Horseshoe crabs have survived five great extinctions, including one that wiped off 96 percent of all species on Earth, yet they are dying in large numbers off the coast of Japan this year for unknown reasons. The shallow seas near Kitakyushu are normally ideal habitat for one of four crab species (Tachypleus tridentatus), and are regarded the most significant breeding area for crabs, but this year they have been dying in large numbers.
Despite spending most of the year in deeper water, the crabs (which aren’t truly crabs but are more closely related to spiders, scorpions, and trilobites) congregate in enormous aggregations off the coast to mate and deposit their eggs. Some of the ancient animals are expected to die during these episodes, but environmentalists have discovered up to 500 of them littering the beaches – approximately eight times the average mortality rate.
The prehistoric animals are a remarkable example of a “living fossil” since their appearance has stayed unaltered during the 450 million years they have been swimming in the waters, with fossils and trackways revealing nearly-identical crabs throughout. Throughout their historic existence – remember, we as a species have just been existing for 200,000 years – they have endured five major extinction events, even surviving the meteorite that killed the dinosaurs. However, it appears that the amazing survival has met its match.
Crabs have been declining for decades as rapid urbanization along shorelines damages their breeding sites and puts them at risk of exploitation. They were once common around Taiwan, for example, but 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, which means that as coastal areas continue to be developed, crabs continue to suffer.
What caused the recent mass die-off remains unknown, with specialists speculating on everything from low oxygen levels caused by a rising ocean owing to climate change to a particularly high load of parasites, or even a sickness peculiar to horseshoe crabs. It might be a mix of circumstances, but let’s hope they can also get through this one.