Horseshoe crabs are ancient mariners that have existed for millions of years. These arthropods are more closely related to spiders and scorpions than true crabs. Horseshoe crabs play a vital role in coastal ecosystems, and their unique physiology has made them invaluable to the biomedical industry. This article delves into the fascinating world of horseshoe crabs, exploring their dietary preferences, feeding strategies, and ecological significance.
Horseshoe Crab Classification and Distribution
Horseshoe crabs are arthropods of the family Limulidae and the only surviving xiphosurans. Despite their name, they are not crabs or even crustaceans; they are chelicerates, more closely related to arachnids like spiders, ticks, and scorpions. There are only four species of horseshoe crab extant today. The North American species, Limulus polyphemus, is commonly found along the Atlantic and Gulf coasts from Maine to Mexico, including Florida's beaches. The other three species reside in Southeast Asia. Most are marine, though the mangrove horseshoe crab is often found in brackish water, and the Atlantic horseshoe crab is resident in brackish estuarine ecosystems such as the Delaware and Chesapeake bays. Additionally, certain extinct species transitioned to living solely in freshwater.
Anatomy and Sensory Perception
The body of a horseshoe crab is divided into three main parts: the cephalothorax, abdomen, and telson. The largest of these, the cephalothorax, houses most of the animal's eyes, limbs, and internal organs. It is also where the animal gets its name, as its shape somewhat resembles that of a horseshoe. The upper side is smoothly curved. Like all arthropods, horseshoe crabs have segmented bodies with jointed limbs, which are covered by a protective cuticle made of chitin. Horseshoe crabs are chelicerates, meaning their bodies are composed of two main parts (tagmata): the cephalothorax and the opisthosoma. The first tagma, the cephalothorax or prosoma, is a fusion of the head and thorax. This tagma is also covered by a large, semicircular carapace that acts as a shield around the animal's body. In total, horseshoe crabs have 6 pairs of appendages on their cephalothorax.
The first of these are the chelicerae, which give chelicerates their name. In horseshoe crabs, these look like tiny pincers in front of the mouth. Behind the chelicerae are the pedipalps, which are primarily used as legs. In the final molt of males, the ends of the pedipalps are modified into specialized, grasping claws used in mating. Following the pedipalps are three pairs of walking legs and a set of pusher legs for moving through soft sediment. Each of these pusher legs is biramous or divided into two separate branches. The branch closest to the front bears a flat end that looks like a leaf. This end is called the flabellum. The branch towards the back is far longer and looks similar to a walking leg. However, rather than ending in just a claw, the back branch has four leaf-like ends that are arranged like a petal. The final segment of the cephalothorax was originally part of the abdomen but fused in the embryo.
The opisthosoma or abdomen of a horseshoe crab is composed of several fused segments. Similar to a trilobite, the abdomen is made up of three lobes: a medial lobe in the middle, and a pleural lobe on either side. Attached to the perimeter of each pleural lobe is a flat, serrated structure known as the flange. The flange on either side is connected by the telson embayment, which itself is attached to the medial lobe. Along the line where these lobes meet are six sets of indentations known as apodeme. On the underside of the abdomen are several biramous limbs. The branches closest to the outside are flat and broad, while the ones on the inside are more narrow. Closest to the front is a plate-like structure made of two fused appendages. This is the genital operculum and is where horseshoe crabs keep their reproductive organs. Following the operculum are five pairs of book gills. While mainly used for breathing, horseshoe crabs can also use their book gills to swim. At the end of a horseshoe crab's abdomen is a long, tail-like spine known as a telson.
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Despite the ferocious look of the tail, it is not used as a weapon. Instead, horseshoe crabs use their tails for righting themselves if they are flipped over by a wave. *Never pick up a horseshoe crab by its tail, as it can harm the animal. Instead, gently pick it up by both sides of the prosoma using both hands. Though the horseshoe crab's shell is hard, it is very sensitive to the world around it. The crabs are especially sensitive to light. Horseshoe crabs have a variety of eyes that provide them with useful visual information. The most obvious of these are two large compound eyes found on top of the carapace. This feature is unusual, as all other living chelicerates have lost them in their evolution. In adult horseshoe crabs, the compound eyes comprise around 1,000 individual units known as ommatidia. Each ommatidium is made up of a ring of retinal and pigment cells that surround something known as the eccentric cell. A horseshoe crab's compound eyes are less complex and organized than those of most other arthropods.
Ommatidia are arranged messily in what's been deemed an "imperfect hexagonal array" and have a highly variable number of photoreceptors (between 4 and 20) in their retina. Although each ommatidium typically has one eccentric cell, there are sometimes two, and occasionally more. All the eye's photoreceptors, both rods and cones, have a single visual pigment with a peak absorption of around 525 nanometers. At the front of the animal along the cardiac ridge are a pair of eyes known as median ocelli. Their retina is even less organized than those of the compound eyes having between 5 and 11 photoreceptors paired with one or two secondary visual cells called arhabdomeric cells. Arhabdomeric cells are equivalent to eccentric cells as they function identically. The median ocelli are unique due to having two distinct visual pigments. Other, more rudimentary eyes in horseshoe crabs include the endoparietal ocelli, the two lateral ocelli, two ventral ocelli, and a cluster of photoreceptors on the abdomen and telson.
The endoparietal, lateral, and ventral ocelli are very similar to the median ocelli, except like the compound eyes, they only see in visual light with a peak absorbance of around 525 nanometers. The endoparietal eye further differs due to being a fusion of two separate ocelli. This eye is found not far behind the median eyes and sits directly on the cardiac ridge. The two ventral ocelli are located on the underside of the cephalothorax near the mouth and likely help to orient the animal when walking around or swimming. The lateral eyes can be found directly behind the compound eyes and become functional just before a horseshoe crab larvae hatch. The telson's photoreceptors are unique as they're spaced throughout the structure rather than located in a fixed spot. In spite of all these “eyes”, the crab’s vision is poor! Only movement is detected in the daylight and perhaps a blurry image by the compound eyes.
Dietary Habits and Feeding Strategies
Horseshoe crabs are omnivores that primarily feed on smaller sea creatures. They eat worms, small crustaceans, and algae. Their diet mainly consists of worms, clams, crustaceans, and other small animals they can find. They also feed on small mollusks, (mostly clams), but also feed on worms, crustaceans, and dead fish. Additionally, they are believed to occasionally eat algae when the situation demands it. Because they have no mandibles or teeth, they crush hard food between their legs before passing it to their mouth. Like birds, horseshoe crabs also have gizzards for grinding food before it reaches their stomachs.
Horseshoe crabs live a primarily benthic lifestyle, preferring to stay at the water's bottom. During the evening hours, they scurry along the seafloor, looking for morsels and other small animals that they can crush up and eat. They generally hunt at night and sweep the seafloor and coastal edges, looking for any slow-moving or stationary prey, things like clams or worms. Once a horseshoe crab finds prey, it will grasp it with strong legs and begin breaking it apart. The goal is to crush the food up so they can pass it into their mouths in small pieces. The only exception to this method of eating is algae since it’s soft enough to be eaten without being crushed up. A secondary method that horseshoe crabs use to process their food is through a primitive version of a gizzard. It’s similar to that of a bird and contains small hard things that the food then passes through. Like a grinder, the gizzard helps to further process the food before it passes into the stomach.
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Ecological Importance
Horseshoe crabs are an important part of the ecology of coastal communities. Their eggs are the major food source for shorebirds migrating north, including the federally-threatened red knot. These shorebirds have evolved to time their migrations to coincide with peak horseshoe crab spawning activity, especially in the Delaware and Chesapeake Bay areas. They use these horseshoe crab beaches as a gas station, to fuel up and continue their journey. A low horseshoe crab population in Delaware Bay is hypothesized to endanger the future of the red knot. The eggs of the horseshoe crab are an essential element to many species’ diets around the world, and with potentially declining numbers of crabs, other species populations are threatened. One example is the red knot, a migratory bird that relies on its eggs as they travel 9,000 each migratory cycle. With reduced crab numbers, the red knot is threatened.
Threats to Horseshoe Crabs
Horseshoe crab numbers are declining throughout much of their range. Threats to horseshoe crabs include habitat destruction for shoreline development, use in fishing, plastic pollution, status as a culinary delicacy, and use in research and medicine. Development along shorelines is dangerous to horseshoe crab spawning, limiting available space and degrading habitat.
In recent years, horseshoe crabs have experienced a population decline. This is mainly due to coastal habitat destruction and overharvesting. Horseshoe crabs are often caught for their blood, which contains Limulus amebocyte lysate, a chemical used to detect bacterial endotoxins. Additionally, the animals are used as fishing bait in the United States and eaten as a delicacy in some parts of Asia. In 1998, The Atlantic States Marine Fisheries Commission developed a Horseshoe Crab Fishery Management Plan that requires all Atlantic coastal states to identify horseshoe crab nesting beaches.
The population of Indo-Pacific horseshoe crabs (Tachypleus gigas) in Malaysia and Indonesia has decreased dramatically since 2010. This is primarily due to overharvesting, as horseshoe crabs are considered a delicacy in countries like Thailand. The individuals most likely to be targeted are gravid females, as they can be sold for both their meat and eggs. One species, the tri-spine horseshoe crab (Tachypleus tridentatus), has already been declared locally extinct in Taiwan. Facing a greater than 90% decrease in T. tridentatus juveniles, it is suspected that Hong Kong will be the next to declare tri-spine horseshoe crabs as extinct from the area.
Horseshoe Crabs and the Biomedical Industry
Horseshoe crabs are also extremely important to the biomedical industry because their unique, copper-based blue blood contains a substance called "Limulus Amebocyte Lysate", or "LAL". This compound coagulates or clumps up in the presence of small amounts of bacterial toxins and is used to test for sterility of medical equipment and virtually all injectable drugs. That way, when you get a vaccine you know it hasn’t been contaminated by any bacteria. Anyone who has had an injection, vaccination, or surgery has benefited from horseshoe crabs! Only horseshoe crabs have a blood-clotting agent known as Limulus Amebocyte Lysate, or LAL, which clots in the presence of certain groups of bacteria. These bacteria are difficult to detect by other means. According to the biomedical industry, up to 30% of an individual's blood is removed. The good news is that up to one-third of a horseshoe crab's blood can be removed without killing the animal. Horseshoe crabs provide an interesting medical substance through their blood. Their blood contains important immune cells that are used to detect certain antibodies, allowing humans to test for certain diseases. Biomedical groups harvest the crabs, bleed them for around 20-30% of their blood content, and then release them 2-3 days later. There have been some reports that the collectors are just selling the crabs for bait instead of releasing them, further threatening populations.
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Reproduction and Life Cycle
Horseshoe crabs are known to gather in large nesting aggregations, or groups, on beaches particularly in the mid-Atlantic states such as Delaware, New Jersey and Maryland in the spring and summer, where their populations are largest. Spawning occurs in late spring or early summer at high tides typically during new and full moons. Adult crabs migrate from deeper water to shallow flats or beaches in large numbers, females first. Females then secrete a pheromone to attract a male who uses the hook-like claspers at the tips of his first pair of walking “legs” to attach to the female, hitching a ride to the high tide line. When mating, the smaller male crab hooks himself to the top of the larger female’s shell by using his specialized front claws, and together they crawl to the beach. Often other males will attach to the first male until a chain has formed that the female has to drag to the spawning area. Some males (called satellite males) do not attach to females but still have success in fertilizing the female's eggs by hanging around the attached pair.
The female digs four to five nests in the sand every few feet in which she deposits a cluster of as many as 3,600 eggs that are externally fertilized by the male. In the meantime, the female digs a hole in the sediment and lays between 2,000 and 30,000 large eggs. Following fertilization, the female covers the eggs with sandy. Tailless green, free-swimming larval typically hatch two to four weeks later. Horseshoe crab larvae emerge from their nests several weeks after the eggs are laid. Molting begins during the egg stage. Embryos moll four times at 4, 8, 11 and 13 days after egg fertilization. The next molting occurs 20 days after the larvae emerge. At each molt they grow about 25 percent in length. There are 16 molts in the male and 17 in the female before adulthood is reached. Juvenile horseshoe crabs look a lot like adults except that their tails are smaller. The young and adult horseshoe crabs spend most of their time on the sandy bottoms of inter-tidal flats or zones above the low tide mark and feed on various invertebrates.
Baby horseshoe crabs begin their lives as a "trilobite larvae", a name given due to their resemblance to a trilobite. Upon hatching, larva typically measure around 1 cm (1⁄2 in) long. As the larvae molt into juveniles, their telson gets longer and they gain their missing book gills. Juveniles can attain a carapace width of around 4 cm (1+1⁄2 in) in their first year. 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. On average, males of C. rotundicauda are about 30 centimeters (12 inches) long, including a telson that is about 15 cm (6 in), and a carapace about 15 cm (6 in) wide. Some southern populations (in the Yucatán Peninsula) of L. polyphemus are somewhat smaller, but otherwise, this species is larger. In the largest species, T. tridentatus, females can reach as much as 79.5 cm (31+1⁄4 in) long, including their telson, and up to 4 kg (9 lb) in weight. This is only about 10-20 cm (4-8 in) longer than the largest females of L. polyphemus and T. When mating, the smaller male clings to the back or opisthosoma of the larger female using specialized pedipalps. This typically leaves scars, the absence of which allows younger females to be easily identified.
Horseshoe Crabs as Food
Though they have little meat, horseshoe crabs are valued as a delicacy in some parts of East and Southeast Asia. The meat is white, has a rubbery texture similar to that of lobsters, and possesses a slightly salty aftertaste. Horseshoe crab can be eaten both raw and cooked, but must be properly prepared to prevent food poisoning. Furthermore, only certain species can be eaten. In addition to their meat, horseshoe crabs are valued for their eggs. Much like the meat, only the eggs of specific species can be eaten.
Conservation Efforts
Currently, with the help of the public, biologists at the Fish and Wildlife Research Institute are documenting nesting sites of horseshoe crabs throughout the state. If you see horseshoe crabs mating and would like to report a sighting, please visit the Report Sightings page for more information. Impacts of climate change may affect horseshoe crabs-loss of spewing areas due to sea level rise and ocean acidification’s affecting shell development.