Kal et Krab

Horseshoe crabs - King of Crabs?

"The blood of horseshoe crabs is blue, which is a result of its high content in copper-based hemocyanin instead of the iron-based hemoglobin found, for example, in humans."

http://en.wikipedia.org/wiki/Horse_shoe_crab

Is that why they are considered the king of crabs?

The ironic thing being that they aren't crabs at all.