Horseshoe crab
family of arthropods
Horseshoe crabs are arthropods of the family Limulidae and the only surviving xiphosurans. Despite their name, they are not crabs or even crustaceans; they are chelicerates, more closely related to arachnids like spiders, ticks, and scorpions. The body of a horseshoe crab is divided into three main parts: the cephalothorax, abdomen, and telson. The largest of these, the cephalothorax, houses most of the animal's eyes, limbs, and internal organs. It is also where the animal gets its name, as its shape somewhat resembles that of a horseshoe. Horseshoe crabs have been described as "living fossils", having changed little since they first appeared in the Triassic around 250 million years ago, and similar-looking fossil xiphosurans extend back to the Ordovician around 445 million years ago.
Only four species of horseshoe crab are extant today, the Atlantic horseshoe crab (Limulus polyphemus), native to the eastern coasts of Mexico and the United States, as well as the mangrove horseshoe crab (Carcinoscorpius rotundicauda), tri-spine horseshoe crab (Tachypleus tridentatus) and Indo-Pacific horseshoe crab (Tachypleus gigas), which are native to South, South East, and East Asia.
Most horseshoe crabs are marine, though the mangrove horseshoe crab is often found in brackish water, and the Atlantic horseshoe crab is resident in brackish estuarine ecosystems, such as the Delaware and Chesapeake bays. Additionally, certain extinct species transitioned to living solely in freshwater. Horseshoe crabs primarily live at the water's bottom but they can swim if needed.
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This entry incorporates text from “Horseshoe crab” on English Wikipedia. Contributors are listed in the page history. Text is available under the Creative Commons Attribution-ShareAlike 4.0 License. Selected authority identifiers and statements are retrieved from Wikidata under CC0; their references and qualifiers remain part of the verification path.