A relict species whose roots date back to the middle Silurian Era
These very distinctive animals were once much more widespread on earth. They belong to order Xiphosura , of which four species have survived to the present day. The most common of these is actually the Horseshoe crab, which is found in Atlantic and Pacific waters. These animals are used in fisheries and in the biomedical industry. They are also an important part of the Delaware Bay ecosystem, where the eggs they lay are one of the staple foods in the diets of the bay's resident birds.
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Despite their appearance, they are more related to spiders, ticks and scorpions than to crabs.
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Limulus is a genus of horseshoe crab, with one extant species, the Atlantic horseshoe crab (Limulus polyphemus). Limulus first appeared on earth about 425 million years ago during the Silurian period.
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They live in the Atlantic Ocean near the coast of North and Central America and near the Asian coasts in the Pacific.
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Limulus living today are saltwater animals. Fossil specimens also inhabited freshwater enviroments.
Currently living horseshoe crabs evolved 244 million years ago and are now considered living fossils as they didn't change a lot since then.
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The chitinous armor of the horseshoe crabs is bisected, similar to that of scorpions.
It consists of an anterior part or prosoma and a posterior part or opisthosoma. Prosoma of horseshoe crabs consists of 6-7 segments, while opisthosoma consists of 10 segments. The posterior-most division of the body of animal is called telson which is a long spike.
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They swim mostly upside down.
When swimming, they tilt at an angle of about 30° to the horizontal plane and move at a speed of about 10-15 cm/s.
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The horseshoe tail spike helps them swim and acts as a rudder. It is also helpful in burying the animal.
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Horseshoe crab can grow up to 60 cm in length.
Females are approximately 20% - 30% larger than males.
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An adult animal weighs up to 4.5 kilograms.
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During the breeding season, horseshoe crabs migrate to shallow coastal waters.
The smaller male clings to the back of the larger female with specialized front claws and fertilizes the eggs, which are then deposited in the sand.
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Horseshoe crab has 10 eyes, including a pair of multi-lens eyes.
There are 1000 lenses in multi-lens eyes. In addition to the eyes, horseshoe crabs also have photoreceptors on the telson. They can see perfectly in very dark conditions because their cones and rods in the eyes are the largest of all animals, 100 times larger than in humans.
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They feed on clams, polychaetes, small fish and decayed organic debris.
The mouth opening is located on the underside of the body between the legs. They move the food into the mouth with chelicerae—a pair of small appendages.
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Like many crustaceans, the blood of the horseshoe crab is blue.
Hemocyanin is responsible for the transport of oxygen in these animals.
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Horseshoe crabs breathe using book gills.
This organ is made of gills covered with large surface area for absorbing oxygen. It's located in the rear part of the animal body between the legs and the telson.
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The blood of the horseshoe crab shows a very interesting feature that is used in the biomedical industry.
Due to the environment in which these animals live, which is full of Gram-negative bacteria, their immune system is sensitive to the endotoxins of these microbes. When exposed to endotoxins, horseshoe crab blood coagulates, leading to the formation of a clot. That is why their blood is harvested for microorganism detection tests production. Those tests allow even small amounts of bacteria to be detected in a short time.
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Blood is being extracted from living animals.
It is estimated that between 10% and 30% of the animals do not survive the process. Approximately 500,000 horseshoe crabs are harvested for blood annually.
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They are caught and used as bait to catch eels.
Scientists have been able to isolate a substance from the horseshoe crabs organism that is responsible for attracting eels, but it has not been synthesised yet.
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The female is able to lay 60 to 120 thousand eggs at a time.
Spawning takes place in late spring and early summer. In addition to the male attached to the female, there may be other males around the copulating pair who are also involved in egg fertilization.
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Horseshoe crabs are animals that are most active at night.
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The blood of these arthropod is sterile, which means it has no microorganisms in it.
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Bacterial tests made from their blood are used on the International Space Station to check for the presence of unwanted microorganisms.
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The horseshoe crab is currently not an endangered species.
However, the capture and destruction of their habitats raises some concerns about the future of these animals.
