horseshoe crab (Limulidae) is not a crab, despite their name, but are related to Spiders y scorpions.

These invertebrates marine they have a horseshoe-shaped shell or carapace. They are often described as'living fossils'Because they haven't changed much in almost 300 million years. They are close living relatives of a group of invertebrates now extinct called trilobites.

The horseshoe crab is not a crab, as you might believe.
The horseshoe crab is not a crab, as you might believe.

Features

The body of the horseshoe crab is made up of two parts: the cefalotórax and abdomen. The cephalothorax is basically the fused head and thorax of the crab. Under the cephalothorax, there are six body segments, each equipped with a pair of limbs. Beneath the abdominal shell are the circulatory, respiratory, reproductive, and nervous systems. In addition, the abdomen houses part of the crab's digestive system and an abundant number of glands.

Like all members of the subphylum ChelicerataBut unlike other anthropods, the horseshoe crab does not have antennae. Instead, it uses its first pair of appendages (called chelicerae), located in the front and on the sides of the mouth, to feed. The chelicerae, and all their appendages except the walking legs, are equipped with pincers (called chelas) with which the animal takes food from the bottom of the sea. The second pair of legs (called pedipalpo) was evidently used for walking, but over time it developed more specialized functions. Currently, the second pair of legs is used in different ways, depending on the species; basically these paws can be used for grasping, chewing or feeling.

While the horseshoe crab does not have a conventional jaw, its four pairs of walking legs have special equipment attached to them. Known as gnathobases, are primitive devices that the crab uses to manipulate and grind the food before passing it to the mouth. The last pair of walking legs can be used to break shells and crush hard food. Because the crab often swallows sand and shell fragments, its gizzard is quite powerful and can grind up almost anything it consumes.

The horseshoe crab has two pairs of eyes. The first pair are large and compound, which means they are made up of numerous closely grouped simple eyes. These large eyes are widely spaced in the front of the dorsal plate. Much less noticeable, the two small, simple eyes are located fairly close to each other on the front of the crab's back. Little is known about the other senses of the animal.

Adult horseshoe crabs are 89 to 850 millimeters long. The females are bigger than the males.

Behavior

Every spring during the high tides of the new and full moons, thousands of horseshoe crabs congregate along the beaches during sunrise. The males, two-thirds the size of their mates, cluster along the water's edge as the females arrive. With glove-like claws on its first pair of legs, the male hangs from the female's shell and is dragged along the beach to the high tide line.

The female pauses every few meters to dig a hole and deposit up to 20.000 eggs pearl green the size of a partridge. The male then fertilizes the eggs when placed on the nest. Once spawning is complete, the crabs leave and the waves wash sand over the nest.

Habitat

The horseshoe crab lives at the bottom of muddy and sandy bays and estuaries. In the Gulf of Mexico, individuals have been found to depths of 30 meters, with concentrations of five to six meters. They require sloping sandy beaches on which to lay their eggs.

Distribution

Along the Atlantic Coast, from Nova Scotia to Yucatán. This species is found along the east coast of North America from Maine to the Yucatan Peninsula, passing through southern Florida and the Gulf of Mexico. Throughout this range there are distinct populations, with the largest population in the Delaware Bay.

Food

They eat almost anything. The horseshoe crab is primarily a predator. They feed on small clams, crustaceans and worms; however, they also eat other animals and even algae. Because they don't have jaws or teeth, they smash hard food between their legs before putting it in their mouths. Like the birdsHorseshoe crabs also have gizzards to grind food before it reaches their stomachs.

horseshoe crab upside down
horseshoe crab upside down

Predators

Various types of shorebirds eat horseshoe crab eggs, various fish, invertebrates and sea ​​turtles feed on the larvae, and the human catch adult horseshoe crabs for use as bait and for medical research thanks to their blue blood.

Reproduction

Horseshoe crabs can live 20 to 25 years. They migrate to shore in late spring, with males arriving first. The females arrive and make their nests at a depth of 15-20 centimeters in the sand. The females lay eggs in the nests, which are later fertilized by the male. The number of eggs depends on the body size of the female and ranges between 15.000 and 64.000 eggs per female.

After the eggs hatch, the larvae form and then swim for about 5 to 7 days. After swimming they settle down and begin the first molt. This occurs approximately 20 days after the formation of the egg capsule. As the young horseshoe crabs grow, they move to deeper water, where the molting continues. Horseshoe crabs reach sexual maturity in about 11 years and can live another 10 - 14 years.

State of conservation

It is a species listed as vulnerable (VU) and the horseshoe crab population has decreased dramatically. In the past, clam hunters killed them in large numbers, as the species feeds on clams. They were also used for animal feed. In the 1920s and 1930s, between four and five million individuals were harvested each year.

Currently, they are collected in large quantities for use as bait for eels y snails; In 1996 alone, at least two million horseshoe crabs were killed for this reason.

Horseshoe crabs have also been used extensively in the biomedical industry for the manufacture of surgical sutures, the manufacture of dressings for burn victims, and in eye research. Furthermore, the copper-based blue blood of this species coagulates when it comes in contact with toxins released by bacteria. Studies have shown that between 10 and 15 percent of individuals who bleed this way die as a result, accounting for the loss of between 20.000 and 37.500 horseshoe crabs each year.

Other threats this ancient species faces are habitat loss and coastal development, as well as pollution. This unique species is exceptionally vulnerable as it matures very slowly, assembles in large numbers, making it an 'easy prey', and changes in abundance are not easy to detect. In addition, once the population has been reduced, it can take up to ten years to recover after harvest, which roughly corresponds to the time it takes for individuals to reach maturity.

If its status continues in the same way, it will be classified as an Endangered animal.

Curiosities

These animals are the subject of many questions and urban legends, we are going to answer the most frequent questions that we may have.

Are horseshoe crabs really crabs?

No, horseshoe crabs are in a class by themselves; they are most closely related to spiders, scorpions, and ticks. They differ from true crabs in that they do not have antennae or jaws (mouthparts for grinding food). Like spiders, they have a pair of chelicerae (small appendages to carry food to the mouth).

Are they really old?

Yes and no, that idea comes from the fact that 445 million years ago, the ancestors of horseshoe crabs were abundant. The anatomy of the species that we have today has not changed much from the older forms. The life of these animals is also remarkable: it can live up to 20 years.

Can a horseshoe crab hurt me?

Horseshoe crabs do not bite or sting. Despite the fierce appearance of the tail, it is not used as a weapon. Instead, they use their tails to straighten out if they are flipped over by a wave. They have spines along the edge of their shell, so if you have to handle them, be careful to pick them up by the sides of the shell, not by the tail.

What's so special about horseshoe crab blood?

Only these types of animals have a blood clotting agent known as Limulus amebocyte lysate, or LAL, that clots in the presence of certain bacterial toxins. These toxins are difficult to detect by other means. The FDA requires the use of LAL to test all injectable and intravenous drugs produced in the US The good news is that up to a third of a horseshoe crab's blood can be drawn without killing the animal.

Why are so many dead on the beach?

Horseshoe crabs are often knocked over by high wave action during spawning and may not be able to right themselves. This often leads to the death of the animal (They can be helped by gently lifting it from both sides of the shell and dropping it back into the water).

Others have mistaken molting horseshoe crabs for dead crabs. Like all arthropods (including crustaceans and insects), horseshoe crabs have a hard exoskeleton (shell) on the outside of their body. To grow, the crab must shed its old exoskeleton and form a new, larger one. Unlike true crabs, which recoil from their old exoskeletons, horseshoe crabs advance, leaving their molts behind, leaving a crack in front.

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