Horseshoe crab characteristics, habitat, feeding, reproduction

Alexander Pearson

The horseshoe crab (Limulus polyphemus) is an arthropod belonging to the Limulidae family. This species is more closely related to ticks, spiders, and scorpions than to crabs..

It is characterized by having nine eyes. On each side of the prostoma there is a large compound eye with monochrome vision. In the shell there are five simple eyes and another two in the lower part of the body, just in front of the mouth. Despite this, this species has an underdeveloped sense of sight..

As for your blood, it contains the protein hemocyanin, which is responsible for transporting oxygen in the extracellular fluid. This compound is made up of high concentrations of copper, so when it is oxygenated it turns blue and when it does not contain oxygen it is colorless..

The Limulus polyphemus it is distributed along the Atlantic coast of the United States and in the Gulf of Mexico. In these regions it lives in shallow coastal areas, such as mangroves and estuaries. However, they can live in deeper areas, less than 30 meters..

Article index

  • 1 Evolution
  • 2 Features
    • 2.1 Size
    • 2.2 Body
    • 2.3 Carapace
  • 3 Habitat and distribution
    • 3.1 Distribution
    • 3.2 Habitat
  • 4 Taxonomy
  • 5 Conservation status
    • 5.1 Threats
  • 6 Food
    • 6.1 Prey capture and digestive process
  • 7 Playback
    • 7.1 Mating
    • 7.2 Development of hatchlings
  • 8 Behavior
    • 8.1 Breeding behaviors
  • 9 References 


Traditionally, the Limulus polyphemus it was grouped together with the extinct eurypterids, within the superclass Merostomata. However, recent studies suggest a relationship between eurypterids and arachnids, leaving Xiphosura as part of the Prosomapoda.

Trilobite fossil. Trilobite_tracks_at_World_Museum_Liverpool.JPG: Rept0n1xderivative work: JMCC1 [CC BY-SA (]

Possibly, the horseshoe crab evolved in the shallow waters of existing seas in the Paleozoic Era, around 570-248 million years ago. This could occur in conjunction with other primitive arthropods, such as trilobites..

As for the four extant species, they constitute the only remaining members of the Xiphosura subclass. This is one of the oldest clades, as far as marine arthropods are concerned.

Recently, researchers identified the Limulus darwini, a species of horseshoe crab that lived in the Upper Jurassic (ca. 148 Ma). This fossil was found in the sediments near the Kcynia Formation, in Poland..

Experts point out that the sample does not have any important morphological differences with the juvenile species that make up the genus Limulus..



In the horseshoe crab, both sexes are similar in appearance. However, the female is usually 25-30% larger than the male. Thus, the adult female can be 60 centimeters long and its body mass can reach up to 5 kilograms..

On the other hand, body dimensions show latitudinal variations. In this way, the largest animals are located towards the center of the range and the smallest ones are at the extremes..

For example, those who live between Cape Cod and Georgia are larger. As for those that live north of Cape Cod and south of Georgia, they are smaller.


The Limulus polyphemus has the body divided into two parts: the prosoma or head and the opistosoma or abdominal region.

The prosome contains the heart and the brain. In addition, it is made up of six pairs of legs. They all have claws except the last pair. The first appendix is ​​used to bring food to the mouth. The four remaining legs are used for locomotion

As for the opistosome, it has six additional pairs of appendages, which are involved in reproduction, respiration, and locomotion. The first pair forms a genital operculum, where the genital pores meet.

Relative to the remaining five pairs, they are modified into a series of overlapping plates, folded into folds, known as book gills. In addition to this respiratory function, the appendages function as paddles in locomotion..

Behind the opistosome is a long spine, called the tail or telson. This is used as a lever to get up, if the horseshoe crab is upside down. Also serves as a tool for digging in the sand.


The shell is shaped like a U or a horseshoe. It is smooth in texture and its color can be from dark brown to greenish gray. Often, this structure is covered with various marine species, such as algae, mollusks, flatworms and barnacles. These can grow so large that they prevent the shield from being seen.

During its development, the Limulus polyphemus sheds the shell periodically. This occurs so that the structure can adjust to the changes that the body has undergone. The new skeleton is flexible, hardening and changing color over time.

Habitat and distribution


The horseshoe crab is found along the entire east Atlantic coast of the United States, from 19 ° N to 42 ° N. Thus, this species ranges from Maine to Florida. In addition, it extends on the east, west and north coast of the Yucatan peninsula, in Mexico..

Infrequently, it is sighted outside the breeding range. However, experts have registered their presence on the Atlantic coast of Canada, Cuba, the Bahamas and west of the Gulf of Mexico, in Veracruz and Texas..


The Limulus polyphemus it can live in shallow coastal areas such as estuaries, lagoons, and mangroves. However, it is also located in deeper areas, more than 200 meters and up to 56 kilometers offshore. In any case, according to experts, this animal is commonly found at depths of less than 30 meters..

Horseshoe crabs can live in brackish water, almost fresh water, up to hypersaline, whose salinity is almost twice that of the sea. However, its optimal growth occurs when the salinity is slightly below that of seawater..

On the other hand, the preference for the water temperature has variations, according to each species. Thus, the populations that live in the Greater Bay of New Hampshire have a greater activity when the temperature is above 10.5 ° C.

As for those that live in the Delaware Bay, they are most active when the water is above 15 ° C.


-Animal Kingdom.

-Subkingdom: Bilateria.

-Superfilum: Ecdysozoa.

-Phylum: Arthropoda.

-Subphylum: Chelicerata.

-Class: Euchelicerata.

-Subclass: Xiphosura.

-Order: Xiphosurida.

-Suborder: Limulina

-Family: Limulidae.

-Gender: Limulus.

-Species: Limulus Polyphemus.

State of conservation


Horseshoe crab populations are declining, mainly due to overexploitation. This situation has caused the IUCN to include this species within the group of animals that are vulnerable to extinction..


Bait for the fishery

The Limulus polyphemus It is commercially hunted to be used as bait in American eel fisheries (Anguilla rostrata), sea snails (Busycon spp.) and in artisanal fishing for red octopus (Mayan octopus).

Use in medicine

Members of this species are used by the biomedical industry for the manufacture of LAL (Limulus Amebocyte Lysate). This is used in the detection of the presence of gram-negative bacteria in implantable medical devices and injectable drugs..

The animal is returned alive to its environment, after having extracted a portion of its blood, however, between 10 and 30% die after its release.

Incidental catch

Historically, horseshoe crabs have been caught incidentally in commercial fisheries, targeting other species. The animal is returned to the water, although when it is caught in the nets, its body can be injured.

These damages can cause death or cause alterations that prevent it from reproducing, among other things incidents.

Climate change

Climate change creates a terrible threat to the coastal habitat, as it causes the rise in sea level. As a consequence of this, the loss of the sandy beach has repercussions on the reproductive process of the Limulus polyphemus, because this area is an optimal spawning area.


The larvae do not feed. It is from the first molt of the first juvenile stage that horseshoe crabs start in this feeding behavior. Thus, the adult feeds on a wide range of benthic invertebrates.

Among its prey are mollusks, small bivalves and polychaete worms, such as those belonging to the genera Nereis, Cerebratulus Y Cistenides.

On the other hand, this species can be a scavenger, by including pieces of dead fish in its diet. Also, eventually, you can eat seaweed.

Prey capture and the digestive process

This arthropod lacks a jaw, so it uses other methods to grind the organisms it is going to ingest. To feed, the horseshoe crab generally digs through sediment to capture its prey..

It does this using its legs, which contain thick, inward-facing bristles. The animal that has been hunted, is transferred to the base of the limbs, where it is crushed.

Subsequently, the first pair of legs brings the food to the mouth, which is located at the base of the limbs. The food reaches the esophagus, where there is a structure similar to the gizzard of birds. This is responsible for crushing the ingested prey even more.

The course of the food bolus continues in the stomach and intestines, where the digestion and absorption of water and nutrients takes place. As for the waste, they are excreted through the anus, which is located on the ventral side, in front of the tail..


The male horseshoe crab reaches sexual maturity when it is between 9 and 11 years old, while the female does it around 10 and 12 years. Mating season varies by geographic region.

Thus, northern populations, with the exception of southern Florida, reproduce from spring to fall. In relation to the southern communities, including the Yucatan peninsula and those of Florida, they can reproduce throughout the year.

In the north, reproduction is triggered by an increase in water temperature, an aspect that is reversed in the Yucatan peninsula. In this area, the decrease in temperature stimulates mating..


When the mating season approaches, the males, who generally outnumber the females, patrol the waters, all along the beach, waiting for the females. These, on the contrary, move directly from the deep waters where they live to the nesting beach.

Spawning occurs in the intertidal zone and is correlated with the highest tides of the month. Once on the beach, the female lays between 2,000 and 30,000 eggs in each nest, which is approximately 15 to 20 centimeters deep..

In addition, the male releases the sperm, thus fertilizing them. Later, the fertile eggs are buried to protect them from migratory birds..

Hatchling development

On the other hand, when the larvae reach a centimeter in length, they hatch. They then crawl from the nest to the sea water, where they swim for 5 to 7 days. After this, they settle down and their first molt begins..

As young horseshoe crabs develop, they head into deeper water, where they continue to molt. Before reaching sexual maturity, the Limulus polyphemus molts about 17 times.

On the other hand, in the first two or three years, the juveniles remain on the coast, in shallow waters.

In this video you can see a group of horseshoe crabs in mating season and laying eggs:


Horseshoe crab. Source:

After the larvae of the horseshoe crab molt to the juvenile phase, it stops swimming at night, becoming benthic. This begins to crawl on the surface of the substrate and burrow into the sand. Thus, both the young and the adult exhibit patterns of diurnal activity.

However, the adult could present some type of activity at night, while the juvenile buries at night.

Breeding behaviors

Male horseshoe crabs have two breeding tactics. Some come ashore tied to a female, attached to her shell with their first appendages. In some cases, a female may reach the sand with several males on her shell..

Other males hit the beach alone, gathering around nesting pairs. In the same way as the males that are on the females, this group releases their sperm on the eggs deposited by the female in the nest.

The experts showed that approximately 40% of the eggs are fertilized by the male “satellites”, while 51% were fertilized by the males that were attached to the females..

The high reproductive success rate of satellite males may be due to their position in the nest, relative to that of the attached male. In this regard, some experts suggest that, to ensure reproduction, males that cluster around the nest may push and occasionally displace the attached males..


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