Horseshoe crabs have survived five mass extinctions, including one that wiped out 96 percent of all animals on Earth, but for some reason this year they are dying in massive numbers off the coast of Japan. The shallow waters off Kitakyushu are usually the perfect habitat for one of the four species of the crab (Tachypleus tridentatus), and is considered the most important breeding ground for them, yet this year they have been dying in scores.

Usually living in deeper water for most of the year, the crabs (which aren’t actually crabs, but more closely related to spiders, scorpions, and trilobites) come into the shallower, sandy waters off the coast in massive aggregations in order to mate and lay their eggs. It’s normal for some of the ancient creatures to die during these events, but so far conservationists have found up to 500 of them littering the beaches – roughly eight times the normal mortality rate.

The prehistoric animals are a prime example of a “living fossil” as their appearance has remained unchanged for the 450 million years they have been swimming in the oceans, with fossils and trackways showing near-identical crabs for this entire time. During their epic history – bear in mind that we as a species have only been around for 200,000 years – they have faced five major extinction events, even managing to battle their way through the asteroid that did in the dinosaurs. But it seems that nowadays, the incredible survivor may have met its match.

The crabs have actually been in decline for decades, as rapid development along the shorelines destroys their breeding grounds as well as puts them at risk from overfishing. Once common around Taiwan for example, they are now rare, and while they have been listed as endangered by the Japanese environment ministry for some time, there are no concrete laws that actually protect them, meaning that as the coastal areas continue to be built on, the crabs continue to lose out.

What has been behind this recent mass die-off is still a mystery, with experts variously postulating that it could be anything from low oxygen levels due to a warming ocean from climate change to a particularly heavy load of parasites, or even simply a disease specific to the horseshoe crabs. It may well be a combination of factors, but let’s hope they can pull through this one as well.