horseshoe crab
noun
: any of several closely related marine arthropods (order Xiphosura and class Merostomata) with a broad crescentic cephalothorax, six pairs of legs, and a spikelike tail
especially : one (Limulus polyphemus) of coastal waters from Maine to the Yucatán Peninsula
Note: The horseshoe crab is often said to have five pairs of legs since the first pair is greatly reduced in size and used mainly for feeding. All the legs of the horseshoe crab end in claws except for the last pair of legs which have leaflike expansions used mainly for pushing.
… a horseshoe crab … is a Swiss army knife of jointed limbs—for walking, swimming, grasping, shredding, and defending. 
Carl Zimmer
Carl Zimmer … the horseshoe crab has changed little since an ancestor was fossilized 150 million years ago … National Geographic
National Geographic called also king crab
Example Sentences
Recent Examples on the Web Wellfleet Bay Wildlife Sanctuary, which conducts surveys of the horseshoe crab population on Cape Cod. David Abel, BostonGlobe.com, 17 July 2022 Mass Audubon’s science coordinator Mark Faherty examined a horseshoe crab in Pleasant Bay in Orleans. David Abel, BostonGlobe.com, 17 July 2022 The mayor first realized the rise in such tourism as the calendar approached horseshoe crab-mating season, typically between May and June. Joseph Edelen, Baltimore Sun, 4 July 2022 The similarity between Jimmy Carr and Milhouse petting the horseshoe crab is uncanny. Bethy Squires, Vulture, 1 Jan. 2022 The best comparison is the 2003 SARS outbreak, which was caused by a close cousin of the virus that causes COVID-19 and eventually traced back to a single population of horseshoe crab bats. Erin Schumaker, ABC News, 8 Apr. 2021 Researchers pointed to the 2003 SARS outbreak, which was caused by a close cousin of the virus that causes COVID-19 and eventually traced back to a single population of horseshoe crab bats. Erin Schumaker, ABC News, 30 Mar. 2021 The race to a vaccine: The horseshoe crab's fluorescent blue blood is its best line of defense against toxins. Ashley Shaffer, USA TODAY, 22 Sep. 2020 The story of the horseshoe crab is a lesson in humility. Tim Prudente, baltimoresun.com, 31 Dec. 2020 See More
David Abel, BostonGlobe.com, 17 July 2022 Mass Audubon’s science coordinator Mark Faherty examined a horseshoe crab in Pleasant Bay in Orleans. David Abel, BostonGlobe.com, 17 July 2022 The mayor first realized the rise in such tourism as the calendar approached horseshoe crab-mating season, typically between May and June. Joseph Edelen, Baltimore Sun, 4 July 2022 The similarity between Jimmy Carr and Milhouse petting the horseshoe crab is uncanny. Bethy Squires, Vulture, 1 Jan. 2022 The best comparison is the 2003 SARS outbreak, which was caused by a close cousin of the virus that causes COVID-19 and eventually traced back to a single population of horseshoe crab bats. Erin Schumaker, ABC News, 8 Apr. 2021 Researchers pointed to the 2003 SARS outbreak, which was caused by a close cousin of the virus that causes COVID-19 and eventually traced back to a single population of horseshoe crab bats. Erin Schumaker, ABC News, 30 Mar. 2021 The race to a vaccine: The horseshoe crab's fluorescent blue blood is its best line of defense against toxins. Ashley Shaffer, USA TODAY, 22 Sep. 2020 The story of the horseshoe crab is a lesson in humility. Tim Prudente, baltimoresun.com, 31 Dec. 2020 See More Word History
First Known Use
1797, in the meaning defined above