The Atlantic Horseshoe Crab (Limulus polyphemus)

Taxonomic ClassificationKingdom: Animalia

Phylum: Arthropoda

Sub-phylum: Cheilcerata

Class: Merostomata

Order: Xiphosurida

Family: Limulidae

Genus: Limulus

Species: L. polyphemus

Description and BehavoirThe Atlantic Horseshoe Crab (Limulus polyphemus) is a strange creature that is usually found on the eastern part of the North American coast. They are considered living fossils that have been virtually unchanged in form for over 200 million years. They can range in color from dark brown to a pale grey. Although it is thought to be a type of crab it is actually more closely related to spiders and scorpions. On average they can grow to about 24 inches long and 12 inches wide with females of this species being larger than the males. The tail of these animal may look slightly menacing but in reality the spine-like tail is not used as a weapon instead it is used to help turn themselves over when upside down and used a rudder when moving along the bottom of the ocean. Under their protective spiny horseshoe shaped shell they have 6 pairs of appendages 5 pairs of legs that have small claws attached to manually break down food and to help place food in their mouths and pair chelicerae which are basically modified pincer like claws. Over the course of their lives horseshoe crabs molt about 16 to 17 times in their life time until they reach adult size. Horseshoe crabs are divided into three regions the upper horseshoe shaped part the prosoma, the spiky rear region the opithosoma and the tail region called the telson.

Habitat and FeedingFound on the eastern part of the North American coast, from approximately Maine to Mexico. They are tolerant of a broad range of conditions because of this they are considered to be very sturdy creatures.They spend the winter in deep waters.When temperature rises and sunlight penetrates deeper they migrate their way to the sandy shores to spawn.

They feed at night by digging around on the ocean floor their prey is primary mollusks, crustaceans and worms. When feeding it uses its chelicerae to grasp its prey and uses it legs to crush its prey. Horseshoe carbs have centrally located mouth under the shell makes it very easy for Horseshoe crabs to eat.

Reproduction and PopulationWhen Temperature and sunlight conditions are right horseshoe crabs gather on shore lines to mate. Male horseshoe crabs usually outnumber female horseshoe crabs on spawning beaches (Shuster, 1958), this causes male-male based competition for mates.

Males use their modified, claw-like pedipalps to grasp onto females’ terminal spines (Brockmann and Penn, 1992). The male continues to hold on to the female sometimes for days until spawning occurs. Females dug a small cavity under their body in an inter tidal zone so male sperm in the water gets brought to the eggs so fertilization occurs. Females are able lay over 90,000 eggs in a single season.

Before 1960 horseshoe crabs were severely over fished because they were used as fertilizer and used as food for eel farming. In the same period horseshoe crabs were being used in the medical field due to their blood containing certain unique properties. Because of this there numbers were declining at a concerning rate. After 1960 horseshoe carbs were no longer being used as animal feed so the populations began to recover.

Predators Horseshoe crabs have many predators such as sharks, birds, eels, sea turtles and humans. The horseshoe crab is considered a keystone species because many animal depend on it being a part of there diet for survival. Setting aside their hard spiky shell they are pretty easy prey since they have no offensive mechanisms. In addition to the animal itself its eggs are also highly sought after by migrating birds such as the red knot during the horseshoe crabs spawning season.