Horseshoe Crab

Horseshoe crabs are the sole living members of the order Xiphosura and belong to the Limulidae family of marine and brackish water arthropods. Their common name is a misnomer, as they are not real crabs or even crustaceans, as crabs are, but belong to a separate arthropod order. Horseshoe crab scientific name is Limulidae. Horseshoe crabs have been found in fossil records dating back 240 million years, with modern varieties being living fossils. Hence, they are pretty prehistoric crabs. They are the sister group of Ricinulei within Arachnida, according to a molecular investigation published in 2019. An image of a horseshoe crab is given below:

(Image will be uploaded soon)

Basic Characteristics Of Horseshoe Crab

Horseshoe crabs normally prefer to live on soft, sandy, or muddy bottoms in shallow coastal waters. During spring high tides, they breed in the intertidal zone. They're commonly consumed in Asia, and they're also used for fishing bait, fertiliser, and research (especially Limulus amebocyte lysate). Population losses have occurred in recent years as a result of coastal habitat damage and overharvesting. Carcinoscorpius rotundicauda, a type of horseshoe crab, may contain tetrodotoxin.

Horseshoe crabs are similar to crustaceans, however they belong to the Chelicerata subphylum of arthropods. Horseshoe crabs are connected to extinct eurypterids (sea scorpions), which include some of the world's largest arthropods, and the two families may be sister groups. Other research has linked eurypterids to the Merostomata, a group of arachnids. Horseshoe crabs are supposed to be closely linked to the mysterious Chasmata Spiders. The first horseshoe crab fossils were discovered in Lower Ordovician strata, some 480 million years ago.

The Limulidae is the newest family of the order Xiphosura, and it includes all four live horseshoe crab species:

• The mangrove horseshoe crab, Carcinoscorpius rotundicauda, is found in South and Southeast Asia.

• The Atlantic horseshoe crab or American horseshoe crab, Limulus polyphemus, is found along the Atlantic coast of the United States and in the Southeast Gulf of Mexico. These horseshoe crabs are also known as king crabs or Limulus king crab.

• The Indo-Pacific, Indonesian, Indian, or southern horseshoe crab, Tachypleus gigas, is found in South and Southeast Asia.

• The Chinese, Japanese, or tri-spine horseshoe crab, Tachypleus tridentatus, is found in Southeast and East Asia.

Their Biology & Their Behaviour

The horseshoe crab's entire body is protected by a strong carapace. It has two compound lateral eyes with roughly 1,000 ommatidia each, as well as a pair of middle eyes that can detect both visible and ultraviolet light, a single endo parietal eye, and two rudimentary lateral eyes on top. Just before the embryo hatches, the latter becomes functioning. A pair of ventral eyes, as well as a cluster of photoreceptors on the telson, are also found near the mouth.The animals have the largest rods and cones of any known mammal, nearly 100 times the size of humans, and their eyes are a million times more sensitive to light at night than during the day, despite having very poor eyesight. They move food into their mouth with their chelicerae, a pair of tiny appendages.

The next five pairs of the limbs are used for moving around i.e. for locomotion. They are known as ambulatory legs and the first ones are the pedipalps.The mouth is situated at the center of the legs the bases of which are known as gnathobases, and have the same function as jaws and help grind up food. In the species that have gone extinct the appendages of the limbs are uniramous. But there are exceptions such as the fossil genus Disasterium having four pairs of branched legs used for walking.  During a male's terminal moult, his pedipalps change shape, becoming boxing glove-like claspers that are used to grab the female during mating. The major legs utilised for pushing when walking on the ocean floor are the last pair of legs for both male and females. At the tip of the remaining leg pairs is a feeble claw. Lost limbs or the telson (tail) may recover over time, and body shell fissures may repair.

Behind the legs of the horseshoe crab are the book gills which are used for the process of respiration and also occasionally used for swimming. Just like the other arthropods, they also lack a true endoskeleton. But their body does have an endoskeletal structure that is made up of cartilaginous plates that support the book gills. They prefer to seek the ocean floor for worms and mollusks, which are their primary food sources. Crustaceans and even small fish may be eaten by them.

Females are approximately 20–30% larger than males. C. rotundicauda is the smallest species, while T. tridentatus is the largest. Males of C. rotundicauda are around 30 centimetres long on average, with a 15-centimetre tail (telson) and a 15-centimetre wide carapace (prosoma). Although some southern populations of L. polyphemus (on the Yucatán Peninsula) are smaller, the species as a whole is larger. Females of the largest species, T. tridentatus, can grow to be 79.5 cm long, including their tail, and weigh up to 4 kg. The largest females of L. polyphemus and T. gigas are only 10–20 cm longer, but weigh about twice as much. Every moult increases the growth of the juveniles by roughly 33% until they reach adult size. Late in July, Atlantic horseshoe crabs moult.

Reproduction In Horseshoe Crabs

Horseshoe crabs move to shallow coastal waters during the breeding season. The smaller male horseshoe crab uses its specialised front claws to cling to the back of the larger female to fertilise the eggs as they are placed in the sand. Additional males known as "satellite males" may surround the pair and have some success fertilising eggs even if they are not linked to the female. The lack of mating scars on female horseshoe crabs can be used to identify them. In batches of a few thousand eggs, the female can lay between 60,000 and 120,000 eggs. The eggs of L. polyphemus take about two weeks to hatch, and many of them are eaten by shore birds before the baby horseshoe crabs hatch. The larvae moult six times during the first year and once every three or four years after that.

Horseshoe crabs in captivity have proven impossible to breed naturally. According to some research, mating occurs exclusively in the vicinity of the sand or mud in which the horseshoe crab's eggs were laid. It's unclear what the crabs can detect in the sand or how they do it. In captivity, artificial insemination and induced spawning have been used on a wide scale, and eggs and juveniles (baby horseshoe crab) obtained from the wild are frequently grown to adulthood.

Conservation Status Of Horseshoe Crabs

Hemocyanin is used by horseshoe crabs to transport oxygen through their blood. Their blood is blue because of the copper in hemocyanin. Hence, they are blue blooded crab. Their blood contains amebocytes, which act similarly to vertebrates' white blood cells in defending the organism against parasites. L. amebocytes from his blood Limulus amebocyte lysate (LAL) is made from polyphemus and is utilised to identify bacterial endotoxins in medicinal applications. The blood is in high demand, and it is harvested by catching and bleeding the creatures before releasing them back into the water. The majority of the animals survive the procedure; mortality is linked to the amount of blood collected from each animal as well as the degree of stress encountered during handling and transportation. Death rates after blood collecting have been estimated to range from 3–15 percent to 10–30 percent.  Every year, about 500,000 King Crab Limulus are taken for this purpose.

Bleeding can also make it difficult for female horseshoe crabs to spawn or reduce the quantity of eggs they can lay. According to the biomedical sector, up to 30% of an individual's blood is extracted, and horseshoe crabs spend one to three days away from the water before being returned. They can survive on land for four days as long as their gills stay moist. Some scientists doubt that certain companies return horseshoe crabs to the water at all, suspecting them of selling the crabs as fishing bait.

The pharmaceutical sector is reducing its use of horseshoe crab blood as the horseshoe crab endangered species. Researchers at Kyushu University revealed in 1986 that the same test could be performed with isolated Limulus clotting factor C (rFC), an enzyme contained in LAL, as well as LAL itself. Jeak Ling Ding, a researcher at the National University of Singapore, registered a method for producing rFC, and on May 8, 2003, synthetic isolated rFC produced using her patented method became available for the first time. Industry was initially uninterested in the new product because it was patent-protected, had not yet been approved by authorities, and was only marketed by one company, Lonza Group. Hyglos GmbH, on the other hand, began producing its own rFC product in 2013. This, combined with European regulators' endorsement of rFC, the cost of LAL and rFC being equivalent, and support from Eli Lilly and Company, which has committed to using rFC instead of LAL, is expected to put a stop to the practise of blood collection from horseshoe crabs.

PETA supported a report presented by the US Senate in December 2019 that encouraged the Food and Drug Administration to "develop protocols for reviewing alternate pyrogenicity tests and report back on efforts taken to boost their use."

The United States Pharmacopeia refused to give rFC the same status as horseshoe crab blood in June 2020. Without certification for classification as an industry standard testing material, US companies will have to prove that rFC is safe and effective for their intended uses, which might be a deterrent to the use of the horseshoe crab blood substitute.

Horseshoe crabs are used as bait for eels and whelk (or conch) fishing (primarily in the United States). In the United States, almost 1 million (1,000,000) crabs are captured for bait per year, dwarfing biomedical mortality. In New Jersey, however, horseshoe crab fishing was outlawed indefinitely in 2008, along with a harvesting embargo, to protect the red knot, a shorebird that consumes the crab's eggs. In Delaware, a moratorium on male crabs was imposed, while in South Carolina, a permanent moratorium is in place. In Southeast Asia, Johor, and China, the eggs are consumed.

The red knot's future is thought to be jeopardised by a low horseshoe crab population in the Delaware Bay. During their stopovers on the beaches of New Jersey and Delaware, red knots, long-distance migratory shorebirds, feast on the protein-rich eggs. Adaptive management plans are being developed to govern horseshoe crab harvests in the bay while protecting migrating shorebirds. Horseshoe crab spawning is threatened by development along shorelines, which reduces available space and degrades habitat. Bulkheads can also prevent access to intertidal spawning areas.