Horseshoe Crabs: Everything you need to know about this species

The horseshoe crabs or Limulus polyphemus in scientific terms are the oldest living fossil that exist in our planet, with more than four hundred and forty five million years of existence.

These strange creatures are characterized by having blue blood, and they look as if they had come out of a science fiction movie. Despite being crustaceans, they are more related to arachnids than to crabs, being actually a unique species by their own.

Horseshoe Crabs Main Characteristics

The horseshoe crabs are also known by other common names such as casserole crabs, since they look like an inverted pot, casserole from Moluccas, in reference to the Indonesian archipelago; crab of Japan, sea little casserole, bayonet crab, marine cockroach, given their similarity with this creeping animal, and small sea tank, since they look like an old war tank.

This crustacean is characterized by having a distinctive horseshoe shaped shell, which originates its name. Moreover, such carapace is characterized by being smooth and convex.

The horseshoe crabs are dark brown, and can reach between fifty and sixty centimeters in length. Their carapaces´ peculiar shape , benefits their movements through the sand and mud, and also provides protection to the ventral appendages. The frontal shell is usually called prosoma, and the rear shell is called opisthosoma. The tail of the horseshoe crabs is called telson.horseshoe crabs: anatomy

The lateral edges of the shell have a large compound eye, while the middle edge has one or two eyes that can be small or medium. It has a dorsal surface folded ventrally, which forms a triangular shape on the surface, which narrows towards the mouth. At the edge of the triangle there is a frontal organ and two degenerated eyes.

The horseshoe crab has an upper lip, in which the last segments form a clamp, which gives it the characteristic of chelates; and after this lip the mouth is located, followed by a narrow sternum.

This crab possess five pairs of legs that he employs to walk , which are attached to his belly.  Excepting the last two pair of legs, the remaining ones are all equal and chelated, while their thighs possess thorns. The mouth of these crabs is positioned in the middle of their legs.

A masticating tooth and a lateral projection are located in their last pair of legs, which they employ to direct the water flow that usually enters into their abdomens. To push, sweep and remove the mud, when the horseshoe crab digs, are other common uses of these legs.

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Denying the Horseshoe Crabs´s Tails Myth

The horseshoe crab consists of a long tail (telson) of triangular section, which has the shape of a spine or rod, which is located at the rear end of the abdomen. However, according to the scientists, this tail cannot be considered as a telson, since it does not have an anal orifice, since these crustaceans use it more to propel themselves and rejoin, when they remain face up.

Unlike some myths that haunt popular culture, where they say they have been attacked by crabs with their murderous tail; the tail of the horseshoe crab doesn´t have a defensive function, and for this reason you can grab the crabs without any danger.

The abdomen of this crab has six pairs of appendages, of which the first forms the lower genital operculum, where the genital orifices of the female are opened. After their genitals there are five pairs of laminar appendages, which act as gills.

Because of the appearance of these appendages they are usually called gill books, and they serve to maintain a constant circulation of water over the lamellae;  as well as to be employed like oars when the crabs swim.


Horseshoe Crabs Common Habitats

These crabs commonly live in shallow waters of muddy bottom at the North Atlantic, and they spend most of their time in the water.

The juvenile specimens of horseshoe crabs  inhabit in intertidal waters, and as they grow they go deeper into the two hundred meters, although  during the mating period, which happens once a year, they usually invade some coasts of North America, to deposit the eggs in the sand or among the rocks.

These creatures usually gather in large numbers to spawn on the beaches once a year, during spring and fall and the peninsula of Yucatan in Mexico is also a common location of this species. The systems of bays, reefs, and coastal and insular lagoons of this region, especially those of the sub littoral zone of the state of Quintana Roo, up to the border zone with Tabasco are quite useful for the biological cycle of this arthropod.

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Horseshoe Crabs Distribution

Limbs of these creature have also been seen in some other places but only corpses, which may mean that the waters could drag them there, since none alive specimen has been captured in such areas.

It is believed that they prefer these territories because of the warm environment and calcareous structure of the peninsula of the United States, being the focal point the Gulf of Mexico, where they can find the conditions that surely allowed them to survive from millions of years, until today. The altitude, tide, topography, current systems, and sub littoral floor accidents of these two peninsulas, seem to be the most suitable for the horseshoe crabs.

As far as other species of horseshoe crabs, we can mention the following:

Mangrove horseshoe crabs ( Carcinoscorpious rotundicauda)

Southern Horseshoe Crabs or Indonesian  Crabs ( Tachypleus gigas)

Japanese or tri-spine Horseshoe crab (Tachypleus tridentatus).

These three species are distributed throughout the Asian continent.

Horseshoe Crabs Reproductive Method

This species lives mainly at the bottom of the bays and coastal areas, in salty and warm waters where they remain active the whole year. When they inhabit areas where the temperature drops, the horseshoe crabs dig to make a cave, where they stay until spring arrives.

As spring approaches, the horseshoe crabs leave the deep waters, heading towards the beaches, where they will soon spawn their offspring.

The horseshoe crabs are diocics, which means that it is necessary both parents for the reproduction of the species. The gonad is located below the intestine, while the eggs or sperm emerge through short ducts that end under the genital operculum.

The Mating and laying of the eggs takes place when the tide is at the highest, during full moon and new moon of the spring and summer seasons, thanks to the fact that these animals can detect the ultraviolet light of the moon. Spawning can occur during the day and night, but more are those who prefer the night to lay their eggs.

The male specimens arrive long before than the female to the beach, to go along the shore. Once the females arrive, they release a pheromone into the water, in order to attract the males and stimulate them sexually, so that a single female can attract a large number of males.

The male mounts over the female, and then he fixes in her shell by employing his first pair of walking legs, which he modifies in the form of hooks. Then, the female digs a nest in the sand and she partially buried into  it, to lay around two thousand eggs which will be fertilized by the male while the laying lasts.

Finally, the crabs cover the eggs very well, and then they abandon them. However, the statistics of eggs that reach adulthood are usually very low.

The females can lay between fifteen thousand and ninety thousand eggs in of  wells fifteen to twenty centimeters deep, and thirty centimeters wide, located in rows of four meters long.  The soil must be a porous sediment, and have a good drainage, to guarantee the life of the eggs. Each of these wells are inseminated, and covered with sand.

Development of the Larvae

The eggs of the horseshoe crabs measure from two to three millimeters in diameter, and are covered by a thick transparent cover. Those that are located on the surface of the well where they were deposited, develop faster than those that are deeper, since they perceive a greater variation in temperature.

After thirty days have passed from the formation of the egg capsule, it breaks so that a larva measuring about an inch in length emerges. Although it only has two of the five pairs of gill books, this larva swims very well, so it moves to the waters.

As they are molting, the other gill books makes appearance, as well as the caudal spine, so the juvenile crab begins to take the form of an adult.  Then, after a year has elapsed, they have a carapace four centimeters wide, but sexual maturity  is not  reached until they are nine or twelve years old.

This is an arduous process for these crabs, and in the same way than  the Japanese spider crabs , they can lose a limb, or even their life, if they get stuck while the molting of the exoskeleton lasts. However, after several changes, the clamps or lost limbs can be regenerated.

Besides the natural mortality of this species, the biggest cause of death for them comes from the fishing industry, which hunts the horseshoe crab as bait for other animals.

They are also captured and bled for medical jobs, and then they are returned to the ocean, but it is estimated that around 10-15% of them die during the process.

Let´s watch how they molt

What do the Horseshoe Crabs Eat?

These crabs, as we mentioned earlier, have nocturnal habits and usually remain buried in the ground throughout the day. For this reason, it is at night when they feed on mollusks, worms, bivalves, and other benthic invertebrates.

In addition, some other species such as small pieces of fish that they find when they dig in the ground, also comprise their diet. The chemoreceptors that are located on the hips of their walking legs, allow them to accomplish these functions.

Finally, they lack of jaws, and instead have a gizzard with sand and gravel and also have  the ability to expel the parts that they cannot digest.

The Horseshoe Crabs´ Blood Benefits

These animals are a sort of magnificent work of Mother Nature, so much so, that they have managed to survive for so many years, managing to be one of the oldest living beings in our planet.  In addition, they are also famous because of the characteristic blue color of their blood which has several properties for health. For example, this blood is employed to combat the effects of AIDS.

It is believed that this animal has a colorless blood, but when it comes in contact with air, it turns blue, due to its high copper content. Moreover, since it has several valuable compounds for scientific and pharmacological research, it is very useful for both medicine and advances in science.

Horseshoe Crabs´Blood Properties

What makes the blood of the horseshoe crab so unique is a protein called hemocyanin, which contains two copper atoms that transports oxygen through the animal’s organism. This is the same function that hemoglobin performs in vertebrates.

This blue blood has coagulant properties, thanks to the amebocytes, which are blood cells, very similar to the leukocytes of vertebrates. For this reason, it is used in the pharmaceutical industry and medicine, in order to ensure that both treatment and medical devices are out of reach of bacterial contamination.

This wonderful blood also contains amibocytes, which are cells that have the purpose of reacting to a bacterial infection. For this reason, when a horseshoe crab is injured in any way, its blood immediately solidifies, to prevent the entry of any type of microorganism that may cause an infection. This fact makes some scientists speculate, that this could be the secret of their long existence.

In the decade of the fifties, a group of scientists discovered that the blood of the horseshoe crab coagulated in contact with the salmonella ( bacteria), and from there they have continued the tests not only with antibiotics, but also with a large number of drugs that they are used today.

Not only has progress been made producing a reduction in the effects of AIDS, but also for other medications, such as those used daily for influenza, or to fight an infection.

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New discoveries related to the Horseshoe Crabs´ Blood

The blue blood of this animal has been recently implemented, to determine through a rapid test, if possible infections are detected in astronauts, in order to treat them as soon as possible.

Such device is named LOCAD-PTS, and was tested at the International Space Station.  The whole process is related to the enzymes that are present in the horseshoe crabs and it consists in setting a small amount of dried  blood into a channel, and when it comes in contact with a bacterium, the enzyme is rehydrated, changing the color of the solution.

The amount of bacteria present will be determined by the intensity of the solution. It is very possible that, as other tests have been done in this institution, it also applies to the whole society in general. This way a simple test would be enough to diagnose from a throat infection, to a pregnancy test, which would facilitate the work to doctors, who would only be responsible for indicating the appropriate medication.

The blood of the horseshoe crab has also been used to search for life on other planets, through a process that removes the enzymes from it, to be then dried and frozen.

Once it is necessary, it is placed in a measuring instrument designed for this purpose, which when coming into contact with a way of life, causes the blood to transform from blue to yellow.


Consequences  for the Horseshoe Crabs

Unfortunately not everything is good, because the most affected in all these advances are the crabs, which are usually picked up by hand in a boat using a rake of clams, or through dredges  that hurts them, and then they are returned to the sea to let them die .

They are then taken to a laboratory where they undergo a bleeding process in which thirty (30)% of their blood is extracted, but according to the scientist, they will reproduce again when they are returned to their natural habitat in two or three months, so their blood will be extracted again several times a year.

According to the researchers, horseshoe crabs are not in danger of death when they undergo this process of blood extraction, where their heart is drilled to extract one hundred (100) milliliters, and that they can continue normally with their lives, once they are released,

The price of one liter of this blood in the market, exceeds fifteen thousand Euros, and is a business that generates more than fifty million dollars a year.

Unfortunately, these fascinating animals are disappearing from the water where they used to abound. According to the experts the global warming is the main responsible of this fact, since it affects in negative way this species because of the effects of the greenhouse gas emissions produced by the human being.

The overfishing and the catches of these creatures, to be employed in the pharmaceutical industry, have also contributed in big way the disappearance of them.

The fishing of these crabs in order to be employed as a bait to capture whelk and snails, is also a negative aspect for them..

It spite of the benefits of the blood of their blood,  their catches and further use should be carried out in a responsible way.

If no «serious» measures are taken for the protection of this noble animal, it will soon disappear from the face of the earth. This situation would represent a real debacle for the environment where they develop, because when they disappear, so would the other species that feed on this crab.

Horseshoe crabs not only contribute their blood for the benefit of human beings, but they are also the food of some species of birds that eat their eggs, because of the amount of fat they provide. They are also the food of fish and invertebrates that also take advantage of their larvae.

These crabs also serve as a room for certain mollusks and sponges, and a source of substrate for leeches and other mollusks. The worst thing is that they have no defense mechanism against their predators; because it is even a myth that its tail has a powerful poison that can paralyze a man’s heart in minutes.

If this were true, there would not be so many dead horseshoe crabs on the shore of the beaches, and it would be much harder to obtain their blood, and their meat, which they also use as food in some parts of Asia.

Although its fishing also represents a danger for the horseshoe crabs, it does not mean that they will be extinguished by that reason. The real danger for them, lies in the indiscriminate fishing carried out by the pharmaceutical industry.

Several restrictive measures have been created in the United States, to control the employ of the employ of these crabs, but in the same way, when they just can´t fish in their country, they import horseshoe crabs from Asia to continue their studies.

It has also been demonstrated, that the females of this species suffer deterioration in their ability to spawn, once they pass through the bled experience, and in the same way than males,  they suffer a period of weakness and loss of orientation after being released, which makes them more vulnerable than they already are.

The worst thing about this, is that although there are some supposed restrictions, the companies are not obliged to reveal how many horseshoe crabs they employ, nor to carry out a more reliable statistic regarding how many of them die after being returned to the sea.

The statistics from 2016, refers to 426.000 horseshoe crabs captured by the pharmaceutical industry, while others 787.000 were fished to be employed as baits.

Possible Solutions

According to Eric Hallerman, a professor of the American Fisheries Society, a 15 percent of the horseshoe crabs die immediately, as a consequence of the bleeding process.  However, it has been quite difficult to determine how many others die when they return to the ocean.

An experiment was carried out, which consisted in maintaining some horseshoe crabs in tanks for two weeks, after being bled. Only 5 percent of them died, but scientists agree that these are not the same conditions that these crabs face in the ocean, although it is a good reference.

In the meantime, the populations of this species continue to decline, without anyone thinking that the horseshoe crabs are becoming extinct.

However, not everything is bad. Given the importance of their blood, and in order to preserve them, several studies are being developed, in order to determine the feasibility of different techniques of aquaculture or farming of this species.

This is an initiativeof  Kepley BioSystems, Inc., a Greensboro-based life-science startup, which aims to train growers to grow horseshoe crabs on site, which would decrease the deaths of this species, while the benefits of their blood would be always guaranteed.

Let´s be optimist about these researches, in order to enjoy the benefits of their blood without hurting them

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