The quiet coast of Kitakyushu is the largest habitat for the Japanese horseshoe crab. Some 2,000 breeding pairs come to lay eggs on the shore each year, and, in the process, on average, 50 crabs die.

This year has been different. Nearly 500 have died, and nobody knows for sure why.

“Rises in the sea level caused by global warming, shortages of places to lay eggs, and a lack of nutrition could have resulted in their deaths,” Hiroko Koike, a researcher at the Kyushu University Museum told the Asahi Shimbun. “We have to be careful to identify the cause.”

The story is no different in other parts of Asia. Of the four species of horseshoe crab found in the world, two are found in India. At the recent meeting of the International Union for Conservation of Nature (IUCN), researchers said that habitat destruction may be driving horseshoe crabs to extinction on the country’s coasts.

These amazing crab species are among the handful of species referred to as “living fossils,” because their current form resembles those found in the fossil record. Externally at least, the crab hasn’t changed much in nearly 450 million years. In that time, it has survived all five of Earth’s great mass extinctions, the worst of which killed off an estimated 95% of all marine species, and the most recent of which did away with the the dinosaurs.

The rise of humans, however, hasn’t been something the horseshoe crab has been able to adapt to. Across the world, its population has fallen steadily over the last two decades. In some areas, human-induced climate change is to blame.

In others, the blame may lie on the harvesting of the crabs’ blue blood for medical use. As we reported previously: