As strange as that sounds sounds, there is evidence to back that up.
Horseshoe crabs are not related to crabs at all, instead they are close cousins of spiders, ticks and scorpions.
One of the interesting features of the horeshoe crab is that horseshoe crab blood is a light blue instead of red like ours. The reason for this is because their blood is copper based instead of iron based.
Horseshoe crabs are thought to be descendants of trilobites and there is plenty of scientific evidence to back that up. There is still a lot more to find out about these amazing creatures.
The horseshoe crab is becoming threatened by man and we may be the only predator that they have that could challenge their very existence that without many other creatures that depend on them for existence would also become endangered.
So, spider or not we need to do what we can to ensure their survival in the wild.