 Faraday says for decades, they were considered pests. Kids on the Cape could get a couple pennies for each crab they killed, tossing its helmet-shaped body into the dunes to die and snapping off its spiky tail as proof. Bring that in and you could get paid for that. So they were worth more dead than alive. Today, horseshoe crabs are harvested for bait and for use in the biomedical industry. Their blue blood helps test for toxins in medical products.