Horseshoe crab


The horseshoe crab or king crab is a very interesting invertebrate living in oceans all over the world. Although they are called crabs, they are actually not crabs for multiple reasons. First of all, crabs have two pairs of antennae, whereas horseshoe crabs have none. Crabs also have jaws which horseshoe crabs also lack. Finally, crabs have five pairs (ten) legs, and horseshoe crabs have seven pairs (fourteen) legs. One pair of these legs is smaller than the others and is called chilaria.  Since horseshoe crabs are not actually crabs, they are classified in the class merostomata, which is closely related to the order arachnid.

Horseshoe crabs can be visually divided into three sections. The first one (the one farthest to the left in the picture up above) is called the prosoma. The middle section is the abdomen, and the third one (on the right in the picture) is its tail, or telson.

Male horseshoe crabs grow up to only 8 inches (21 cm) wide at the prosoma. Females are much larger at up to 14 inches (36 cm). The largest horseshoe crab species is the only one living in the United States which can grow up to two feet (60 cm) long! This species also weighs up to 10 pounds (4.5 kg).

Horseshoe crabs eat worms, crabs, and mollusks with thin shells which they seize with their first pair of legs. They then transfer the food to the other pairs of legs which tear up the food and transport it to the mouth, which the feet surround.


Starting around age nine, in the spring or summer, female horseshoe crabs scoop out several depressions in the sand and lay 200-300 eggs in each. While they are doing this, one or more males are following them. The males then fertilize the eggs. After several weeks, the .2 inch (5 mm) babies hatch from their eggs. After this, the babies molt up to 16 times before becoming an adult.

Young horseshoe crabs are known to be eaten by fish and birds and adults are sometimes eaten by endangered loggerhead sea turtles. If a horseshoe crab is lucky enough to avoid these predators, it can live for up to 20 years.

Horseshoe crabs are vital for human medicine in a very unusual way. Because horseshoe crab blood turns blue when it comes in contact with a lot of oxygen, medicine companies use this blood to test how sterile their medicine is. This test is called the LAL test. There is currently no known substitute for the LAL test.

Don’t forget to check out the coloring sheet under sources and scroll down to the bottom to leave your guess about what the next animal is!



Photo credits:

  • Horseshoe crab: public domain
  • mystery animal: Richard Ling

2 Responses

  1. Kurt
    Kurt at |

    Bicolor parrotfish

  2. Sharon Madson
    Sharon Madson at |

    This is very interesting, especially about the medical tests. Thank you for all this information. I am not guessing again as I was so wrong last time!

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