American horseshoe crab
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. Despite the name, horseshoe crabs are more closely related to arachnids like spiders and scorpions than they are to crabs (or any crustacean).
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.
Former scientific names include Limulus cyclops, Xiphosura americana, and Polyphemus occidentalis.
It is the tail that earns this order its name Xiphosura, which derives from the Greek for "sword tail".
Studies of microsatellite DNA have revealed several distinct geographic groups 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.
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 60 cm (24 in) in length, including tail, and 4.8 kg (11 lb) 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 25.5 cm (10.0 in) and 20 cm (7.9 in), respectively. In Florida, females average about 21.5 cm (8.5 in) and males 16 cm (6.3 in). 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.mw-parser-output.frac{white-space:nowrap}.mw-parser-output.frac.num,.mw-parser-output.frac.den{font-size:80%;line-height:0;vertical-align:super}.mw-parser-output.frac.den{vertical-align:sub}.mw-parser-output.sr-only{border:0;clip:rect(0,0,0,0);clip-path:polygon(0px 0px,0px 0px,0px 0px);height:1px;margin:-1px;overflow:hidden;padding:0;position:absolute;width:1px}3⁄4 the size of Florida horseshoe crabs.
Horseshoe crabs regrow lost limbs similar to many other arthropods.
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.
The second pair of appendages, the pedipalps, are used as walking legs; in males they are tipped with 'claspers', which are used during mating to hold onto the female's carapace. The remaining four pairs of appendages are the 'pusher legs', also used in locomotion. The first four pairs of legs have claws, the last pair has a leaf-like structure used for pushing.
The opisthosoma bears a further six pairs of appendages; the first pair houses the genital pores, while the remaining five pairs are modified into flattened plates, known as book gills, that allow them to breathe underwater, and can also allow them to breathe on land for short periods of time, provided the gills remain moist.
The telson (i.e., tail or caudal spine) is used to steer in the water and also to flip itself over if stuck upside down.
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.
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 200 m (660 ft) up to 56 km (35 mi) offshore. There are indications that they prefer depths shallower than 30 m (98 ft). 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 10.5 °C (51 °F) and in Delaware Bay they are active above 15 °C (59 °F). 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.
The crab feeds on mollusks, annelid worms, other benthic invertebrates, and bits of fish. Lacking jaws, it grinds up the food with bristles on its legs and a gizzard that contains sand and gravel.
Spawning tends to occur in the intertidal zone and to be correlated with spring tides (the highest tides of the month). The breeding season varies. Northern populations (all in the United States, except those in southern Florida) generally breed from the spring to the autumn, while southern populations (southern Florida and Yucatán Peninsula) breed year-round. In the north, breeding is triggered by a temperature increase, but this is reversed in the Yucatán Peninsula, the southernmost population, where decreasing temperatures stimulate breeding. In Massachusetts, horseshoe crabs spend the winters on the continental shelf and emerge at the shoreline in late spring to spawn, with the males arriving first. The smaller male grabs on to the back of a female with a "boxing glove" like structure on his front claws, often holding on for months at a time. Often several males will hold on to a single female. Females reach the beach at high tide. After the female has laid a batch of eggs in a nest at a depth of 15–20 cm (6–8 in) in the sand, the male or males fertilize them with their sperm. Egg quantity is dependent on the female's body size, and ranges from 15,000 to 64,000 eggs per female.
Development begins when the first egg cover splits and new membrane, secreted by the embryo, forms a transparent spherical capsule. The larvae form and then swim for about five to seven days. After swimming, they settle, and begin the first molt. This occurs about 20 days after the formation of the egg capsule. As young horseshoe crabs grow, they move to deeper waters, where molting continues. Before becoming sexually mature around age 9, they have to shed their shells some 17 times. In the first 2–3 years of their life, the juveniles stay in shallow coastal waters near the breeding beaches. Longevity is difficult to assess, but the average lifespan is thought to be 20–40 years.
Research from the University of New Hampshire gives insight into the circadian rhythm of Atlantic horseshoe crabs. For example, several studies have looked into the effect of a circa tidal rhythm on the locomotion of this species. While it has been known for a while that a circadian clock system controls eye sensitivity, scientists discovered a separate clock system for locomotion. When a sample of Atlantic horseshoe crabs were exposed to artificial tidal cycles in the lab, circa tidal rhythms were observed. The study found that light and dark cycles influence locomotion, but not as much as tidal activity.
Overall the Atlantic horseshoe crab is recognized as vulnerable by the IUCN due primarily to overharvesting and habitat loss. There are, however, significant geographic differences with some populations increasing, some stable and some declining.
