Atlantic horseshoe crabs at the Tulsa Aquarium. Horseshoe crabs have blue blood and it has medicinal purposes, such as curing blood poisoning. As such its very valuable. Sadly this means that horseshoe crabs are farmed and bled to collect it. Interestingly the horseshoe crab isn't really a crab, they're more closely related to arachnids than they are crustaceans.
Found in the Gulf of Mexico and the Atlantic Coast.