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Mass horseshoe crab die-off in Japan

This week over 500 horseshoe crabs have washed up dead on Japan’s southern beaches near Kitakyushu, according to the BBC. The matter is being examined by biologists and the effect has never been seen before.

The horseshoe crab is a remarkable creature, unchanged since prehistoric times (some 445 million years ago) leading to some scientists referring to them as “living fossils.” Horseshoe crabs are marine arthropods of the family Limulidae (and they are not true ‘crabs’, being more closely related to scorpions — arthropods).

Horseshoe crabs are found off the coasts in North America (genus Limulus), Japan (genus Tachypleus) and the Philippines (genus Carcinoscorpius). The crabs have blue blood (due to a copper pigment) and the blood will clot if the crab becomes infected.

Today the creatures are under threat and there is a global decline in numbers. In Japan, for example, coastal habitat destruction is a major contributor; and in North America over-harvesting along the east coast has negatively impacted on the population (here the horseshoe crabs, of the species Limulus polymphemus, are used as a diagnostic test reagent to examine samples for a type of bacterial toxin called endotoxin). Worryingly, nearly a third of all the crabs who give their blood to save human lives die at sea prematurely.

With the test, one liter retails for around $15,000. For more on the medical use, Digital Journal’s Karen Graham gives the remarkable history in her article “Horseshoe crab blood is saving lives – now we need to save them.”

The loss of species numbers has raised alarm bells with the International Union for Conservation of Nature. The conservation body says that two species are now on the brink of extinction.

With the hundreds of dead crabs in Japan (of the species Tachypleus tridentatus), conservationists are trying to work out what has happened, according to the newspaper Asahi Shimbun from an interview with Hiroko Koike from Kyushu University Museum. Possible theories include effects of global warming, a lack of places to lay eggs and disease.

Speaking with ABC News, Kitakyushu city official Kenji Sato said: “The conservation group spotted about five to 10 bodies every day during the egg-laying period, so they started to tally them”, adding “in total the number of dead horseshoe crabs reached about 500.”

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Dr. Tim Sandle is Digital Journal's Editor-at-Large for science news. Tim specializes in science, technology, environmental, business, and health journalism. He is additionally a practising microbiologist; and an author. He is also interested in history, politics and current affairs.

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