I always hate to see people buy horseshoe crabs for their reef tanks. They are really not suited for our tanks. They prefer cooler water than we usually keep, and generally starve fairly quickly.
Here's some FAQ's from WetWebMedia on them:
HorseshoeCrabFAQs
AND:
"Limulus polyphemus, a/the Horseshoe Crab. This species is principally a sand-sifting filter feeder, though there are other horseshoe crabs that are more predatory. Up to a foot in diameter. Most die from a lack of nutrition in captivity, due to insufficient size, depth, culture of live sand organisms."