Tapping the blue blood of horseshoe crabs to test vaccines and medical devices is coming under fire, with environmentalists and a large European investor putting pressure on pharmaceuticals groups to seek alternatives.

The industry uses the blood in millions of tests a year to detect endotoxins, poisons inside bacteria cells that can make people ill. But pressure is mounting on pharma companies to switch to synthetic substances as populations decline for horseshoe crabs and seabirds that eat the creatures’ eggs.

The asset management division of BNP Paribas, the French banking group, has sent letters to 14 of the world’s largest pharmaceutical companies asking them to use a cloned substance called recombinant factor C (rFC) instead of blood from horseshoe crabs for tests.