The oldest species living in Hong Kong is the horseshoe crab. Considered a living fossil, it has lived in Hong Kong for a staggering 400 million years! Despite its name, the Horseshoe Crab is not actually a crab but rather a close relative of scorpions and spiders. Interestingly, its blood contains copper ions and appears blue in colour. It is used as a reagent to test for impurities in medical drugs.