Atlantic horseshoe crab Introduction

Atlantic horseshoe crabs may appear alien, but their history as earthlings is pretty impressive. They’ve been around for 450 million years, predating the dinosaurs by more than 200 million years. They live on the Atlantic coast of North America, from Maine to down and around the Florida coast to Alabama and Mississippi. They also live around the northern Yucatán Peninsula in Mexico. Despite their common name, horseshoe crabs are more closely related to spiders and scorpions than to “true crabs,” which are crustaceans. They’re in the class Merostomata, which means “legs attached to the mouth” because their mouths are literally in the center of all 10 legs. Horseshoe crabs also have 10 eyes—a pair of compound eyes on the front shell and more photo receptors along their tail. Horseshoe crabs’ bodies are divided into three segments. The head, or prosoma, houses most vital organs, including the heart and the brain. The middle segment, the opisthosoma, contains the back gills and ridges that help with movement. The third segment is the tail, or telson, which looks like a big stinger but is used to help these animals flip if overturned and acts as a rudder as they move across the ocean floor. Horseshoe crabs molt their distinctive shells about 17 times before reaching maturity at age 10.