Horseshoe Crab

Reviewed by: BD Editors

An Atlantic horseshoe crab on the beach
Horseshoe crabs are examples of living fossils!

Kingdom Animalia
Phylum Arthropoda
Class Merostomata
Order Xiphosura
Family Limulidae
Genus 3
Species 4
Niche Carnivorous
Length Females: 8 – 19 in (46 – 48 cm), Males: 14 to 15 in (36 to 38 cm)
Weight 2.2 to 9.9 pounds (1 – 4.5 kg)
Lifespan 20+ years
Social Structure Meet in large groups during breeding
Status 1 endangered, 1 vulnerable, and 2 not listed yet
Natural Habitat Ocean floor
Average Litter Size Thousands
Main food item Clams and marine worms
Main threats Habitat loss and overharvesting

The Basics

The Horseshoe Crab is a prehistoric animal that belongs to a family of arthropods that have been around for more than 400 million years. This makes them even older than the dinosaurs! Their common name is misleading as horseshoe crabs are not true crabs nor are they crustaceans. While they look like prehistoric crabs, horseshoe crabs belong to the order Xiphosura and are more closely related to scorpions and spiders. They live in marine and brackish waters.Horseshoe crabs are carnivores that eat crustaceans, mollusks, and marine worms.

There are four species of horseshoe crabs that can be divided into the Western horseshoe crabs, the American horseshoe crab (Limulus polyphemus), and the Eastern horseshoe crabs, tri-spine horseshoe crab (Tachypleus tridentatus), coastal horseshoe crab (Tachypleus gigas), and the mangrove horseshoe crab (Carcinoscorpius rotundicauda). Horseshoe crabs are only found in North America, Central America, and South-East Asia. Limulus polyphemus can be found along the Gulf of Mexico and the Atlantic Ocean. The three Eastern horseshoe crab species have a wide range including along the coast of Japan, China, Java, Sumatra, Philippines, and India.

The American horseshoe crab is listed as vulnerable and the tri-spine horseshoe crab is listed as endangered. The other two species will soon be found on the IUCN’s red list.

Horseshoe Crab Anatomy

The horseshoe crab gets its name from the shape of its head, which is rounded in shape just like the shoe of a horse. Its head, known as the prosoma, is the largest part of its body and contains most of the nervous systems and vital biological organs including the brain, heart, nervous system, mouth, and several glands. The hard plate of its exoskeleton provides a protective layer for organs.

The abdomen, also known as the opisthosoma, is the next part of its body. It is triangular in shape and has spines along the side and a ridge in the center. The spines can move and help protect the horseshoe crab. On the underside of the abdomen, you will find the gills which are used for breathing.

The final section of a horseshoe crabs body is called the telson, or tail. The telson is long and pointed, and although it looks like it could cause harm it is not dangerous at all. Horsehoe crabs mainly use their tail to help right themselves if they end up on their backs. The horseshoe crab has ten legs, which it uses to move across the ocean floor. Females are generally up to a third larger in size, from the head to the tail, than the males.

A group of horseshoe crabs on the shore breeding
Horseshoe crabs breed on the beach.

Life-History

Horseshoe crabs breed in the late spring and early summer when the adults travel from the deep ocean water to the beach to reproduce. The males arrive first and wait for the females – who release pheromones (natural chemicals) when they come ashore. These pheromones signal to the males that they are there and its time to mate. The females dig small nests and deposit their eggs before the males fertilize them. This is usually done at night during high tides. The breeding pair can repeat the process multiple times, depositing tens of thousands of eggs.

Unfortunately, most horseshoe crabs won’t make it to the larval stage, as the eggs are a food source for several birds, fish, and reptiles. If a horseshoe crab egg is one of the lucky ones and avoids being eaten, it hatches after approximately two weeks. The larva looks a lot like the adult horseshoe crab except for the fact that it doesn’t have a tail. Once it has hatched, the larva travels into the water and settle on the sandy floor of a tidal flat for approximately one year. As they continue to develop, they move into deeper water where they consume the same diet as the adult crabs.

It takes a juvenile horseshoe crab approximately ten years to reach adulthood. Over this time, the horseshoe crab will molt 16 or 17 times. This process involves shedding their small exoskeletons and growing larger shells. Once they reach adulthood, they migrate back to the beaches in the spring ready to breed. Horseshoe crabs can live for more than 20 years.

Horseshoe crabs in the water coming in to breed
Adult horseshoe crabs migrate from the deep water to the beach.

Fun Facts about the Horseshoe Crab!

Horseshoe crabs are ancient animals that have existed for millions of years on earth. These animals have been studied intensively and have several biological adaptations that have helped them survive on the earth for so long. Let’s take a closer look!

Biomedical Bleeding

Horseshoe crabs are important to the biomedical industry because their blue blood contains a product known as Limulus amebocyte lysate (LAL), which is used to ensure that biomedical equipment and vaccines are not contaminated with bacteria. This is so important to the industry, that each year the US biomedical industry harvests blood from approximately half a million horseshoe crabs for use in pharmaceuticals.

After the crabs are collected, up to 30% of their blood is harvested before the animals are returned to the ocean floor. While the horseshoe crabs are returned alive and well to the sea, research has shown that this harvesting of their blood can have short-term impacts on their behavior. The scientists demonstrate that the harvesting process causes the crabs to become disorientated and that it could be having a negative impact on their reproduction!

Horseshoe Crabs Have Nine Eyes

Horseshoe crabs have nine eyes and their vision has been the focus of much physiological research. Two large compound eyes are found on the prosoma, one on either side. These eyes have a monochromatic vision and are primarily used for finding mates.

They also have five simple eyes on the carapace – two median eyes, one endoparietal eye, and two rudimentary lateral eyes. These eyes are probably important during embryonic and larval development, as even unhatched embryos appear to be able to detect light levels from within their eggs. The median eyes are sensitive to visible and ultraviolet light. The rudimentary lateral eyes are photoreceptors and they become functional just before the egg hatches. The remaining two eyes are found on the underside of the crab. These two ventral eyes are found near its mouth and are thought to help orientate the animal when it is swimming. As well as its nine eyes, the horseshoe crab also has a number of light-sensing organs along the length of its tail.

The horseshoe crabs eyes have been studied so extensively that one researcher, Dr. Hartline, received the 1967 Nobel Prize in Physiology or Medicine for his discovery of the optical nerve network using horseshoe crabs as one of his models.

A horseshoe crab fossil
Horseshoe crabs are examples of living fossils.

Living Fossils

Horseshoe crabs are ‘living fossils’. They have been around since before the time of the dinosaurs! For over 400 million years horseshoe crabs have remained relatively unchanged and survived mass extinctions.

The terms living fossil is used by scientists to describe animals that have existed for millions of years and have few or no living relatives. Most of these have unusual traits that make them seem like they come from another world and they offer a rare glimpse into how life used to be on Earth. Other animals that are considered to be living fossils include the komodo dragon, the hagfish, the koala, and the pig-nosed turtle.

Cite This Article

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Biologydictionary.net Editors. “Horseshoe Crab.” Biology Dictionary, Biologydictionary.net, 12 Oct. 2020, https://biologydictionary.net/horseshoe-crab/.
Biologydictionary.net Editors. (2020, October 12). Horseshoe Crab. Retrieved from https://biologydictionary.net/horseshoe-crab/
Biologydictionary.net Editors. “Horseshoe Crab.” Biology Dictionary. Biologydictionary.net, October 12, 2020. https://biologydictionary.net/horseshoe-crab/.

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