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Horseshoe Crabs: Merostomata


Horseshoe crabs live just offshore along the coast or in salty estuaries (EHS-chew-air-eez), the wide parts at the lower ends of rivers, where the river meets the sea. They prefer coves and bays, which are small inlets of the sea, or wetland, meaning land that is covered with shallow water or that has very wet soil. The crabs like clean, sandy bottoms or muddy bottoms protected from strong wave action. Adults move onshore to mate on beaches at night.


Despite their name, horseshoe crabs are not related to true crabs. Instead, they are related to sea spiders and true spiders and their relatives. The Merostomata are among the oldest living animals, with fossils (FAH-suhls), the remains of ancient animals, dating back 350 million years. Horseshoe crabs are the only living members of the subclass, which includes the now extinct giant water scorpions, or giant sea scorpions. They are called "living fossils" because they have changed very little over millions of years. Some 290 million years ago they were plentiful on the bottoms of ancient seas, as well as in brackish, or salty, waters and freshwaters. The sea scorpions, also called eurypterids, were similar to horseshoe crabs but thinner and longer and with a smaller shell and an abdomen divided into many segments. They were the largest arthropods ever to have roamed the earth. One species measured 9 feet (2.7 meters) in length. Giant sea scorpions probably ate fish and other giant sea scorpions.

Additional topics

Animal Life ResourceInsects and SpidersHorseshoe Crabs: Merostomata - Physical Characteristics, Habitat, Diet, Behavior And Reproduction, Horseshoe Crabs And People, Horseshoe Crab (limulus Polyphemus): Species Account - GEOGRAPHIC RANGE, CONSERVATION STATUS