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Horseshoe Crabs

Limulidae

Horseshoe crabs are aquatic arthropods of the family Limulidae, the only extant members of the order Xiphosura. Although they are named crabs, they are not true crabs in reality and are more closely related to arachnids like spiders and scorpions than crustaceans. The name ‘horseshoe’ originates from the rounded shape of their cephalothorax, resembling the U-shaped shoe on a horse’s foot.

The fossil records of horseshoe crabs date back over 440 million years ago to the Ordovician period, with the oldest representatives of Lumulidae dating to the Early Triassic Period (250 million years ago). This extensive history has led to horseshoe crabs being called ‘living fossils,’ indicating their remarkable persistence and minimal evolutionary change over long periods.

Description

Size

Of all the four extant species, the smallest is Carcinoscorpius rotundicauda, with males averaging about 30 centimeters (12 inches) in length, including a tail (telson) that is approximately 15 cm (6 in) long and a carapace (prosoma) that is about 15 cm (6 in) wide.

The world’s largest species is Tachypleus tridentatus, the females of which grow up to 79.5 cm (31 inches), including their tail, and weigh up to 4 kg (9 lbs). They are 10 to 20 cm (4 to 8 inches) longer than the largest females of Limulus polyphemus and Tachypleus gigas and roughly twice their weight.

Generally, female horseshoe crabs are 20 to 30% larger than the males, while the juveniles grow about 33% larger with every molt until adulthood.

Body Plan

The body of a horseshoe crab is broadly divided into three segments that are hinged together: the horseshoe-shaped cephalothorax (prosoma), the segmented abdomen (opisthosoma), and the sharp tail-spine (telson). A hard carapace covers all three segments.

  1. Cephalothorax: Its smoothly arched upper surface bears a pair of lateral eyes and a pair of median eyes. On the underside, the cephalothorax bears six pairs of legs, the first being the chelicerae, specialized for seizing prey. The next five pairs (beginning with pedipalps) surround the mouth and assist in feeding and locomotion. In both males and females, the last pair of legs is mainly used for walking on the ocean floor. 
  2. Abdomen: The central, triangular part of the body bearing spines on the sides and a ridge in the center. The gills are located on the underside of the abdomen, along with the locomotory muscles.
  3. Tail-spine: It is a long, pointed structure at the end of the body that, despite its intimidating appearance, is neither poisonous nor used for stinging, unlike other arthropod tails.

Like other arthropods, horseshoe crabs are composed of bone or cartilage and thus lack a true skeleton. Their endoskeletal structure comprises cartilaginous plates that serve as a supportive framework for various internal organs, including the book gills.

Physiology

Circulatory System

The circulatory system in horseshoe crabs is well-developed, with a prominent long tubular heart extending along the prosoma and the abdomen’s central axis. Deoxygenated blood is directed from the heart into specialized respiratory organs known as book gills, located on the underside of the horseshoe crab. Once oxygenated, the blood is returned to the heart and pumped throughout the horseshoe crab’s body.

Their blood contains hemocyanin, a copper-based pigment that imparts a striking blue color to the body fluid. Specialized immune cells in the blood called ‘amoebocytes’ help these arthropods fight bacteria, viruses, parasites, and other external invaders.

Respiratory System

They absorb oxygen from water using five pairs of gills present under the abdomen. Each pair comprises a notable flap-like structure covering the leaf-like membranes known as lamellae. The lamellae serve as sites for gaseous exchange, where oxygen is absorbed, and carbon dioxide is released as the gills move rhythmically. With approximately 150 lamellae per gill (resembling pages in a book), these structures are commonly referred to as ‘book gills.’

Beyond their respiratory function, these gills act as paddles, propelling juvenile horseshoe crabs through the water.

Digestive System

Once the chelicerae and other prosomal appendages have initially processed the prey, further breakdown occurs in the gizzard, an organ specialized for grinding food. A few digestive juices secreted into the gut by the hepatopancreas then act on the partially digested food and help break it into a simple and easily absorbable form.

Excretory System

Horseshoe crabs possess a relatively simple excretory system, primarily composed of a pair of coxal glands, also known as green glands. These glands, located near the base of the legs, filter waste products (ammonia and urea) and excess ions from the hemolymph, thus maintaining internal homeostasis.

Nervous System

In horseshoe crabs, the optic nerves, which originate from the anterior portion of the brain and connect directly to the two lateral eyes, help their sensory perception. These nerves facilitate visual perception, enabling the horseshoe crab to detect light and movement in the surroundings.

In addition to the optic nerves, eight pairs of haemal nerves extend throughout the crab’s body. These nerves contain both motor and sensory fibers and are primarily distributed throughout the body tissues. 

One particularly significant haemal nerve is the ‘lateral line nerve’ that branches at the base of the sixth appendage and extends the entire length of the branchial chamber, sending small nerve branches to the bases of the five gills.

Eyes

Horseshoe crabs have ten eyes, each serving specific functions vital for survival and reproductive behavior. The most prominent among them are the two lateral eyes that help locate mates during the spawning season. These compound eyes comprise roughly 1,000 receptors or ommatidia and exhibit a structure akin to human eyes, albeit about 100 times larger. The ommatidia within these compound eyes adjust their sensitivity to light, enhancing the visibility significantly at night through chemical stimulation, thus facilitating the detection of other horseshoe crabs in the dark. They also have five eyes on the top of the prosoma.

Just behind each lateral eye lies a rudimentary lateral eye, while towards the anterior of the prosoma, a small ridge features three dark spots housing the median and endoparietal eyes. Functioning as detectors of ultraviolet (UV) light from the sun and reflected light from the moon, these eyes help the crabs track the lunar cycle, which is crucial for timing their spawning activities. In addition, a pair of ventral eyes are found near the mouth, and a cluster of photoreceptors is present on the telson.

Taxonomy

Although horseshoe crabs share their name with crabs (crustaceans), these arthropods are taxonomically distinct. They also share a close evolutionary relationship with the extinct eurypterids, also known as sea scorpions. The precise evolutionary relationship between horseshoe crabs and eurypterids has remained debatable, with some studies suggesting that they may be sister groups within the Chelicerata subphylum and other research proposing that eurypterids might be more closely related to arachnids, forming a group called Merostomata.

The present classification places them in the same group as arachnids, spiders, scorpions, and ticks rather than crustaceans like crabs and lobsters.

Horseshoe Crabs

Distribution

The horseshoe crab Limulus polyphemus (Atlantic Horseshoe Crab) is most commonly found in the United States, living along the coastline of the Atlantic Ocean. The other three species are located in the Indian Ocean and along the coast of Asia in the Pacific Ocean.

Habitat

Horseshoe crabs predominantly inhabit shallow coastal waters characterized by soft, sandy, or muddy substrates. Their preferred habitats include areas around intertidal zones, where they can be commonly encountered during spring high tides.

Diet

Although they primarily feed on mollusks and worms, they may consume crustaceans, small fish, and algae when necessary.

Lifespan

Horseshoe crabs usually survive over 20 years in the wild.

Reproduction and Life Cycle

Adult horseshoe crabs travel from deep ocean waters to the sandy beaches along the East and Gulf coasts for their annual breeding ritual during late spring and early summer. The breeding process begins with the arrival of male horseshoe crabs at the breeding grounds, where they patiently await the arrival of the females. When the females reach the shore, they release pheromones that serve as signals to attract males and indicate that it is time to mate. This chemical cue triggers the males to gather around the females and fertilize their eggs.

Preferred breeding times for horseshoe crabs are during high tides and under the influence of new and full moons. These conditions provide optimal environmental cues for successful reproduction. Under the cover of darkness, the males grasp onto the back of the females, and together, they make their way to the shoreline.

Once on the beach, the females dig small nests in the sand using their specialized appendages and deposit 60,000 and 120,000 eggs into these shallow depressions. Within 20 to 30 minutes of egg-laying, the males release sperm to fertilize the eggs externally.

After a few weeks of incubation, the larvae (typically 5mm long) hatch from the eggs. Initially, these larvae lack a telson and rely on the yolk reserve for sustenance. As they progress into the second larval stage, a short telson develops, enabling them to swim for short periods and begin feeding on small marine organisms. 

Some larvae disperse widely during this phase, while others remain near beaches, wintering in the sediment of mud flats. On transitioning into the third stage, the larvae resemble miniature adults, signifying their progression towards maturity. Between each stage, molting occurs, involving the shedding of the outer cuticle, resulting in a 25% increase in length post-molt. Finally, after around 16 molts, typically occurring over 9 to 12 years, the horseshoe crabs attain sexual maturity.

Predators

Their natural predators include sharks and rays, sea turtles, and shorebirds like gulls, terns, and shore plovers. Around May, thousands of red knots, ruddy turnstones, and sanderlings descend on Delaware Bay in the United States to prey on the fat- and protein-rich horseshoe crab eggs.

Adaptations

Commercial Uses

Conservation Status

In the IUCN Red Data List, T. tridentatus is listed as Endangered, while L. polyphemus as Vulnerable, shedding light on the constantly shrinking population of horseshoe crabs. This drastic drop in their numbers could be primarily attributed to overexploitation and habitat loss.

The development of buildings, roads, and seawalls along the shorelines disrupts the natural habitat of horseshoe crabs and shrinks the space available for survival. In particular, bulkheads and seawalls block access to intertidal areas where horseshoe crabs typically spawn. Also, shoreline development through increased sedimentation, pollution runoff, and alterations to coastal hydrology can further lead to habitat degradation.

While most horseshoe crabs survive blood harvesting, fatalities often occur, depending on the amount of blood taken and the stress from handling and transport. The death rate after blood collection is reported to be between 3% and 30% in the United States, with about 500,000 horseshoe crabs harvested each year. Moreover, in countries like Thailand, Indonesia, and Malaysia, female T.gigas are cherished as a local delicacy, further decreasing their numbers.

Horseshoe crabs are a keystone species and are an essential food source for various marine predators, including shorebirds, fish, and crustaceans. 

In 2023, the U.S. Fish and Wildlife Service stopped the collection of horseshoe crabs in the Cape Romain National Wildlife Refuge in South Carolina from March 15 to July 15, keeping in mind the ecological role of these arthropods. Efforts are also being undertaken to create flexible management strategies to control the harvesting of horseshoe crabs.

References Article last updated on 30th March 2024
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