Horseshoe Crab

(Limulus polyphemus)

Horseshoe crabs are amazing creatures that have been around for a very long time. In fact, some form of horseshoe crab has been in existence since before the dinosaurs! The name “crab” is misleading as they are not crustaceans at all but are actually a close relative to arachnids, or the spider family. Far larger than any spider, a female horseshoe crab can be up to 2 feet long and weigh more than 10 pounds! However, not all horseshoe crabs are this large; on average horseshoe crabs are between 12 and 18 inches long and 8 to 10 inches wide. At first glance, you might only see the smooth, tan outer shell of the horseshoe crab, but if you look underneath this armor, you can see the body of the animal. The head and thorax of the horseshoe crab are fused together to form what is called the cephalothorax. Along the center of the body are 5 pairs of walking legs, each ending in a claw. The first four of these walking legs are covered with bristles and are also used in feeding. The last two walking legs are shaped like small paddles and are used for cleaning out the 5 pairs of gills located at the rear of the body. Horseshoe crab gills are called book gills because they are folded over many times and resemble pages of a book. At the front of the row of walking legs are the chelicerae, which are used in feeding. At the rear of the body is a long, strong, spine-like tail called the telson. This tail is not used as a weapon, but instead aids in the movement of the crab. It is mainly used as a rudder as the horseshoe crab swims, but if the horseshoe crab becomes inverted it can also be used it to right itself. Horseshoe crabs have ten “eyes.” The 2 main compound eyes are visible as bumps or ridges on top of the carapace. Each of these two eyes is made up of multiple cells that aid in vision and can adjust to see in the day and night. The remaining 8 smaller “eyes,” some of which can detect ultraviolet light, are located along the top of the carapace, under the body and on the telson.
As the horseshoe crab grows and increases in size, it molts or sheds its shell. The old shell is forced away by the crab, which then simply walks out if it. Incredibly, each time a crab molts it can increase in size by over 25%. They usually stop molting when they reach full size at about 12 to 20 inches long or around 8 to 11 years old. Females are typically larger than the males because they molt at least once more than males do. During the spring months, horseshoe crabs move in to shallower water to mate and lay eggs. Mating activity occurs at night during the late spring and early summer and peaks during the full and new moons. Females come up on the beach first and deposit greenish colored eggs, up to 20,000 a night, in the sand. Males then follow and fertilize the eggs.
There are four different species of horseshoe crabs in the world. The Atlantic horseshoe crab is found all along the intertidal and subtidal zones of the east coast. Horseshoe crabs are nocturnal, scavenging for food like worms and mollusks in the sand and mud during the night. Interestingly, horseshoe crabs must be walking in order to grind their food. As the animal walks, the bristles on their legs grind up the food. This “chewed” food is then put into the mouth by the chelicerae and then travels to the stomach.
The blood of horseshoe crabs is pretty amazing scientifically, and it’s blue! Due to a high concentration of a copper-based oxygen carrying substance called hemocyanin, the blood of horseshoe crabs turns blue when exposed to air. In humans, the oxygen carrying substance is an iron-based substance called hemoglobin and becomes a reddish color after exposure to air. Horseshoe crab blood is very important in medical research because it is an excellent detector of toxins and harmful bacteria and can be used to spot illness in humans. Horseshoe crab blood contains a protein called Limulus Amebocyte Lysate (LAL). Scientists extract this from the blood, add it to the human blood sample they wish to test, and if the sample clots, the human is infected. Removing the blood does not injure the crab and after a small amount of blood is extracted, the crab is released.
If you spot a horseshoe crab up on the beach, you are pretty lucky as they move fairly slow and it will be easy to get a good look at them. Aside from their pinching legs and pointy tail, horseshoe crabs are harmless. The proper way to handle or pick up a horseshoe crab is by grasping the shell along the sides, being careful of the legs and tail. It can be harmful to the crab to handle it by the tail. Sometimes crabs are overturned in rough surf and are laying upside-down with their underside exposed. This is dangerous for them as their protection from predators is their shell, so if you see a crab that is upside-down with its underside exposed, flip it over.