming tide had floated them free and we found them dead in the lapping waves.

Here was a creature that has been on this planet hundreds of millions of years before there were birds, fish and mammals, and millions of years before the dinosaurs, horseshoe crabs were coming ashore to lay and fertilize their eggs. Such animals are built for survival but here they were dead because our “civilized” society makes a habit of dumping its human fecal bacteria into nature’s once pristine rivers.

Later in the day we came across another ancient species, far younger than horseshoe crabs. These were an inconspicuous little species of ground cover called horsetails. They had feathery green stalks along with light brown spore bodies.

But the ancestors of these horsetails had been 100-foot high trees that towered over dinosaur filled swamps and marshes. Horsetail trees were the dominant species on earth and you can still see their remains in chunks of Cretaceous coal.

But where are they now? Sic transit Gloria Mundi! It was their inefficient spores that made horsetails lose dominance in the rapidly cooling climate.

Horsetails have specialized organs called elaters that act like moisture sensitive springs, so when conditions were just right in a warmer, moister world, the elaters helped the spores crawl and hop to neighboring horsetails, not a very efficient dispersal system compared to the wind blown pollen and seed bearing fruit of modern trees adapted to a cooler, less moist climate.

So what happened to horsetails? They devolved into this innocuous little ground cover easily overlooked by herbivores. Some of them even evolved packets of silicates in their leaves which made them excellent sources of fiber for polishing woodwork but indigestible for reptilian and mammalian herbivores.

And what is the lesson for humans? If we don’t heed the warning of horseshoe crabs we may face the fate of horsetails and devolve into a smaller brained, less dexterous species better adapted to moisture, heat and carbon dioxide – perhaps our best hope for survival on this planet we are so rapidly altering if not destroying.

Bill Sargent is a North Shore science writer and author whose column appears regularly in The Daily News.