In the wild Horseshoe crabs feed on a number of worms, mullusks and seaweed. They will also eat bits of fish and shrimp that fall to the bottom.
They dig around in the sand and mud to find whatever they can to eat. Basically they are scavengers.
If you are keeping horseshoe crabs in an aquarium, you can feed your horseshoe crab bits of clams, worms, brine shrimp as well as any type of sea food as long as the pieces are small enough for the size horseshoe crab that you have.
Just let the food fall to the bottom and the horseshoe crab will scavenge for them.
One of the problems with keeping a horseshoe crab is making sure that there is enough for them to eat. One of the most common reasons for a horseshoe crab dieing in captivity is starvation.
Another point to keep in mind when keeping horseshoe crabs is make sure you have a sandy bottom for the horseshoe crab to sift through as it’s feeding.