In the south of Japan, near the city of Kitakyushu, Japanese horseshoe crabs gather every year by the thousands on the Sonehigata tidal flat to lay their eggs. This is a vital place for these ocean arthropods, because it's one of the largest habitats they use during the breeding season. This season, however, something has gone very wrong: horseshoe crabs are dying en masse.

Some deaths are expected during each horseshoe crab egg-laying season, usually around 50 or 60. But according to a count by the Japan Horseshoe Crab Association, around 2,000 breeding pairs gathered at the tidal flat this year, and the death rate was much higher than usual.

"The conservation group spotted about five to ten bodies every day during the egg-laying period, so they started to tally them," Kitakyushu city official Kenji Sato told the Australian Broadcasting Corporation. "In total the number of dead horseshoe crabs reached about 500."

Perhaps the most distressing part of the story is that no one really knows what has caused this mass mortality. "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 newspaper. "We have to be careful to identify the cause."

Whenever horseshoe crabs hit the news, media outlets are quick to dust off the term "living fossil" – but this is a misleading phrase. Horseshoe crabs do have similar-looking relatives in the fossil record going back more than 400 million years, but to say they haven't changed is just plain wrong, and it detracts from the uniqueness of these fascinating little creatures. The four species of horseshoe crab living today are important, recently evolved species unlike any before them.

Horseshoe Crabs 2016 09 16
There are four living species of horseshoe crabs. Image: Paul VanDerWerf, Flickr

Japan isn't the only place where these animals are in trouble. Elsewhere in Asia, as well as on the Atlantic coastlines, they're losing ground to rising ocean temperatures and habitat destruction, and they're often over-harvested for their blood, which contains chemicals that are useful for medical testing. Because these crabs grow and reproduce relatively slowly, they have a hard time bouncing back from these damaging impacts. The mass deaths in Japan are part of a widespread problem.

As news of the Sonehigata crabs has spread, some online commenters have begun wondering if the deaths might be linked to the Fukushima nuclear disaster, which released radioactive material into the environment. But Dr Jim Smith, a University of Portsmouth professor who studies environmental contamination, assures us that the events are almost certainly unrelated. 

"There's no reason why this should be linked to the Fukushima accident. The current radiation dose rates in the marine environment, even near Fukushima, aren't high enough to cause mass mortality of organisms," he told me via email. "Looking at a map of Japan, it looks like the site of the mortality isn't anywhere near Fukushima, making this even less likely as a cause."

The true scale of this season's horseshoe crab deaths has only just been realised, so experts haven't had much time to investigate. The cause may turn out to be something new, such as disease or parasites, as some experts have suggested. On the other hand, it may be that the same habitat loss and over-harvesting that have been plaguing these creatures for so long have simply struck an unusually potent blow this year. 


Top header image: Scott Sherrill-Mix, Flickr