Image copyright AFP Image caption A photo from 27 August shows collected horseshoe crabs near Kitakyushu City

Nearly 500 horseshoe crabs have washed up dead on Japan's southern beaches near Kitakyushu, mystifying experts.

The famously blue-blooded creatures come to the tidal flats in southern and western Japan each year to lay eggs, and some normally die off.

But this year conservationists say up to 10 crabs have died each day, eight times higher than normal, according to the Japanese newspaper Asahi Shimbun.

Some think the die-off means the crabs will lay fewer eggs next year.

The creatures are not true crabs, being most closely related to spiders and scorpions.

They are classified as an endangered species in Japan, where their habitat is being destroyed.

Experts cite the effects of global warming, a lack of places to lay eggs and disease as possible causes for the crabs' demise.

Horseshoe crabs are one of the world's oldest creatures and are prized for their blue blood.

Scientists have harvested the horseshoe's blue blood since the 1970s to test the sterility of medical equipment and intravenous drugs.

The blood coagulates around tiny amounts of bacteria, immobilising the pathogens.

One litre can sell for $15,000 (£11,360).