The Atlantic horseshoe crab (Limulus polyphemus), also known as the American horseshoe crab, is a species of horseshoe crab, a kind of marine and brackish chelicerate arthropod. It is found in the Gulf of Mexico and along the Atlantic coast of North America. The main area of annual migration is Delaware Bay along the South Jersey Delaware Bayshore.
Their eggs were eaten by Native Americans, but today Atlantic horseshoe crabs are caught for use as fishing bait, in biomedicine (especially for Limulus amebocyte lysate) and science. They play a major role in the local ecosystems, with their eggs providing an important food source for shorebirds, and the juveniles and adults being eaten by sea turtles.
The other three extant (living) species in the family Limulidae are also called horseshoe crabs, but they are restricted to Asia. Horseshoe crabs, despite their common name, are not crustaceans, and are more closely related to arachnids like spiders and scorpions.
Names and classification
This group of animals is also known as horsefoot, or saucepan. Some people call the horseshoe crab a "helmet crab", but this common name is more frequently applied to a true crab, a malacostracan, of the species Telmessus cheiragonus. The term "king crab" is sometimes used for horseshoe crabs, but it is more usually applied to a group of decapod crustaceans.
Limulus means "askew" and polyphemus refers to Polyphemus, a giant in Greek mythology. It is based on the misconception that the animal had a single eye. However, horseshoe crabs have multiple eyes, including two large compound eyes and many small ones. These eyes help them detect light and movement and are important for navigation and finding mates during spawning season.
Former scientific names include Limulus cyclops, Xiphosura americana, and Polyphemus occidentalis.
Studies of microsatellite DNA have revealed several discrete geographic populations in the Atlantic horseshoe crab. While there is extensive mixing between neighbouring populations, primarily due to movements by males, there is little or no mixing between the US and isolated Yucatán Peninsula horseshoe crabs, leading some to suggest that a taxonomic review is necessary.
Anatomy and physiology
Horseshoe crabs have three main parts to the body: the head region, known as the "prosoma", the abdominal region or "opisthosoma", and the spine-like tail or "telson". The smooth shell or carapace is shaped like a horseshoe, and is greenish grey to dark brown in colour. The sexes are similar in appearance, but females are typically 25 to 30% larger than the male in length and width, and can reach more than twice the weight. Females can grow up to in length, including tail, and in weight. There are also geographic differences: in the United States, there is a north–south cline in the size. Central animals, between Georgia and Cape Cod, are the largest. North of Cape Cod and south of Georgia they gradually become smaller. In Delaware Bay, females and males have an average carapace width of about and , respectively. In Florida, females average about and males . However, this north-south pattern does not exist in the Yucatán Peninsula, the southernmost population of the species, where some subpopulations are intermediate between the Delaware Bay and Florida horseshoe crabs, and others are smaller, averaging about the size of Florida horseshoe crabs.
Horseshoe crabs are not known to regrow entire limbs, but have been documented with small terminal pincer segments that appear to be regenerated terminal pincer segments on the walking feet.
A wide range of marine species become attached to the carapace, including algae, flat worms, mollusks, barnacles, and bryozoans, and horseshoe crabs have been described as 'walking museums' due to the number of organisms they can support. In areas where Limulus is common, the shells, exoskeletons or exuviae (molted shells) of horseshoe crabs frequently wash up on beaches, either as whole shells, or as disarticulated pieces.
The brain and the heart are located in the prosoma. On the underside of the prosoma, six pairs of appendages occur, the first of which (the small pincers or chelicerae) are used to pass food into the mouth, which is located in the middle of the underside of the cephalothorax, between the chelicerae. Although most arthropods have mandibles, the horseshoe crab is jawless, instead relying on the chelicerae to perform similar functions. The remaining ten additional limbs, the pedipalps, are used for locomotion on the sea floor. Males of the species develop clasping hooks on the first pair of pedipalps in order to hold on to the female during mating. The posterior pair end in structures used for burrowing into the sediment.Anatomy of the Horseshoe Crab, Maryland Department of Natural Resources. Retrieved 12 August 2008.
The telson (i.e., tail) is used by the horseshoe crab to steer during locomotion and to maintain stability while moving along the seafloor. The telson may also be used by L. polyphemus to right itself if flipped upside down. Horseshoe crabs are often flipped over by currents, and in this position they are vulnerable to suffocation.
Among other senses, they have a small chemoreceptor organ that senses smells on the triangular area formed by the exoskeleton beneath the body near the ventral eyes.
Vision Limulus has been extensively used in research into the physiology of vision. The Nobel Prize in Medicine was awarded in 1967 in part for research performed on the horseshoe crab eye.
A large compound eye with monochromatic vision is found on each side of the prosoma;Peak absorption is at 525 nm it has five simple eyes on the carapace, and two simple eyes on the underside, just in front of the mouth, making a total of nine eyes. The simple eyes are probably important during the embryonic or larval stages of the organism, and even unhatched embryos seem to be able to sense light levels from within their buried eggs. The less sensitive compound eyes, and the median ocelli, become the dominant sight organs during adulthood.
In addition, the tail bears a series of light-sensing organs along its length.
Each compound eye is composed of about 1000 receptors called ommatidia, complex structures consisting of upwards of 300 cells. The ommatidia are somewhat messily arranged, not falling into the ordered hexagonal pattern seen in more derived arthropods. Each ommatidium feeds into a single nerve fiber. Furthermore, the nerves are large and relatively accessible. This made it possible for electrophysiologists to record the nervous response to light stimulation easily, and to observe visual phenomena such as lateral inhibition working at the cellular level. More recently, behavioral experiments have investigated the functions of visual perception in Limulus. Habituation and classical conditioning to light stimuli have been demonstrated, as has the use of brightness and shape information by males when recognizing potential mates.
The retinula (literally, "small retina") cells of the ommatidium of the compound eye contain areas from which membranous organelles of conceivable size (rhabdomeres) extend. Rhabdomeres have tiny microvilli (tiny tubes extending out of the retinula) that interlock with neighboring retinular cells. This forms the rhabdom, which contains the dendrite of the eccentric cell, and may also contribute some microvilli. The only other species with an eccentric cell is the silkworm moth. Microvilli are composed of a double layer, 7 nm each and with 3.5 nm space of two electron-deficient boundaries in between. Where the microvilli meet, these outer borders fuse and yield five membranes about 15 nm thick. In all arthropods, there is always a rhabdom below a crystalline cone, on or near the center of the ommatidium, and always aligned with the path of light. At right angles to the length of the rhabdome are the length of the microvilli, which are in line with each other. The microvilli are about 40–150 nm in diameter."Photoreception" McGraw-Hill Encyclopedia of Science & Technology, vol. 13, p. 461 2007
Blood
The blood of horseshoe crabs (as well as that of most mollusks, including cephalopods and gastropods) contains the copper-containing protein hemocyanin at concentrations of about 50 g per liter. These creatures do not have hemoglobin (iron-containing protein), which is the basis of oxygen transport in vertebrates. Hemocyanin is colorless when deoxygenated and dark blue when oxygenated. As they generally live in cold environments with low oxygen tensions, their blood is grey-white to pale yellow, and it turns dark blue when exposed to the oxygen in the air, as seen when they bleed. Hemocyanin carries oxygen in extracellular fluid, which is in contrast to the intracellular oxygen transport in vertebrates by hemoglobin in red blood cells.
The blood of horseshoe crabs contains one type of blood cell, the amebocytes. These play an important role in the defense against pathogens. Amebocytes contain granules with a clotting factor known as coagulogen; this is released outside the cell when bacterial endotoxin is encountered. The resulting coagulation is thought to contain bacterial infections in the animal's semiclosed circulatory system.
Distribution and habitat
The Atlantic horseshoe crab is the only extant (living) species of horseshoe crab native to the Americas, although there are other extinct species only known from fossil remains from this region. The other living species of horseshoe crab are restricted to Asia, but all are quite similar in form and behavior. The Asian species are Tachypleus tridentatus, Tachypleus gigas and Carcinoscorpius rotundicauda.
Most Atlantic horseshoe crabs are found along the Atlantic East Coast of the United States, ranging from Maine to Florida. In the Gulf Coast of the United States, they are found in Florida, Alabama, Mississippi and Louisiana. Outside the United States, the only breeding population is in Mexico's Yucatán Peninsula, where it is found on the western, northern and eastern coasts. Individuals rarely appear outside the breeding range, with a few records from the Atlantic coast of Canada (Lahave Island on Nova Scotia), the Bahamas, the Turks and Caicos, Cuba and the western Gulf of Mexico (Texas and Veracruz). Historic claims of horseshoe crabs on Jamaica and Hispaniola (on the Dominican Republic's southeast coast) have not resulted in any discoveries after expeditions there. There have been attempts of introducing it to Texas, California and the southern North Sea, but these all failed to become established. Records from Europe, Israel and Western Africa are considered releases/escapees of captives.
Atlantic horseshoe crabs range from shallow coastal habitats such as lagoons, estuaries and mangrove to depths of more than up to offshore. There are indications that they prefer depths shallower than . The temperature preference varies depending on the population, with the northernmost being the most cold-resistant: in the Great Bay in New Hampshire they show increased activity above and in Delaware Bay they are active above . In contrast, these northern populations do not tolerate as warm temperatures as southern populations of the species. Atlantic horseshoe crabs can be seen in waters that range from brackish (almost fresh water) to hypersaline (almost twice the salinity of sea water), but their optimum growth is at salinities around or slightly below sea water (20–40‰).
A 2022 study of ancient (Early Pleistocene, 2 million years ago) environmental DNA from the Kap Kobenhavn Formation of northern Greenland identified preserved DNA fragments of horseshoe crabs, assigned to L. polyphemus. This suggests that horseshoe crabs ranged and spawned as far north as Greenland during these warmer conditions. Around this time, the sea surface temperature would have been 8 °C warmer than the present. These are among the oldest DNA fragments ever sequenced.