Every May and June for the last 450 millions years, horseshoe crabs have been mastering the art of precision by laying their eggs along beaches at high tide during new and full moons.

For the last eight years, the Massachusetts Bays Program has been tracking the horseshoe crab population in Duxbury Bay during their spawning season. Ecologist Sara Grady and volunteers seize this small window of time to monitor how these hard-shelled arthropods are holding up.

We are trying to measure the spawning index and the crab density over many years to get a sense of how their populations are doing,” Grady said.

It turns out that horseshoe crabs are a hot commodity. They’re used as bait for the eel and conch fisheries, and on a smaller scale, their blood is harvested for the biomedical industry.

Intern Callie Neaves catches sight of a horseshoe crab, calling out “So, there’s a single male over there.”

Grady, Neaves and volunteer Carolyn Sones keep count by measuring in 5-meter quadrants, using 3 poles with rope attached at each end. And on this particular day, they are counting a lot of single males trolling the shore.

Typically, crabs that are all alone are male crabs because they haven’t found a female to latch onto, and we have a lot more males than females. If there’s a female crab, she’ll almost certainly have a male crab latched onto her shell.” Grady said.

The male has boxer claws in the front that allow him to lock onto the female shell. Once he locks on, she carries him off to shore where she lays her eggs and he promptly fertilizes them.

There was a drop in the horseshoe crab population in Duxbury between 2008 and 2012 but Grady says it’s been increasingly stable since then. 

In places where we read declines, it would probably be attributed to either bait harvesting or loss of habitat.” Grady said.

Seaside development and armoring along the coast can also impact the population. If it isn’t sandy and relatively peaceful, horseshoe crabs are less likely to lay their eggs—and that matters because of the horseshoe crab’s role in modern medicine. Their blood clots immediately when in contact with the tiniest trace of bacteria, making it an invaluable tool in the biomedical field.

Since the 1970s, the FDA has mandated that all injectable drugs and vaccines must be exposed to a small of amount of horseshoe crab blood to ensure their sterility. The crabs are released back into the ocean but about 10-20% of them don’t survive.

At Duxbury Bay, volunteer Carolyn Neaves tallies up the crab count for the noon high tide: 98 crabs altogether; 7 females.

It’s not a high spawning index but Grady says it’s a pretty good density, considering her team counted an overwhelming 1200 horseshoe crabs a couple weeks earlier, and the spawning season is tapering off.

We’ve been seeing a lot of spawning pairs, which is always good. Duxbury tends to have a much higher spawning index than some of the other bays in the state. And we see a much better sex ratio, so we see more females to males.” Grady said.

Meaning these guys should be around for another 450 million years.

To learn more about horseshoe crab research, visit WGBH’s Forum Network where they featured Kathryn Tuxbury and John Dubczak: http://forum-network.org/lectures/living-fossil-and-blue-blood-horseshoe-crab-and-human-health/.