20+ Interesting Facts About Horseshoe Crabs

1. Horseshoe crabs are marine arthropods currently present in Asia and North America. Their name is a misnomer because they are not related to crabs. The closest relatives of horseshoe crabs are scorpions, spiders, and mites.
2. Horseshoe crabs are at least 450 million years old. They survived three extinction events during this period, particularly the Great Dying that exterminated nearly 96% of all marine species 250 million years ago. Reasons behind their survival include their tolerance of various habitats, feeding any available organic matter, unique blood that fights bacteria and heals wounds, and their ability to live in areas with low oxygen levels. Trilobites, their close relatives, had 20,000 species, but all vanished during the Great Dying. (Source)
3. Scientists found the oldest fossil of horseshoe crab in 2008 from Canada. It is 445 million years old, nearly 100 million years older than any previous fossil. This discovery proves the status of horseshoe crabs as “living fossils” because they have almost maintained their form for nearly half a billion years. (Source)
4. There are four living species of horseshoe crabs. These include Atlantic, Japanese (or tri-spine), mangrove, and coastal horseshoe crabs. The initial one is limited to North America, while the remaining three species are present in Asia, from Japan to India.
5. The largest horseshoe crab species is Atlantic horseshoe crab, having a length of up to 60 cm. The mangrove horseshoe crab is the smallest species with a diameter of 15 cm. (Source) (Source)
6. Horseshoe crabs have three main parts: prosoma (cephalothorax), opisthosoma (abdomen), and a long telson (tail-spine). The prosoma is hoof-shaped from where they get their initial name (horseshoe). These invertebrates have hard exoskeletons that protect their internal organs.
7. Horseshoe crabs have six pairs of legs. Various leg pairs perform separate functions, including capturing prey, tearing food (due to the absence of jaws and teeth), moving food inside the mouth, and walking. 

   

   Six pairs of legs in horseshoe crabs have different functions

   
   
8. Adult male horseshoe crabs develop the first pair of their legs into pedipalps that resemble boxing gloves. They use this hooklike structure to clasp females during the breeding season.
9. Horseshoe crabs have ten eyes with distinctive functions. Two lateral compound eyes have 1,000 receptors each, while their cones and rods are nearly 100 times larger than those in human eyes. Their function is to find potential mates. Five additional eyes on the top of the prosoma detect UV light from the sun and reflected light from the moon. They support the animal to follow the lunar cycle. The function of two ventral eyes near the mouth is unknown. Photoreceptors on the telson (tail) act as the last eye and distinguish between light and darkness.
10. Sea scorpions, extinct relatives of horseshoe crabs, appeared 470 million years ago and died out 250 million years ago. Research proves that the eyes of horseshoe crabs are similar to those of sea scorpions. Unlike compound eyes of insects with simple lenses, complex eyes in horseshoe crabs perceive contours clearly and assist in better sight despite low visibility underwater. Horseshoe crabs are the only living animals with these unique eyes that initially formed 400 million years ago. (Source)
11. Haldan Keffer Hartline, an American physiologist, won a Nobel Prize in 1967 for analyzing the process of primary signals from visual cells. His research involved the eyes of horseshoe crabs. (Source)
12. Horseshoe crabs forage at night. Their food includes worms, algae, crustaceans, small mollusks, and dead fish. Adult animals eat mollusks, while smaller ones prefer worms and other soft food. Their favored habitat is estuarine waters due to the abundance of their food.
13. Horseshoe crabs breed during spring and summer. It usually happens during high spring tides after sunset. Female lays nearly 20,000 eggs a night in a clutch of 4,000 eggs and around 100,000 eggs each season. One or more males cover these eggs with sperm. However, most eggs do not hatch and become food for migratory birds, sea turtles, and fish. Horseshoe crabs remain solitary except during the breeding season.
14. Horseshoe crabs can survive on land for up to 4 days if their gills remain moist. Animals stuck on the beach during the breeding season bury themselves in the sand to conserve water before the arrival of a tide.
15. The color of horseshoe crab’s blood is blue due to hemocyanin, a copper-based respiratory pigment. Scientists use this blood to produce LAL (limulus amebocyte lysate) that detects endotoxins in intravenous drugs. These toxic bacterial substances can prove fatal for humans. The accidental discovery of this method in the 1950s and 1960s replaced the rabbit pyrogen test. The blood harvesting process drains up to 30% of blood from these animals, while nearly 30% of crabs die or face complications after this procedure. It negatively impacts migratory shorebirds that rely on the eggs of horseshoe crabs to complete their journey. The recent approval of synthetic compounds in various countries has slightly reduced the burden on horseshoe crabs. (Source)
16. Three horseshoe crab species in Asia contribute only 20% to bacterial endotoxin test (BET). However, their mortality rate is 100% compared to less than 30% in Atlantic horseshoe crabs. After taking blood from these crabs, they are used as food, fertilizer, or traditional Chinese medicines. They are present in over 15 Asian countries, and many protect them due to their continuous falling numbers. (Source)
17. Two Asian horseshoe crab species (mangrove and Japanese) are unsafe to consume due to tetrodotoxin, a neurotoxin. The highest amount of this poison is in their hemolymph (a blood-like fluid). (Source)
18. The predators of horseshoe crabs are large marine animals such as sea turtles, sharks, and gulls. These arachnids use their hard shells to defend against their enemies.
19. Young horseshoe crabs grow with molting, while each molt increases their length by around 25%. They reach maturity between 9 to 12 years, and females gain more length due to one or two additional molting than males. These animals usually live up to 25 years.
20. According to IUCN (the International Union for Conservation of Nature), Atlantic horseshoe crabs are vulnerable, while the status of Japanese horseshoe crabs is endangered. The other two species are not on the red list due to lack of information. The reasons behind the continuous decline of the horseshoe population include habitat loss and their usage as food, fishing bait (for eels and sea snails), and biomedical applications.
21. It is a misconception that pointed tails in horseshoe crabs act as stingers. The primary function of a telson is to work as a rudder while swimming and to overturn the animal if it becomes upside down by a wave. However, becoming upside down is not dangerous, and they can swim this way.
22. Picking a horseshow crab with its tail can harm the animal. Use both hands to lift it from both sides of the prosoma. These marine animals are safe for humans as they do not bite or sting.