Hm, horseshoe crabs are interesting little creatures, not ones I would choose to keep in an aquarium though, as once they release eggs, I imagine theres going to be trouble, as their eggs contain tetrodotoxin, its a sort of similar situation to keeping Palythoa genus corals, theyre fine, until one gets damaged and releases its internal fluids into your aquarium tank, in which case, your'e buggered, and everything dies.