Horseshoe Crab

Update of post from February 22, 2014


The horseshoe crab is a large invertebrate living off the coasts of North America. Although they are called crabs, these animals are actually arachnids, a group which includes spiders, scorpions, and ticks, but not crabs.

These creatures have been called living fossils because of how much they resemble actual fossils. Their body is divided into three sections: the head, the abdomen, and the tail. If you want to get technical you can call them the prosoma, opisthosoma, and telson, but I prefer the easier names. This animal’s name came from the rounded, u-shaped head it has that looks like a horse’s foot. The main uses for this crab’s tail are to flip itself right side up in case it gets turned on its back and to steer when swimming.

Under the horseshoe crab’s head, it has six pairs of appendages. This first pair is used for getting food to its mouth while the other five are used for getting around. Surprisingly, this creature has nine eyes! One of them can be seen relatively well in the picture above, and there is a second one in the same place on the other side of its body. It also has five smaller ones on top of its head, and the other two are actually under its body!


Female horseshoe crabs can be up to 33% larger than the males. The females can be about 19 inches (48 cm) across at their widest point while males are closer to 15 inches (38 cm). They are probably lighter than you would think for their size, reaching only four pounds (1.8 kg).


Horseshoe crabs are active mostly at night, and their activity peaks during full and new moons. They feed mostly on invertebrates, such as worms, clams, and crustaceans, but they will also feed on algae.

Habitat and range

Horseshoe crabs live mostly on the North American coasts of the Atlantic Ocean and Caribbean Sea. Here they are found as far south as Nova Scotia in Canada and as far south as the Yucatan Peninsula in Mexico.

While their North American population are the most known, these creatures also live on the Indian Ocean’s coasts in southeast Asia. No matter which continent they live on horseshoe crabs live mostly on the ocean floor. They typically occur around 17 feet (5.2 m) deep, but they have been observed as far as 100 feet (30 m) below the surface.

Status and threats

The horseshoe crab is classified as Near Threatened on the IUCN Redlist. Their main threats occur when they are in their egg and larva form. At this time shore birds eat them frequently, but sea turtles will also prey on them. As adults, they are eaten by sharks, gulls, and sea turtles. They are also used by humans for various reasons.

Use for humans

Different parts of horseshoe crabs are used for bait and fertilizer, but their main value to humans comes from their blood. It has a special quality that it clots when it comes in contact with toxins released by bacteria. This special property is used to test whether drugs and medical fluids are bacteria free.

Normally they are caught when breeding, and about one-third of their blood is taken out. They are then released back into the ocean. While this is normally relatively harmless to the horseshoe crab, around 30,000 crabs, or 10-15 percent of crabs used for this, die because of it.


Around May and June, the adults migrate the short distances from the deepest waters in their range to coastal beaches. The males usually arrive first, and when the females arrive, their pheromones help the males find them.

Their breeding time occurs at night, normally during the new moon or full moon, both of these are high tides. The females dig small nests in the sand and deposit their eggs as the males fertilize them. The females can lay up to 90,000 eggs each year, but they are laid in multiple nests. When the tides go back down, the crabs return to the ocean.

These eggs hatch after about five weeks, but the exact time goes by the temperature. However, they will remain buried in the sand until the next high tide. Then they will move to the ocean where they will float around in the water. About a week later, they molt for the first time and start to look more like the adults. It is then that they move down to the bottom of the ocean to live.

These creatures are rather slow growing. They become mature between the age of 9 and 12. The exact age of horseshoe crabs is hard to tell, but they are thought to live between 20 and 40 years old.

Don’t forget to scroll down and comment your guess about what the next animal is!


Photo credits:

  • Horseshoe crab – Public domain
  • Mystery animal – Wikipedia user: Nhobgood
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