Horseshoe crabs are arthropods in the family Limulidae and the only surviving xiphosurans. Although their name includes "crab," they are not crabs or crustaceans. Instead, they are chelicerates, closely related to arachnids such as spiders, ticks, and scorpions. A horseshoe crab’s body has three main parts: the cephalothorax, abdomen, and telson. The largest part, the cephalothorax, contains most of the animal’s eyes, limbs, and internal organs. This section also gives the animal its name because its shape resembles a horseshoe. Horseshoe crabs are called "living fossils" because they have changed little since appearing in the Triassic period about 250 million years ago. Similar fossils of xiphosurans date back to the Ordovician period around 445 million years ago.

Four horseshoe crab species exist today. The Atlantic horseshoe crab (Limulus polyphemus) lives along the eastern coasts of North and Central America. The mangrove horseshoe crab (Carcinoscorpius rotundicauda), tri-spine horseshoe crab (Tachypleus tridentatus), and Indo-Pacific horseshoe crab (Tachypleus gigas) are found in South, Southeast, and East Asia. Most horseshoe crabs live in marine environments. However, the mangrove horseshoe crab often lives in brackish water, and the Atlantic horseshoe crab is found in brackish estuaries like Delaware and Chesapeake bays. Some extinct species lived only in freshwater. Horseshoe crabs typically stay near the bottom of water bodies but can swim when needed.

Horseshoe crabs are often caught for their blood, which contains Limulus amebocyte lysate, a substance used to test for harmful bacteria. They are also used as fishing bait in the United States and eaten as food in parts of Asia. In recent years, horseshoe crab populations have declined due to loss of coastal habitats and overharvesting. To protect them, many regions have created rules to limit harvesting and started programs to breed them in captivity.

Phylogeny and evolution

The fossil record of Xiphosura, a group that includes horseshoe crabs and their extinct relatives, dates back to the Early Ordovician period, about 480 million years ago. Ordovician xiphosurans, such as Lunataspis, looked very similar to modern horseshoe crabs. Modern horseshoe crabs first appeared around 250 million years ago during the Early Triassic period. Because they have changed very little since then, living horseshoe crabs are often called "living fossils."

Horseshoe crabs look like crustaceans but belong to a different group of arthropods called Chelicerata. They are closely related to extinct eurypterids, which were some of the largest arthropods ever. Horseshoe crabs may be closely related to eurypterids as sister groups. Another group, the chasmataspidids, is also thought to be closely related to horseshoe crabs.

The diversification of horseshoe crabs led to 22 known species, but only 4 remain today. The Atlantic species is closely related to the three Asian species, which likely split into separate groups recently. The most recent common ancestor of the four living species is estimated to have lived about 135 million years ago during the Cretaceous period.

Limulidae is the only living family in the order Xiphosura and includes all four modern horseshoe crab species:

After Bicknell et al. 2021 and Lamsdell et al. 2020

The position of horseshoe crabs within Chelicerata is complex. Most studies based on physical traits place them outside the group Arachnida. However, a genetic study suggested they are closely related to ricinuleids, making them part of Arachnida. A more recent study, using more complete genetic data and a larger sample of species, again placed horseshoe crabs outside Arachnida.

Below is a cladogram showing the relationships among members of Limulidae (modern horseshoe crabs), including both living and extinct species:

† Yunnanolimulus henkeli
† Yunnanolimulus luopingensis
† Tarracolimulus rieki
† Keuperlimulus vicensis
† Allolimulus woodwardi
† Casterolimulus kletti
† Victalimulus mcqueeni
† Mesolimulus crespelli
† Mesolimulus sibiricus
† Mesolimulus tafraoutensis
† Crenatolimulus darwini
† Crenatolimus paluxyensis
† Volanalimulus madagascarensis
† Heterolimulus gadeai
Carcinoscorpius rotundicauda
† Tachypleus syriacus
Tachypleus tridentatus

The common ancestor of arachnids and xiphosurans (the group that includes horseshoe crabs) experienced a whole-genome duplication event. This was followed by at least two, possibly three, whole-genome duplications in the ancestor of living horseshoe crabs. These events resulted in unusually large genomes for invertebrates, such as those of C. rotundicauda and T. tridentatus, which are about 1.72 Gb each. Evidence of these duplications includes similarities in chromosome structure (synteny) and clusters of homeobox genes. Over time, many duplicated genes changed through processes like neofunctionalization or subfunctionalization, meaning their functions became different from their original roles.

Several hypotheses explain why female horseshoe crabs are generally larger than males. This difference, called sexual size dimorphism, is likely caused by a combination of two factors:

Anatomy and physiology

Horseshoe crabs, like all arthropods, have bodies divided into parts with legs that can move. These legs are covered by a tough, protective layer made of a material called chitin. Their heads are made of several parts that join together during development.

Horseshoe crabs are chelicerates, meaning their bodies are divided into two main sections: the cephalothorax and the opisthosoma. The cephalothorax, also called the prosoma, is where the head and thorax join. This section is protected by a large, curved shell that looks like a horse's hoof, which is why the animal is named "horseshoe crab." The opisthosoma, or abdomen, is made of several fused parts. It has three main sections: one in the middle and two on the sides. Attached to each side is a flat, serrated edge called a flange. These flanges are connected by a structure called the telson embayment, which is attached to the middle section. Along the edges where these sections meet are six areas called apodemes, which help muscles attach to the animal’s twelve movable spines.

On the underside of the abdomen are several limbs. The outer parts of these limbs are flat and wide, while the inner parts are narrower. Near the front is a plate-like structure made of two fused appendages called the genital operculum, which holds the reproductive organs. Following this are five pairs of book gills. These structures help the crab breathe and can also be used for swimming. At the end of the abdomen is a long, tail-like spine called the telson. This part is highly mobile and has many uses.

Horseshoe crabs have several types of eyes that help them see. The most noticeable are two large compound eyes on top of the shell. These are unusual because other chelicerates have lost similar eyes during evolution. In adult horseshoe crabs, each compound eye has about 1,000 tiny units called ommatidia. Each ommatidium contains a ring of light-sensitive cells around a special cell called an eccentric cell. This cell works with other cells to detect light.

The compound eyes of horseshoe crabs are not as organized as those of other arthropods. The ommatidia are arranged in a messy pattern called an "imperfect hexagonal array." Each ommatidium has between 4 and 20 photoreceptors, and sometimes has one or two eccentric cells. All the photoreceptors in the eyes use a single type of light-sensitive pigment that works best with light around 525 nanometers. This is different from insects or crabs, which have photoreceptors that detect different light colors. Horseshoe crabs have poor vision, but their eyes have the largest rods and cones of any known animal, about 100 times bigger than in humans. Their eyes are also a million times more sensitive to light at night than during the day.

Along the front of the body near the heart are two eyes called median ocelli. These eyes have fewer photoreceptors and use special cells called arhabdomeric cells, which work like eccentric cells. The median ocelli have two types of light-sensitive pigments. One works like the pigment in compound eyes, and the other detects ultraviolet light, which has a peak absorption of about 360 nanometers.

Other eyes in horseshoe crabs include the endoparietal ocelli, lateral ocelli, ventral ocelli, and a group of photoreceptors on the abdomen and telson. The endoparietal ocelli are similar to the median ocelli but are made by joining two separate eyes. These eyes are located near the median ocelli on the heart ridge. The ventral ocelli are on the underside of the cephalothorax near the mouth and help the crab move. The lateral ocelli are behind the compound eyes and become active when the crab hatches. The telson’s photoreceptors are spread throughout its structure and help control the crab’s daily activities.

Like all arthropods, horseshoe crabs have an open circulatory system. This means their blood moves through a body cavity called the hemocoel instead of through closed veins and arteries. The hemocoel contains hemolymph, a fluid that acts as the crab’s blood. Instead of using iron-based hemoglobin, horseshoe crabs use a copper-based protein called hemocyanin, which gives their blood a blue color. The blood also contains two types of cells: amebocytes, which help blood clot, and cyanocytes, which produce hemocyanin.

Horseshoe crabs have a long, tubular heart in the middle of their body. Like vertebrates, their heart has two states: systole (when it contracts) and diastole (when it relaxes). During systole, blood leaves the heart through a large artery called the aorta and smaller arteries. This blood then flows into large spaces in the hemocoel, which surround the crab’s tissues. These spaces lead to smaller spaces, allowing blood to reach all parts of the body. During diastole, blood flows from the hemocoel into a cavity called the pericardial sinus and then back into the heart, restarting the cycle.

Horseshoe crabs breathe using modified swimming appendages called book gills, which are located under their abdomen. These gills look smooth on the outside but have complex structures inside that help with breathing.

Distribution and habitat

Today, horseshoe crabs live in only a few areas. The three Asian species are mainly found in South and Southeast Asia near the Bay of Bengal and along the coasts of Indonesia. An exception is the tri-spine horseshoe crab, which lives as far north as the coasts of China, Taiwan, and southern Japan. The American species is found from Nova Scotia to the northern Gulf of Mexico, with another group living near the Yucatán Peninsula. Most living horseshoe crabs live in saltwater, though one species, the mangrove horseshoe crab (Carcinoscorpius), is often found in areas where saltwater and freshwater mix.

As recently as 2 million years ago, during the Early Pleistocene, Limulus polyphemus lived in the Kap Kobenhavn Formation in northern Greenland, as shown by environmental DNA. At that time, the sea surface temperature was 8°C warmer than it is now.

A 2015 study showed that now-extinct xiphosurans, which are related to horseshoe crabs, moved to freshwater at least five times in their history. This transition happened twice in the horseshoe crabs Victalimulus and Limulitella, which lived in environments like swamps and rivers.

Behavior and life history

Horseshoe crabs primarily eat worms and mollusks that live on the ocean floor. They may also eat crustaceans and small fish. They usually forage for food at night.

Horseshoe crabs live on the ocean floor most of the time. However, they can swim. Young horseshoe crabs and those traveling to the shore to breed often swim. When swimming, they move upside-down with their bodies angled downward. They use their telson like a rudder to change direction. To swim, their retracted legs move to the front of their cephalothorax, extend, and stroke backward. This motion happens together with the genital operculum and the first three pairs of book gills. While the front legs reset, the back two book gills make a smaller stroke.

Horseshoe crabs have several ways to flip themselves over if they are upside-down. The most common method is arching their opisthosoma toward their carapace and balancing their telson on the ocean floor. They then move their telson while beating their legs and gills, which causes them to tilt and flip over. They can also right themselves while swimming by moving to the bottom, rolling on their side, and touching the bottom with their pusher legs while still in the water.

Baby horseshoe crabs start life as "trilobite larvae," named for their resemblance to trilobites. When they hatch, they are about 1 cm (1⁄2 in) long. Their telson is small, and they lack three pairs of book gills. Otherwise, they look like tiny adults. Baby horseshoe crabs can swim and burrow in sediment after hatching.

As larvae molt into juveniles, their telson grows longer, and they gain their missing book gills. Juveniles can reach a carapace width of about 4 cm (1 + 1⁄2 in) in their first year. For each molt, they grow about 33% larger. This process continues until they reach adult size.

When mature, female horseshoe crabs are typically 20–30% larger than males. The smallest species is the mangrove horseshoe crab (C. rotundicauda), and the largest is the tri-spine horseshoe crab (T. tridentatus).

On average, males of C. rotundicauda are about 30 cm (12 in) long, including a telson about 15 cm (6 in) and a carapace about 15 cm (6 in) wide. Some southern populations of L. polyphemus (in the Yucatán Peninsula) are slightly smaller, but otherwise, this species is larger. In the largest species, T. tridentatus, females can grow up to 79.5 cm (31 + 1⁄4 in) long, including their telson, and weigh up to 4 kg (9 lb). This is only about 10–20 cm (4–8 in) longer than the largest females of L. polyphemus and T. gigas, but roughly twice their weight.

During the breeding season (spring and summer in the Northeast US, year-round in warmer areas), horseshoe crabs migrate to shallow coastal waters. Nesting usually occurs during high tides around full or new moons. When nesting, they lay eggs on beaches and salt marshes.

When mating, the smaller male attaches to the back or opisthosoma of the larger female using specialized pedipalps. This often leaves scars, which help identify younger females. The female digs a hole in the sediment and lays between 2,000 and 30,000 large eggs. Fertilization happens outside the body, which is unusual for arthropods. In most species, both the main male and additional "satellite males" help fertilize eggs. Satellite males surround the main pair and may fertilize some eggs. In L. polyphemus, the eggs hatch in about two weeks, and many are eaten by shore birds.

Natural breeding of horseshoe crabs in captivity has been difficult. Some evidence suggests that mating occurs only in the presence of sand or mud where horseshoe crab eggs have previously hatched. However, it is unclear what the animals sense in the sand, how they sense it, or why they mate only in its presence. Artificial insemination and induced spawning have been used since the 1980s. Eggs and juveniles collected from the wild can be raised to adulthood in captivity.

Relationship with humans

Horseshoe crabs are valued as a delicacy in some regions of East and Southeast Asia, even though their meat is not very large. The meat is white and has a rubbery texture similar to that of lobster meat. It also has a slightly salty taste. People can eat horseshoe crabs raw or cooked, but they must be prepared carefully to avoid food poisoning. Only certain species of horseshoe crabs are safe to eat. For example, the meat of mangrove horseshoe crabs (Carcinoscorpius rotundicauda) contains a toxin called tetrodotoxin, which can cause serious illness if eaten.

Horseshoe crab meat is often grilled or stewed, but it can also be pickled in vinegar or stir-fried with vegetables. Many recipes use spices, herbs, and chilies to add flavor.

In addition to their meat, horseshoe crab eggs are also eaten in some areas. However, only the eggs of specific species are safe to consume. The eggs of mangrove horseshoe crabs contain tetrodotoxin and can cause food poisoning if eaten.

In the United States, horseshoe crabs are commonly used as bait for fishing eels, whelk, or conch. Nearly 1 million crabs are harvested each year for this purpose, which is much greater than the number used for biomedical purposes. However, in 2008, New Jersey banned the use of horseshoe crabs as bait indefinitely to protect the red knot, a bird that eats crab eggs. Delaware banned the harvesting of female crabs, and South Carolina has a permanent ban on harvesting horseshoe crabs.

The blood of horseshoe crabs contains special cells called amebocytes, which help protect the crabs from disease. Amebocytes from the blood of Limulus polyphemus are used to make Limulus amebocyte lysate (LAL), a substance used to detect harmful bacteria in medical products. To collect LAL, crabs are bled, then released back into the ocean. Most crabs survive this process, but some die. The number of deaths depends on how much blood is taken and how much stress the crabs experience. Mortality rates range from 3–15% to 10–30%. About 500,000 Limulus crabs are harvested each year for this purpose. Some reports claim that up to 30% of a crab’s blood is removed, while others say it can take more than half of their blood. Blood harvesting may also harm female crabs by reducing their ability to lay eggs or decrease their activity levels.

Horseshoe crabs are kept away from the ocean for one to three days during blood collection. If their gills remain moist, they can survive on land for up to four days. Some scientists believe that certain companies may not return crabs to the ocean after harvesting but instead sell them as fishing bait.

The use of horseshoe crab blood in the pharmaceutical industry is decreasing. In 1986, researchers discovered that a synthetic version of a protein in LAL, called rFC, could be used for the same purpose. A scientist from the National University of Singapore patented a method to produce rFC, and the first synthetic rFC became available in 2003. At first, the industry was slow to adopt rFC because of patent restrictions and lack of approval. However, in 2013, another company began producing rFC, and European regulators approved its use. In 2026, Amgen and Abbott Laboratories announced plans to stop using horseshoe crab blood for testing.

The development of vaccines during the COVID-19 pandemic has increased pressure on horseshoe crab populations. In 2019, a U.S. Senate report urged the Food and Drug Administration to find alternatives to horseshoe crab blood for testing. PETA supported this effort. In 2020, U.S. Pharmacopeia did not approve rFC as an equal alternative to horseshoe crab blood. Without this approval, U.S. companies may face challenges in proving that rFC is safe and effective for their uses, which could slow the shift away from horseshoe crab blood.

Conservation status

Building along shorelines harms horseshoe crab spawning by reducing space and damaging their homes. Structures like bulkheads can block access to areas where crabs lay their eggs during high and low tides.

The number of Indo-Pacific horseshoe crabs (Tachypleus gigas) in Malaysia and Indonesia has dropped sharply since 2010. This is mainly because people are catching too many, especially in countries like Thailand where they are considered a delicacy. Pregnant females are often targeted because their meat and eggs are valuable. This overharvesting has created an unbalanced number of males and females in the wild, which contributes to the population decline.

Horseshoe crabs face dangers from shoreline development, use in fishing, plastic pollution, their role as a food source, and their use in research and medicine. One species, the tri-spine horseshoe crab (Tachypleus tridentatus), has already disappeared in Taiwan. Scientists believe Hong Kong may soon declare this species extinct in its area due to a more than 90% drop in young crabs. This species is listed as endangered by the IUCN Red List because of overharvesting and loss of habitat.

To help protect horseshoe crabs, a breeding center was built in Johor, Malaysia. There, crabs are raised and released into the ocean in large numbers every two years. It takes about 12 years before these crabs can be used for food.

Fewer horseshoe crabs in Delaware Bay might harm the future of the red knot, a long-distance migratory bird. Red knots rely on the protein-rich eggs of horseshoe crabs during their stopovers on beaches in New Jersey and Delaware. Scientists are working to create plans that balance horseshoe crab harvesting with protecting migrating birds. In 2023, the US Fish and Wildlife Service stopped harvesting horseshoe crabs in the Cape Romain National Wildlife Refuge, South Carolina, from March 15 to July 15 to help them reproduce. This decision was based on the importance of horseshoe crab eggs for birds, their use as bait, and their blood’s role in medical products. The refuge, which covers 66,000 acres of marshes, beaches, and islands near Charleston, supports these conservation efforts.