How do horseshoe crabs eat?
Horseshoe crabs are arthropods which include the insects, spiders, true crabs, and
centipedes. Arthropods are divided into two major groups based on the first set of "postoral" (after the mouth) appendages: chelicerates and mandibulates. Mandibulate arthropods have "mandibles," in which the postoral appendages are jawlike (think of what an ant bites you with). Spiders, horseshoe crabs, and scorpions have chelicera (KELL-ISS-ERR-AH) which are feeding appendages which are jointed and sometimes possess a set of claws. The chelicera are used to push food into the mouth while mandibles are capable of cutting or grasping food.
Horseshoe crabs are jawless. The mouth is essentially a hole into the esophagus with muscles that open and close this opening. The mouth is located at the base of the legs which effectively surround it. The first segments of the legs are enlarged and covered with thick and hard bristles pointing inward toward the mouth. These bristles (gnathobases) act like a set of grinding tools as the animal is walking and they are used to "chew" food and direct it toward the mouth where the chelicera then finish the job of pushing the material into the mouth. From the mouth the food passes into the esophagus, a crop, a gizzard that grinds the food, the stomach, intestine and terminates in an anus located on the ventral side just in front of the telson.
Horseshoe crabs are scavengers and feed on mollusks, worms and other benthic organisms. Because they grind food with their spiny leg segments they actually have to be walking to chew their food.
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