American Horseshoe Crab

El límul^[1] (Limulus polyphemus) és una espècie de xifosur de la família Limulidae. És l'únic membre del gènere Limulus.

Característiques

Malgrat el seu aspecte de cranc és un artròpode quelicerat, i per tant més proper als aràcnids (aranyes, escorpins, etc.) que als crancs (crustacis). Viu al mar, a les costes atlàntiques de Nord-amèrica fins al golf de Mèxic.

Característiques

Un nen amb un límul, mostrant-ne la mida

Els límuls tenen el cos aplanat i estan recobertes d'un gruixut exoesquelet que forma una autèntica cuirassa. Poden assolir una llarga de 60 cm. El seu cos té dues regions ben definides, el prosoma o cefalotòrax i l'opistosoma o abdomen, que acaba en una llarga "cua".

• Prosoma o cefalotòrax. El prosoma té forma de ferradura i està cobert per una closca dura en forma d'escut que a la part dorsal té un ull compost a cada banda, i un parell de petits ulls simples al mig; a la part ventral es troba la boca, al voltant de la qual hi ha un parell de quelícers acabats en pinça; a continuació, hi ha els pedipalps i quatre parells de potes també acabades en una pinça, excepte l'últim que té la punta aplanada i serveix per a recolzar-se sobre els fons tous. A l'interior del prosoma està el cervell, el cor i la part anterior del tub digestiu.
• Opistosoma o abdomen. Té forma hexagonal i sis espines a cada costat; a la part ventral, té les obertures genitals cobertes per una placa denominada opercle i cinc parells de brànquies en llibre, anomenades així perquè estan formades per moltes làmines superposades que recorden les fulles d'un llibre; les brànquies els permeten respirar sota l'aigua, però també sobreviure a terra ferma durant curts períodes de temps. A l'última part de l'abdomen, tenen una "cua" llarga, rígida i acabada en punta denominada telson.

La sang dels Limulus és de color blau, ja que conté hemocianina, un pigment amb coure que li atorga aquest color; l'hemocianina transporta l'oxigen des de les brànquies als òrgans de l'animal. La seva carn és verinosa.

Biologia i ecologia

Els límuls viuen al mar, en aigües poc profundes, generalment en fons sorrencs entre els 3 i els 9 metres de fondària, per on repten i excaven, enterrant-se just per sota de la superfície. Són carnívores i s'alimenten de mol·luscs, cucs (anèl·lids), altres invertebrats marins i d'animals morts, que subjecten i esmicolen amb els quelícers. Neden en posició invertida, amb les potes cap amunt, agitant les brànquies, i s'ajuden amb la cua per girar-se quan volen caminar pel fons.

Són ovípares i amb fecundació externa. Al moment de la reproducció, abandonen el mar i migren cap a la platja, on s'apleguen un gran nombre d'individus; els primers a arribar són els mascles; després arriben les femelles, que fan un forat d'uns 15 o 20 cm a la sorra; en aquests nius, dipositen entre 15.000 i 64.000 ous,^[2] que seran fertilitzats pels mascles. De l'ou neix una larva, anomenada trilobítica perquè recorda un trilobit, que retorna al mar, i que només té dos parells de brànquies i no té la llarga cua de l'adult; després de nombroses mudes de l'exoesquelet assoleixen l'estat adult. Al cap d'11 anys, retornen a la platja per reproduir-se. Poden viure fins a 25 anys.

Els limuls poden regenerar les potes perdudes, cosa gens habitual entre els artròpodes.

Evolució

Els límuls són parents llunyans dels aràcnids actuals (aranyes, escorpins) i possiblement són descendents dels antics euriptèrids (escorpins marins), que van evolucionar en mars poc profunds durant el Paleozoic, fa entre 540 i 280 milions d'anys, junt amb altres artròpodes primitius, com ara el trilobits.

Són un dels artròpodes actuals més antics, ja que van aparèixer fa uns 350 o 400 milions d'anys i, des de llavors, han canviat molt poc; és per això que es consideren com a fòssils vivents.

Espècies semblants

En l'actualitat existeixen altres tres espècies de xifosurs: Tachypleus gigas del Japó, Tachypleus tridentatus i Carcinoscorpius rotundicauda, que es troben al sud-est asiàtic.

• Una femella vista per la part ventral

• Limulus polyphemus, segons Haeckel

• Limulus polyphemus vist de front

• Limulus polyphemus

Referències

Vegeu també

• Paraisobuthus

Ostrorep americký (Limulus polyphemus), známý též pod názvem krab trnitý, je hrotnatec, který žije zejména v Mexickém zálivu a na severním atlantském pobřeží Severní Ameriky. Spolu s ostatními ostrorepy je považován za možného nejbližšího žijícího příbuzného trilobitů.^[2]

Inspirace v umění

Francouzský autor Jacques Tardi využil ostrorepa amerického jako působivou rekvizitu. V komiksu Le Mystère des profondeurs (Tajemství hlubin), osmém dílu série Les Aventures extraordinaires d'Adèle Blanc-Sec (Neobyčejná dobrodružství Adély Blanc–Sec), přemnožení obrovští ostrorepové strašili Paříž v roce 1922.^[3]

Odkazy

Reference

1. ↑ Červený seznam IUCN 2018.1. 5. července 2018. Dostupné online. [cit. 2018-08-10]
2. ↑ ZOO Olomouc: Ostrorep americký
3. ↑ TARDI, Jacques. Les Aventures extraordinaires d'Adèle Blanc-Sec, Le Mystère des profondeurs. Paris: Castermann, 1998. Dostupné online.

Externí odkazy

• Obrázky, zvuky či videa k tématu ostrorep americký ve Wikimedia Commons

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.^[1] It is found in the Gulf of Mexico and along the Atlantic coast of North America.^[1] The main area of annual migration is Delaware Bay along the South Jersey Delaware Bayshore.^[2]

Their eggs were eaten by Native Americans,^[3] but today Atlantic horseshoe crabs are caught for use as fishing bait, in biomedicine (especially for Limulus amebocyte lysate) and science.^[1] 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.^[1][2]

The other three extant (living) species in the family Limulidae are also called horseshoe crabs, but they are restricted to Asia.^[4] Horseshoe crabs are more closely related to spiders and scorpions than they are to crabs ^[5]

Names and classification

Atlantic horseshoe crab with attached Crepidula shells on the Delaware Bay beach in Villas, New Jersey.

Underside view of a living male crab, showing the mouth, gills and legs

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"^[6] and polyphemus refers to Polyphemus, a giant in Greek mythology.^[6] 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.^[3][7]

Anatomy and physiology

Underside view: The mouth opening is between the legs, and the gills are visible below.

Underside of a female showing the legs and book gills.

Underside of a male, showing the first leg modified for grasping the female during copulation.

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.^[8][9] Females can grow up to 60 cm (24 in) in length, including tail,^[10] and 4.8 kg (11 lb) in weight.^[9] 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.^[11][12] 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).^[3] 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 3⁄4 the size of Florida horseshoe crabs.^[3]

Horseshoe crabs possess the rare ability to regrow lost limbs, in a manner similar to sea stars.^[13]

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.^[14] 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.^[15]

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.^[16]

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;^{[note 1][17]} it has five simple eyes on the carapace, and two simple eyes on the underside, just in front of the mouth,^[17] making a total of nine eyes. The simple eyes are probably important during the embryonic or larval stages of the organism,^[17] and even unhatched embryos seem to be able to sense light levels from within their buried eggs.^[18] The less sensitive compound eyes, and the median ocelli, become the dominant sight organs during adulthood.^[17]

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,^[15] complex structures consisting of upwards of 300 cells.^[18] The ommatidia are somewhat messily arranged, not falling into the ordered hexagonal pattern seen in more derived arthropods.^[17] 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.

• Clam shells
• Chiton shells
• Snail shells
• Tusk shells

• Conchology
• Nacre
• Valve

• Brachiopod shells
• Crustacean shells
• Horseshoe crabs
• Sea urchin tests

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.^[19]

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.^[20] 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. The blood in the circulation of these creatures, which generally live in cold environments with low oxygen tensions, is grey-white to pale yellow,^[20] and it turns dark blue when exposed to the oxygen in the air, as seen when they bleed.^[20] Hemocyanin carries oxygen in extracellular fluid, which is in contrast to the intracellular oxygen transport in vertebrates by hemoglobin in red blood cells.^[20]

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.^[21] The resulting coagulation is thought to contain bacterial infections in the animal's semiclosed circulatory system.^[22]

Distribution and habitat

Atlantic horseshoe crabs in Mexico, such as this pair at Holbox Island, mostly breed in lagoons with mangrove and seagrass^[3]

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.^[4][23]

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.^[1] 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.^[3] 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.^[1][3] There have been attempts of introducing it to Texas, California and the southern North Sea, but these all failed to become established.^[1] Records from Europe, Israel and Western Africa are considered releases/escapees of captives.^[1]

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.^[1] There are indications that they prefer depths shallower than 30 m (98 ft).^[24] 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).^[1] In contrast, these northern populations do not tolerate as warm temperatures as southern populations of the species.^[12] 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‰).^[1][25][26]

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.^[27][28]

Life cycle and behavior

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.^[8]

Spawning tends to occur in the intertidal zone and to be correlated with spring tides (the highest tides of the month).^[1] 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.^[1] 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.^[1] 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.^[29] Females reach the beach at high tide.^[29] 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.^[29] Egg quantity is dependent on the female's body size, and ranges from 15,000 to 64,000 eggs per female.^[30]

"Development begins when the first egg cover splits and new membrane, secreted by the embryo, forms a transparent spherical capsule" (Sturtevant). 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.^[8] In the first 2–3 years of their life, the juveniles stay in shallow coastal waters near the breeding beaches.^[3] Longevity is difficult to assess, but the average lifespan is thought to be 20–40 years.^[31]

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.^[32] 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.^[33]

Evolution

Horseshoe crabs were traditionally grouped with the extinct eurypterids (sea scorpions) as the Merostomata. However, recent studies suggest a relationship between the eurypterids and the arachnids, leaving Xiphosura in the clade Prosomapoda.^[34] They may have evolved in the shallow seas of the Paleozoic Era (541–252 million years ago) with other primitive arthropods like the trilobites. The four species of horseshoe crab are the only remaining members of the Xiphosura, one of the oldest classes of marine arthropods.

The oldest known horseshoe crab, Lunataspis aurora, 4 centimetres (1.6 in) from head to tail-tip, has been identified in 445-million-year-old Ordovician strata in Manitoba.^[35]

Horseshoe crabs are often referred to as living fossils, as they have changed little in the last 445 million years.^[8] Forms almost identical to this species were present during the Triassic period 230 million years ago, and similar species were present in the Devonian, 400 million years ago. However, horseshoe crab preservation in the fossil record is extremely rare^[36] and the Atlantic horseshoe crab itself has no fossil record at all. Until recently it was thought that the genus Limulus "ranges back only some 20 million years, not 200 million".^[37]

The oldest member of the subfamily Limulinae are known from the Late Jurassic (Tithonian), belonging to the species Crenatolimulus darwini from Poland. The oldest and currently only other known species in the genus Limulus is Limulus coffini from the Late Cretaceous (Maastrichtian) of the United States.^[38]

Medical uses

Horseshoe crabs are valuable as a species to the medical research community, and in medical testing. An extract of blood cells (amoebocytes) from Limulus polyphemus is a critical component in the widely used Limulus amebocyte lysate (LAL) test to detect and quantify bacterial endotoxins in pharmaceuticals and to test for several bacterial diseases.^[6] A protein from horseshoe crab blood is also under investigation as a novel antibiotic.^[29]

Procuring the raw materials for LAL testing involves collecting and bleeding horseshoe crabs from wild populations. Horseshoe crabs are returned to the ocean after bleeding, however, there is a level of mortality and sub-lethal impact involved. It is estimated that between 10 and 30 percent of horseshoe crabs die after bleeding.^[39][40]

Studies show the blood volume returns to normal in about a week, though blood cell count can take two to three months to fully rebound.^[41] Animals that survive the process may be more lethargic when released and less likely to mate – which has prompted concerns about the longer-term impacts of harvesting horseshoe crabs.^[42]

The LAL test is a major source of animal product dependence in the biomedical industry, and a challenge to the Three Rs of science in relation to the use of animals in testing.^[43] There are efforts to reduce the dependence on horseshoe crabs, refine the process of collecting blood from the animals (including through aquaculture),^[44] and even replace the use of animal-derived assays utilizing synthetic approaches, such as the recombinant factor C (rFC) assay.^[45]

Conservation and management

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.^[1]

United States

Horseshoe crab harvest, Delaware, circa 1926. Millions of crabs were harvested for fertilizer around Delaware Bay. Some were fed to hogs.^[46]

The loggerhead sea turtle has suffered from the reduction in numbers of Atlantic horseshoe crabs

Early in the 20th century and possibly before, there was the mistaken belief in some areas that horseshoe crabs were destructive to fisheries, folklore held that they used their long spines to drill into some shellfish. Because of this mistaken belief and folklore, bounties were sometimes offered by authorities for them. On Cape Cod in the early 20th century five cents was offered for every dead horseshoe crab turned in.

The Atlantic horseshoe crab is not presently endangered, but harvesting and habitat destruction have reduced its numbers at some locations and caused some concern for this animal's future. Since the 1970s, the horseshoe crab population has been decreasing in some areas, due to several factors, including the use of the crab as bait in eel, whelk and conch trapping. According to the Horseshoe Crab Benchmark Stock Assessment Peer Review Report published by the Atlantic States Marine Fisheries Commission (ASMFC), the population continues to remain stable where biomedical is present in the Northeast and thrive and grow in the Southeast due to protection efforts – a trend spanning decades.^[47]

Red knots feeding on horseshoe crab eggs in Delaware

Conservationists have also voiced concerns about the declining population of shorebirds, such as red knots, which rely heavily on the horseshoe crabs' eggs for food during their spring migration. Precipitous declines in the population of the red knots have been observed in recent years. Predators of horseshoe crabs, such as the currently threatened Atlantic loggerhead turtle, have also suffered as crab populations diminish.^[48]

In 1991, the species was provided legislated protection from bait fishing in South Carolina by calling on the management and regulation of the horseshoe crab fisheries, allowing only hand-collecting for biomedical applications and marine biological research.^[49] Without the need for LAL in biomedical use, the legal protection of the horseshoe crab is not guaranteed in the future, and they would again fall prey to overfishing and use as bait. In 1995, the nonprofit Ecological Research and Development Group (ERDG) was founded with the aim of preserving the four remaining species of horseshoe crab.^[50] Since its inception, the ERDG has made significant contributions to horseshoe crab conservation. ERDG founder Glenn Gauvry designed a mesh bag for whelk/conch traps, to prevent other species from removing the bait. This has led to a decrease in the amount of bait needed by approximately 50%. In the state of Virginia, these mesh bags are mandatory in whelk/conch fishery. The Atlantic States Marine Fisheries Commission in 2006 considered several conservation options, among them being a two-year ban on harvesting the animals, affecting both Delaware and New Jersey shores of Delaware Bay.^[51] In June 2007, Delaware Superior Court Judge Richard Stokes has allowed limited harvesting of 100,000 males. He ruled that while the crab population was seriously depleted by overharvesting through 1998, it has since stabilized, and that this limited take of males will not adversely affect either horseshoe crab or red knot populations. In opposition, Delaware environmental secretary John Hughes concluded that a decline in the red knot bird population was so significant that extreme measures were needed to ensure a supply of crab eggs when the birds arrived.^[52][53] Harvesting of the crabs was banned in New Jersey on March 25, 2008.^[54]

Every year, about 10% of the horseshoe crab breeding population dies when rough surf flips the creatures onto their backs, a position from which they often cannot right themselves. In response, the ERDG launched a "Just Flip 'Em" campaign, in the hopes that beachgoers will simply turn the crabs back over.^[55] New Jersey beaches campaign "ReTURN the Favor," trains volunteers to rescue impinged and overturned horseshoe crabs while collecting data on natural and man-made hazards.^[56]

A large-scale project to tag and count horseshoe crabs along the North American coast was started in 2008, termed Project Limulus.^[8] Due to the lack of information and knowledge regarding horseshoe crab populations, the management policies lack any abundance of rules and regulations. To implement management policies for the species, more population information needs to be obtained.^[57]

Mexico

Since 1994, the populations in the Yucatán Peninsula have been recognized as endangered under Mexican law. They have declined since the 1960s and remaining significant Yucatán populations are mostly within protected areas.^[3]

Notes

References

El cangrejo herradura del Atlántico o cangrejo cacerola (Limulus polyphemus) es una especie de quelicerado de la clase Merostomata. Llega a alcanzar 60 cm de largo y 30 cm de ancho. A pesar de su nombre, esta especie está más próxima a las arañas y escorpiones (arácnidos), que a los cangrejos (crustáceos),^[3]​ con los que no guarda ninguna relación. Habita en las zonas costeras y los estuarios fluviales.

Se encuentra amenazado de extinción y reduciendo sus poblaciones de una forma lenta pero constante. Las causas que se han relacionado con esta situación son el cambio climático que acelera las emisiones de gases de invernadero y también la sobrepesca y captura para aprovechamiento por las farmacéuticas de su hemolinfa (sangre) en test de toxicidad de medicamentos.^[4]​^[5]​

Nombres comunes

Los cangrejos herradura ^[6]​ se conocen también como cangrejos cacerola,^[7]​ cangrejos bayoneta,^[8]​^[9]​ cacerola de Las Molucas,^[10]​ cacerolitas de mar,^[11]​ cangrejos del Japón, tanquecitos de mar, cucarachas marinas (nombre vulgar que comparte con las especies Chiton y Squilla mantis), ostroreps^[12]​ o límulos.^[13]​^[14]​ Aunque, como sucede habitualmente con los nombres comunes, estas denominaciones tienen carácter regional y refieren indistintamente a individuos de diversas especies dentro de la misma familia.

Descripción

Vistos desde arriba, tienen un caparazón quitinizado en forma de cúpula, de la que surge una larga cola puntiaguda sin funciones defensivas, sino para permitir la alimentación cuando está entrerrado en la arena del fondo marino. Presentan el cuerpo divido en dos segmentos (tagmas), que, desde adelante hacia atrás, son llamados prosoma (cabeza y tórax indiferenciados) y opistosoma. No presentan antenas ni mandíbulas, pero en cambio tienen un par de quelíceros bucales en posición ventral, junto a los 6 pares de patas muy modificadas con funciones en la alimentación además del desplazamieno del animal.

Se alimenta de moluscos, gusanos y otros invertebrados. Pasa gran parte de su vida enterrado en la arena, donde captura a sus presas. Las hembras ponen unos 100.000 huevos a lo largo de su vida.

En cuanto a distribución, el cangrejo herradura del Atlántico se encuentra normalmente en el golfo de México, a lo largo de las costas del Atlántico Norte. La población de cangrejo herradura se extiende desde la Península de Yucatán hasta el norte de las costas del estado de Maine (EE. UU.), pero es más común encontrarlos en la región del Atlántico entre Virginia y Nueva Jersey.

Otras especies

Hay otras tres especies incluidas bajo el nombre de cangrejos herradura.^[15]​ El cangrejo herradura japonés (Tachypleus tridentatus) que se encuentra en el mar interior de Seto y se considera una especie en peligro debido a la pérdida de hábitat. Otras dos especies se encuentran a lo largo de la costa este de la India e Indonesia: Tachypleus gigas y Carcinoscorpius rotundicauda.^[16]​ Las cuatro especies son bastante similares en forma y comportamiento y fuente de diferencias entre distintos taxónomos.^[2]​

Consumido como alimento

Otra fuente de amenaza a la supervivencia de estos animales reside en su captura excesiva como alimento, de acuerdo con las costumbres gastronómicas en algunos países de Extremo Oriente, principalmente Vietnam.^[17]​^[18]​ Aunque en realidad, son las especies asiáticas las amenazadas para ingesta humana, la especie americana (L. polyphemus) está sometida a sobrecaptura para su uso como carnada o cebo en la pesca del pulpo.^[19]​^[20]​

Sangre y sus usos médicos

La sangre o hemolinfa de los cangrejos herradura contiene una proteína llamada hemocianina (que incluye dos átomos de cobre), responsable del transporte del oxígeno a través del organismo del animal, similar a la hemoglobina de los vertebrados, que contiene un átomo de hierro. La hemocianina es incolora cuando no contiene oxígeno, mientras que se vuelve azul oscuro cuando sí lo transporta o cuando el animal sangra y la hemolinfa se ve expuesta al aire.^[21]​^[22]​

Pero desde el punto de vista del uso farmacológico, son un tipo de células contenidas en la hemolinfa las realmente importantes. Dichas células sanguíneas llamadas amebocitos, semejantes a los leucocitos de los vertebrados, reaccionan ante las endotoxinas bacterianas produciendo la coagulación de la hemolinfa como reacción de protección. El extracto de amebocitos de este animal, que en inglés se conoce como Limulus Amebocyte Lysate (LAL) se usa para verificar la presencia de toxinas en inyectables parenterales, particularmente los lipopolisacáridos (LPS), encontrados en la membrana exterior de las bacterias Gram negativas, los pirógenos más importantes.^[23]​

Este método se usa como alternativa al Rabbit Pyrogen Test o RPT, que consiste en inocular a un conejo con la sustancia de ensayo y verificar posteriormente si el conejo desarrolla un proceso febril. Frente al método LAL, el RPT es más lento, no ofrece resultados cuantitativos y además pueden existir otros factores que alteren el estado del conejo, aunque el espectro de aplicación sea mayor.^[23]​

Otro novedoso uso de la sangre de este animal es un test rápido de detección de posibles infecciones en astronautas para su tratamiento inmediato. El dispositivo, llamado LOCAD-PTS (Lab-On-A-Chip Application Development Portable Test System, en inglés), ha sido probado en la Estación Espacial Internacional, ISS.^[24]​^[25]​

Origen

Los cangrejos herradura son parientes de los Eurípteros (escorpiones marinos),^[26]​ cuyos primeros fósiles aparecieron al mismo tiempo. Son bastante antiguos. Estaban definitivamente presentes en el primer Ordovícico, hace 450 millones de años, en un medio tipo Esquisto de Burgess.^[27]​ Su forma corporal no ha cambiado mucho desde entonces, aunque los fósiles de este grupo son escasos. El cangrejo herradura del Atlántico en sí no tiene ningún registro fósil, y el género Limulus se remonta sólo unos 20 millones de años, no 200 millones.^[28]​

El orden al que pertenecen, Xiphosura, es basal a un clado de Eurypterida y de Arachnida.^[29]​ Se estima que el Xiphosura se apartó del Arácnido hace 480 millones de años.^[30]​

Evolución

Debido a que son famosos como "fósiles vivientes", el registro evolutivo y la genética de los cangrejos de herradura han sido estudiados con interés.

1. Está claro que ha habido evolución y adaptación dentro de la forma corporal general.^[31]​^[32]​
2. Desde hace unos 200 millones de años, se han producido pocos cambios visibles. "Limulus... es morfológicamente muy similar a formas que vivieron hace unos 200 millones de años".^[31]​ "Las especies vivientes del género Limulus, son virtualmente idénticas a las especies fósiles de una familia diferente que existió hace 150 millones de años".^[33]​
3. El análisis genético y molecular de Limulus deja claro que el animal tiene tanta variabilidad genética como cualquier otra especie normal.^[34]​

Referencias

Molukkirapu (Limulus polyphemus) on elävä fossiili, joka elää valtamerissä. Sen sukulaisia eli merissä jo 400–350 miljoonaa vuotta sitten. Nykyisin heimosta elää neljä lajia, muut niistä ovat pienikokoisempia ja niitä tavataan Aasian rannikoilla.^[2] Molukkiravut kehittyivät ilmeisesti meriskorpioneista. Nimestään huolimatta molukkirapu on läheisempää sukua hämähäkeille ja skorpioneille kuin ravuille. Molukkirapu kykenee kasvattamaan hävinneen raajan tilalle uuden.

Ulkonäkö ja koko

Molukkiravut ovat väriltään vihertäviä tai ruskeita. Naaras kasvaa suuremmaksi kuin koiras, sen ruumiin halkaisija voi olla 30 cm.^[3] Ravun pituus otsasta pyrstön kärkeen on noin kaksi kertaa sen leveys. Molukkiravun veri on kuparipitoinen koska siinä happea kuljettaa hemoglobiinin asemasta hemosyaniini ja siksi sen väri on sinertävä.

Molukkiravun vartalo on kolmejaokkeinen. Sileässä, puolipyöreässä etuosassa ovat viisi paria silmiä, pikku sakset joiden avulla se siirtää ruokaa suuhunsa, viisi paria jalkoja liikkumiseen ja suu jalkojen välissä. Takimmaiset jalat ovat melamaiset, ja auttavat rapua lipumaan vedessä. Keskiosassa on maha, kidukset ja sukupuolielimet sekä iso osa eläimen lihaksista. Takaosa on pitkä kapea pyrstö.^[4]

Kasvaessaan molukkirapu joutuu vaihtamaan kuorta. Se kehittää uuden kuoren vanhan alle ja pumppaa sitten kiduksillaan niin, että vanha kuori halkeaa kahden etummaisen jaokkeen saumasta. Uusi kuori on pehmeä ensimmäisen vuorokauden ajan. Jokaisessa kuorenvaihdossa molukkirapu kasvaa 25–30 %.^[5]

Levinneisyys

Molukkirapuja tavataan Pohjois-Amerikan itärannikolla Mainesta Floridan eteläosaan, Meksikonlahdelle ja Jukatanin niemimaalle asti.^[6]

Pieniä molukkirapuja voi pitää meriakvaariossa, mutta harva allas on tarpeeksi suuri täysikokoiselle ravulle.^[7]

Lähteet

Aiheesta muualla

• Peripiatus - an overview of arthropod relationships.
• Paleos - a site with a synoptic account of the Xiphosura, focused on fossils.
• http://www.saltwater-fish-tanks.com/fish/horseshoe-crab-conservation.php The Alarming Decrease in Population.
• http://www.ocean.udel.edu/horseshoecrab/Research/eye.html Biomedical Eye Research

Limulus polyphemus est une des quatre espèces encore vivantes de limules, la seule du genre Limulus. Elle se répartit le long de la côte est de l'Amérique du Nord et de l'Amérique centrale.

Synonyme pour le genre Limulus

Xiphosura (Gronovius, 1764) qui sert aujourd'hui à désigner l'ordre.

Anatomie

La limule Limulus polyphemus mesure jusqu'à 50 cm de long^[1].

Vision

La limule dispose de deux grands yeux composés de part et d'autre de sa carapace, ainsi que de cinq yeux simples sur la carapace. Deux autres yeux simples sont situés près de la bouche.

Sang

Son sang n'est pas composé d'hémoglobine, mais d'hémocyanine, à base de cuivre au lieu du fer.

Galerie

• En mouvement

Usage médical

Les limules sont une espèce importante dans la communauté de recherche médicale et dans les tests médicaux. En effet, le système immunitaire de la limule fonctionnant par coagulation de son sang sur les éléments étrangers, on peut en extraire le test au Lysat de Limule (LAL) pour détecter les endotoxines bactériennes dans les produits pharmaceutiques.

Certaines enzymes de limule sont utilisées par les astronautes dans la Station Spatiale Internationale pour tester la présence de bactéries ou de champignons indésirables.

Une des protéines de limule est également sujet de recherche en tant que nouvel antibiotique.

Le LAL est obtenu à partir du sang des limules. Après prélèvement, les individus sont relâchés. Les études montrent que le volume sanguin revient à la normale après une semaine environ. Cela dit, les cellules sanguines peuvent prendre deux à trois mois pour revenir à un niveau normal.

Néanmoins, ce prélèvement n'est pas sans impact. Il est en effet estimé qu'entre 10% et 30% des individus meurent après prélèvement. Par exemple, les chercheurs de l'université du New Hampshire et de l'université de Plymouth ont constaté que 18% d'un groupe de limules femelles sont décédées peu après leur relâchement des suites du prélèvement. Les survivants sont plus léthargiques et moins enclins à se reproduire.

Étude scientifique

Le génome de cette limule a été séquencé en 2017^[2].

Notes et références

Notes

En partie traduit de l'article en anglais.

Références

Limulus polyphemus é unha especie de artrópodo quelicerado da orde dos xifosuros, unha das catro aínda viventes destes aniamis, e a única do xénero Limulus.

Vive ao longo da costa leste de América do Norte e de América Central, principalmente en augas pouco profundas, en fondos brandos areosos ou lamacentos.

Na bibliografía internacional coñécese, entre outros, co nome vulgar de cangrexo de ferradura do Atlántico. Pero, a pesar deste nome, a especie está máis próxima aos arácnidos (arañas, carrachas e escorpións) que aos cangrexos, cos que non garda ningunha relación.^[1]

Características

Ten un corpo segmentado en tres lóbos divididos por sucos lonxitudinais e patas ventrais das que xorden apéndices e ollos compostos. Pode alcanzar os 50 cm de lonxitude.

Non se coñecen moito os seus hábitos. Pero sábese que pasa gran parte da súa vida enterrado na area ou na lama, en augas superficiais, onde captura ás súas presas, xa que é un deprerdador que se alimenta de moluscos, vermes e outros invertebrados (vivos ou mortos: tamén é necrófago).

Hábitat e distribución

Este cangrexo de ferradura se encóntrase comunmente no Golfo de México, ao longo das costas do atlánticas de Norteamérica. Realiza migracións, sendo o seu principal desprazamento, cara ao norte, a baía de Delaware, aínda que algúns individuos de cando en vez achéganse até as costas europeas.^[2]

O número de individuos desta especie está diminuíndo de forma lenta pero constante. Un recente estudo realizado polo Servizo de Estudos Xeolóxicos dos Estados Unidos de América sinala como culpábel deste declive ao cambio climático que está ocorrendo no noso planeta desde o final da era glaciar e que se está acelerando coas emisións de gases de efecto invernadoiro que facemos os humanos. Porén, estudos anteriores aseguran que a sobrepesca e as capturas para a industria farmacéutica tamén contribúen en gran medida á mingua das súas poboacións.^{[Cómpre referencia]}

Taxonomía

Descrición

Xénero

O xénero Limulus foi descrito en 1785 polo naturalista danés Otto Friedrich Müller,^[3] na súa obra Entomostraca seu Insecta Testacea, quae in aquis Daniae et Norvegiae reperit, descripsit et iconibus illustravit.

Especie

En canto á especie, fora descrita en 1758 polo naturalista sueco Carl von Linné (Linneo), na 10ª edición do seu Systema Naturae, co nome de Monoculus polyphemus.^[4]

Etimoloxías

Xénero

O nome Limulus é un diminutivo do adxectivo latino līmus, -a, -um, que significa "oblicuo", "torto", "atravesado" (limis [oculis] spectare = "mirar de esguello"). Literalmente: "un pouco birollo".^[5]

Especie

O epíteto específico, polyphemus, fai referencia ao máis famoso dos ciclopes da mitoloxía grega, Polifemo (en grego antigo Πολύφημος Polýphēmos, "de moitas palabras"), que adoita representarse como un xigante barbudo cun só ollo na fronte. Isto debeuese á idea errónea que se tiña no século XVIII de que o animal tiña un só ollo.^[5] Por iso tamén Linneo, cando describiu a especie, deulle ao xénero o nome de Monoculus, é dicir, "dun só ollo".

Sinónimos

Xénero

Ademais de polo nome actual, o xénero coñécese tamén polo sinónimo Xiphosura Gronovius, 1764.^[3]

Especie

Ademais de polo nome actualmente válido, a especie coñeceuse tamén polos sinónimos seguintes:^[4]

• Limulus albus Bosc, 1802
• Limulus americanus Leach, 1819
• Limulus cyclops Fabricius, 1793
• Limulus occidentalis Lamarck, 1801
• Limulus sowerbii Leach, 1815
• Monoculus polyphemus Linnaeus, 1758 (basónimo)

O sangue desta especie e os seus usos médicos

O sangue ou hemolinfa dos cangrexos de ferradura contén unha proteína chamada hemocianina, na que dous átomos de cobre fan a función de transporte do oxíxeno a través do organismo do animal, función que nos vertebrados realiza a hemoglobina, proteína que contén un átomo de ferro. A hemocianina é incolora cando non contén oxíxeno, mentres que se volve de cor azul escura cando si o transporta, ou cando o animal sangra a a hemolinfa queda exposta ao aire.^[6][7]

Ademais, este sangue contén unhas células sanguíneas chamadas amebocitos, semellantes aos leucocitos dos vertebrados, as cales reaccionan ante as endotoxinas bacterianas, coagulando. O extracto do sangue deste animal, o Limulus amebocyte lysate ou LAL, úsase para verificar a presenza en inxectábeis parenterais destas endotoxinas, particulamente os lipopolisacáridos (LPS) que se encontran na membrana exterior das bacterias gramnegativas, os piróxenos máis importantes.^[8]

Este método úsase como alternativa ao Rabbit Pyrogen Test ou RPT, que consiste en inocular a un coello a substancia de ensaio e verificar posteriormente se o conello desenvolve un proceso febril. Fronte ao método LAL, o RPT é máis lento, non ofrece resultados cuantitativos e, ademais, poden existir outros factores que alteren o estado do coello, aínda que o seu espectro de aplicación sexa maior.^[8]

Outro novo uso do sangue deste animal é un test rápido de detección de posíbeis infeccións en astronautas para o seu tratamento inmediato. O dispositivo, chamado LOCAD-PTS (Lab-On-A-Chip Application Development Portable Test System), foi probado na Estación Espacial Internacional.^[9][10]

Galería

• Esquemas da morfoloxía.

• Vista superior.

• Vista inferior.

• Macho enganchado é femia para a desova.

• Fósil de cangrexo de ferradura.

Notas

Véxase tamén

Bibliografía

• Brusca, R. C. & G. J. Brusca (1990): Invertebrates. Sunderland, MA, USA: Sinauer Associates. Tradución ao castelán en 2005, por McGraw-Hill Interamericana de España, Madrid. ISBN 978-84-486-0246-8.
• Owen, Richard (1873). Anatomy of the king crab (Limulus polyphemus, Latr.). London: Taylor and Francis.
• Rupppert, E. E.; R. S. Fox & R. D. Barnes (2004): Invertebrate Zoology, 7ª ed. Stamford, Connecticut (EE.UU.): Brooks/Cole ISBN 0-03-025982-7.

Rak bodljaš (potkovičasti rak, američki bodljaš; Limulus polyphemus), vrsta morskog člankonošca iz reda Xiphosaura, razred Marostomata. Domovina su mu pjeskovite plaže atlantske obale Meksika, Sjedinjenih Država i Kanade, gdje se sakupljaju zbog mriještenja po pličacima.

Tijelo raka bodljaša sa gornje strane je oklopljeno a ima dugačak rep kojim se najviše služi kao polugom. Rak bodljaš nalikuje na rakove ali to nije, enego pripada podcarstvu kliještara (Chelicerata) i najsrodniji je arachnidima ili paučnjacima. Najstariji foril ove vrste je iz vremena ordovicija, odnosno prije nekih 450 milijuna godina

Njihova jedinstvana karakteristika je plava krv koja ima tu boju zbog bakra u hemocijaninu, proteinu koji prenosi kisik. Za njihovu krv je ustanovljeno da otkriva prisutnosti bakterija, pa se koristi za otkrivanje onečišćenja u farmaceutskim lijekovima i medicinskim pomagalama. Kako bi došli do krvi godišnje sa sakupi oko 500 000 ovih rakova kojima se uzme 30% krvi nakon čega ih vraćaju nazad u vodu. Zbpog unosnog biznisa populacija ovih rakova postaje sve ugroženija^[1].

Meso rakova bodljaša također se koisti kao hrana u Kini, Vijetnamu i SAD.-u na Aljaski i Cape Codu^[2].

• Donji dio tijela ženke

Izvori

Limulus polyphemus (precedentemente classificato come Limulus cyclops, Xiphosura americana, Polyphemus occidentalis, comunemente detto limulo), è un artropode chelicerato, unico rappresentante del genere Limulus.

Nonostante il suo nome comune in inglese (Atlantic horseshoe crab, cioè "granchio atlantico a ferro di cavallo"), derivante dall'aspetto corazzato e dalla forma particolare del corpo, è più strettamente imparentato con ragni, zecche e scorpioni che con i veri e propri granchi. I limuli si nutrono di molluschi, vermi anellidi e altri organismi bentonici dei fondali marini.

Storia

Si fa riferimento alla specie del Limulus polyphemus associandola a specie fossili vissute nel Triassico, circa 200 milioni di anni fa, e ad altre simili vissute nel Devoniano, 400 milioni di anni fa, tuttavia il gruppo degli xifosuri è comparso sulla faccia della terra nel Miocene, circa 20 milioni di anni fa.

"Noi consideriamo erroneamente gli xifosuri come "fossili viventi" perché il gruppo non ha mai prodotto molte specie, e perciò non ha sviluppato un grande potenziale evolutivo di diversificazione; di conseguenza le specie moderne sono morfologicamente simili a forme antiche, ma le specie stesse non sono notevolmente vecchie."^[2]

Habitat

I limuli sono diffusi prevalentemente sulla costa est del nord America, dal Maine fino al sud della Florida, e nel golfo del Messico fino alla penisola dello Yucatán. Nel periodo riproduttivo, in primavera, un'area principale di migrazione e deposizione delle uova è la baia del Delaware. Esistono tre altre specie appartenenti alla famiglia dei Limulidae, e sono il granchio a ferro di cavallo giapponese (Tachypleus gigas), diffuso nel Mare Interno di Seto, e due specie (Tachypleus tridentatus e Carcinoscorpius rotundicauda) presenti sulla costa est dell'India.

L'habitat dei limuli adulti non è ben definito, ma sembra comprendere i fondali marini fino ad una profondità di 200 metri, con una preferenza per quelli di 30 metri. Nel periodo riproduttivo, gli adulti si avvicinano alla riva, preferendo spiagge sabbiose e riparate dal moto ondoso per la deposizione delle uova. Gli esemplari giovani trascorrono i primi due anni di vita nelle acque basse, riparate e prossime alla costa delle baie, spostandosi poi verso fondali più profondi.

Caratteristiche

Anatomia e morfologia

Visuale dorsale, frontale e ventrale

Visione ventrale di una femmina

Visione ventrale di un maschio, che mostra i pedipalpi modificati per aggrapparsi alla femmina

Il corpo dei limuli si suddivide in tre parti: la porzione della testa (prosoma o cefalotorace), dalla forma caratteristica a ferro di cavallo, contenente gli occhi, il cervello, il cuore, la bocca, i cheliceri e le zampe locomotrici, la porzione centrale (opistosoma o addome) presentante delle spine laterali, che racchiude le cinque paia di branchie a libro e l'opercolo genitale, e la porzione del telson, ovvero la lunga coda rigida. Il colore del carapace va dal grigio verdastro al marrone scuro, e le dimensioni dell'adulto possono arrivare sino a 60 cm.

Il limulo è dotato di 6 paia di appendici, di cui le prime due, più piccole e a forma di artiglio, sono dette cheliceri. Il secondo paio di appendici prende il nome di pedipalpi, e nei maschi è modificato in modo da terminare in un artiglio che consente loro di aggrapparsi alle femmine durante l'accoppiamento. Le tre paia di zampe ambulacrali terminano in chele e vengono usate per spezzettare il cibo e portarlo alla bocca, situata al centro del cefalotorace e priva di mandibole, mentre l'ultimo paia di zampe motorie ha dimensioni maggiori, ed è adattato per consentire la locomozione sul terreno sabbioso delle spiagge con degli speroni tarsali che si aprono ad ombrello e vengono utilizzati anche per scavare nel fango. Proprio al centro dell'addome si notano i chilari, appendici piccole ed inarticolate che corrispondono alle chele presenti sulle tre paia di zampe.^[3] All'attaccatura di ciascuna delle zampe dell'ultimo paio è presente il flabellum, un organo a spatola che saggia la composizione dell'acqua che filtra attraverso le branchie.

Delle sei paia di branchie a libro, il primo paio, detto opercolo, è di dimensioni maggiori, contiene l'apertura dei pori genitali e funge da copertura per le altre cinque paia di organi respiratori. Oltre che per l'assorbimento dell'ossigeno dall'acqua, le appendici branchiali sono utilizzate anche per nuotare, e portano dei chemiosensori sensibili alle caratteristiche dell'acqua. Il limulo è in grado di respirare anche sulla terraferma per un breve periodo di tempo, finché le branchie rimangono umide.

La coda, lunga e rigida, serve al limulo per potersi rigirare in caso di capovolgimento, ed è dotata di fotorecettori che intervengono nella regolazione del ritmo circadiano^[4].

I maschi dei limuli si differenziano dalle femmine per la loro dimensione, inferiore del 20%, ed i loro pedipalpi modificati per consentire loro di tenersi aggrappati alle compagne durante l'accoppiamento per fecondare le uova. Inoltre, la parte anteriore del carapace è più larga nelle femmine che nei maschi.

Riproduzione

La riproduzione dei limuli avviene in primavera, in coincidenza di maree particolarmente alte durante i pleniluni. Le femmine producono fra le 15.000 e le 64.000 uova^[5], a seconda della loro taglia, prima dell'accoppiamento, conservandole in masse dense nella parte anteriore del carapace, e le depongono in 4-5 buche profonde 15–20 cm scavate nella sabbia durante ogni marea. Ogni covata contiene circa 4000 uova, e una femmina ne depone circa 20 in un anno durante il corso di più maree.

Visione

Il limulo rappresenta un organismo modello per quanto riguarda gli studi sulla fisiologia della visione. Il suo apparato visivo è costituito da due occhi laterali composti situati in cima al carapace e 5 occhi semplici sensibili alla luce di cui due mediani, uno endoparietale e due laterali rudimentali. Gli occhi composti rappresentano un'eccezione fra i chelicerati, poiché nessun'altra specie appartenente allo stesso subphylum ne possiede, e vengono utilizzati principalmente per la localizzazione del partner. Gli occhi mediani sono sensibili alla luce ultravioletta, oltre a quella visibile, e insieme agli occhi laterali rudimentali variano la propria sensibilità in base a segnali inviati dal cervello in relazione a un orologio interno. Di notte la capacità visiva del limulo arriva ad amplificarsi di un milione di volte. Una serie di fotorecettori presenti sulla parte superiore e sui lati del telson inviano segnali di sincronizzazione al cervello con i cicli di luce e buio, rafforzati da segnali addizionali provenienti dai piccoli occhi mediani che aumentano il grado di adattamento all'oscurità in relazione all'intensità di luce ultravioletta. Sul ventre del limulo, inoltre, sono presenti due occhi, localizzati vicino alla bocca, che favoriscono l'orientamento durante il nuoto.

Nei due occhi composti, ciascun ommatidio è collegato ad una singola fibra nervosa, molto grande, e ciò ha consentito lo studio a livello cellulare della risposta nervosa alla stimolazione. Questi studi, condotti sui limuli sin dagli anni venti del ventesimo secolo, hanno permesso l'individuazione dei meccanismi di funzionamento di fenomeni visivi come l'inibizione laterale, ovvero la capacità di distinguere linee, forme e contorni degli oggetti.

Utilizzo nella ricerca medica

Il sistema immunitario del limulo, per quanto semplice e primitivo, è in grado di riconoscere efficacemente i lipopolisaccaridi presenti sulla parete dei batteri Gram negativi ed eliminare questi ultimi racchiudendoli in un coagulo. Questa capacità ha portato allo sviluppo del test in vitro LAL (saggio del lisato di amebociti di limulus) o più semplicemente limulus test, usato comunemente per l'individuazione di endotossine batteriche nelle materie prime industriali e nell'acqua, così come in farmacologia e per l'individuazione di alcune malattie batteriche. Il sangue, materia prima del test, viene estratto dai limuli senza provocarne la morte, rigettando in mare gli esemplari al termine del salasso. Dal sangue si ottiene poi un lisato di amebociti (uniche cellule in circolo nel Limulus) che contiene gli enzimi che permettono la gelificazione in presenza del lipopolisaccaride batterico (LPS).

Il sangue dei limuli è quasi incolore, ma a contatto con l'aria assume una colorazione azzurra dovuta all'ossidazione dell'emocianina, un pigmento respiratorio contenente rame, che ha le stesse funzioni di trasporto di ossigeno dell'emoglobina. Diversi peptidi isolati da emociti di limulo si sono dimostrati, in test in vitro, in grado di inibire la proliferazione del virus dell'HIV^[6].

Note

-2 Latinitas huius rei dubia est. Corrige si potes. Vide {{latinitas}}.

Limulus polyphemus est animal antiquissimum aquatile, quod etiam in terram venit. Solitissime invenitur in Sinu Mexicano, et in septentrionalibus litoribus Atlanticis Americae Septentrionalis. Unus ex primis locis migrationis annuae est Sinus Delavarianus, in America Septentrionalis, dum perditi^? in Europa aliquando invenuntur.

Limuli polyphemi sunt unus es speciebus quattuor in familia Limulidae, et species minime excultus est per plurimos annos centum milliones.

Nexus externi

Vicimedia Communia plura habent quae ad Limulum polyphemum spectant.

Belangkas Atlantik (Limulus polyphemus) merupakan sejenis artropod keliserat laut. Spesies ini lebih berkait rapat dengan labah-labah, kutu dan kala jengking berbanding dengan ketam.^[2] Belangkas ini paling banyak dijumpai di Teluk Mexico dan sepanjang pesisir Atlantik utara di Amerika Utara. Kawasan utama penghijrahan tahunan belangkas ini adalah di Teluk Delaware dan kadang-kala ia juga ditemui di Eropah.^[3]

Rujukan

Bibliografi

• Richard Owen (1873). Anatomi belangkas (Limulus polyphemus, Latr.). London: Taylor and Francis.

Pautan luar

• Timeless Traveller - The Horseshoe Crab
• Anatomi belangkas

De Atlantische degenkrab (Limulus polyphemus) is een geleedpotig dier dat behoort tot de familie van de degenkrabben (Limulidae). Ondanks de naam is de degenkrab meer verwant aan de spinachtigen dan aan de krabben. De meeste degenkrabben kunnen gevonden worden in de Golf van Mexico en langs de rand van de noordelijke Atlantische kust van Noord-Amerika. Tijdens het paringsseizoen zijn veel degenkrabben te vinden in Delaware Bay en heel soms in Europa.

Degenkrabben bezitten vijf paar boeklongen waarmee ze onder water kunnen ademen, maar ook voor een korte tijd op het land. Hoewel de meeste geleedpotigen mandibula's hebben, bezit de degenkrab geen kaak. De kop bevindt zich in het midden van het kopborststuk en ze hebben tien poten. Degenkrabben kunnen, net zoals zeesterren, reptielen en bepaalde amfibieën weer ledematen aangroeien als deze zijn verloren.

Tot aan de leeftijd van 9 jaar vervellen ze waardoor ze ongeveer 17 keer hun exoskelet verwisselen. Op de plekken waar de degenkrabben voorkomen worden op stranden veel exoskeletten gevonden. Het bloed van een degenkrab is blauw en bevat hemocyanine, ongeveer 50 gram per liter bloed.

O límulo (Limulus polyphemus) é um artrópode quelicerado, também conhecido como caranguejo-ferradura-do-atlântico. Apesar do nome, esta espécie está mais próxima das aranhas e escorpiões que dos caranguejos (Crustacea) propriamente ditos.

Comportamento

Os límulos são normalmente encontrados no Golfo do México e ao longo das costas do Atlântico Norte (Baía de Delaware), para onde é comum migrarem ano após ano. Durante toda a primavera esses animais sobem, aos milhares, até as praias para desovar, durante as marés altas, nas noites de lua nova e cheia. As fêmeas desovam em média 20 mil ovos por cova que cavam na areia da praia; as larvas eclodem após duas semanas. Podem atingir os 51 cm. Alimentam-se de moluscos, vermes e outros invertebrados. Seu habitat são as águas marinhas costeiras rasas, sobre fundos arenosos areia e lodosos.

Um variante japonês (Tachypleus tridentatus) pode ser encontrado em alguns mares, mas é considerada uma espécie sob risco de extinção devido à perda do habitat. Há ainda várias fazendas peixeiras onde os Límulos são criados para ser posteriormente vendidos como comida. Em cativeiro sua dieta pode ser composta de nacos de carne, tais como pedaços de camarão e de lula (Foster and Smith, 2004). Sua boca é encontrada no centro que corresponde à área inferior do tórax. Um par de pinças que os ajuda a puxar sua comida pode ser encontrado de cada lado da boca. Tempo médio de vida entre 20 e 40 anos.

Regeneração do Artrópode

Límulos possuem a rara habilidade de regenerar seus membros perdidos, de uma forma similar ao que fazem as estrelas-do-mar. Esse atributo foi recentemente provado por Sue Shaller, do Serviço Americano de Vida Selvagem.

Pesquisa médica

Estes animais são extremamente valiosos como espécies para a comunidade de pesquisas médicas. Desde 1964 uma substância feita através do sangue (que é azul) dos Límulos, chamada LAL (Limulus Amebocyte Lysate, em inglês) vem sendo testada contra endotoxinas bacterianas e na cura de várias doenças causadas por bactérias.^[1] Os animais podem ser devolvidos à água após a extração de uma certa quantidade de seu sangue, fazendo com que essa busca não se torne um risco à sobrevivência destes artrópodes.^[2]

A vida de um único límulo para extração sanguínea periódica pode valer até 2,5 mil dólares.^{[carece de fontes?]} O sangue destas criaturas é azul, o que é um resultado da alta concentração de hemocianina cuprosa em vez da hemoglobina ferrosa encontrada, por exemplo, nos humanos. O fato de os Límulos terem evoluído tão pouco ao longo desses 300 ou 400 milhões de anos, é uma das razões que faz deste um animal tão diferente dos demais.

Referências

Dolksvans (Limulus polyphemus), ibland kallad hästskokrabba^[2], är ett havslevande leddjur vanligt i Mexikanska golfen och längs med USA:s atlantkust, bland annat utmed kusterna av Maryland och Delaware. Dolksvansen är en av fyra arter inom familjen dolksvansar och då arten utvecklats mycket lite under flera hundra miljoner år brukar den kallas för ett levande fossil.

Systematik

Dolksvansen ingår i ordningen Xiphosura, inom leddjuren, och är avlägset släkt med spindlar, och på längre håll med kräftdjuren. Deras närmaste släkting tros vara den nu utdöda sjöskorpionen. Dolksvansarna utvecklades under paleozoikum för 542–251 miljoner år sedan, tillsammans med andra primitiva leddjur som exempelvis trilobiter. Dolksvansen anses vara en av de äldsta marina leddjuren och kategoriseras som ett levande fossil eftersom de inte har utvecklats nämnvärt de sista 350 till 400 miljoner åren. Dolksvansen placeras som ensam art i släktet Limulus och är en av fyra arter inom familjen dolksvansar (Limulidae).

Utseende och anatomi

Dolksvansen har ett yttre skal uppdelat i tre delar. Den första delen är det släta frontskalet som döljer djurets ögon, ben, mun, hjärna och hjärta. I mittendelen sitter dolksvansens gälar och könsorgan, och den sista delen är svansen som kan användas som hävstång om den skulle råka hamna upp och ner. Dolksvansen kan bli upp till 50 centimeter i diameter. Munnen sitter mitt på undersidan och den har ett par små klor på varsin sida om munnen som den kan hålla fast maten med. Dolksvansens fem gälar är så kallade bokgälar och består av 100-tals tunna membran som sitter som bladen i en bok. De har fyra fasettögon som är oberoende av varandra och studier indikerar att den kan se ultraviolett ljus. Dolksvansar har blått blod, då de har kopparbaserat hemocyanin istället för järnbaserat hemoglobin, som människor har. Dolksvansens immunsystem är enkelt men mycket effektivt. När en främmande bakterie tar sig in i ett sår hos dolksvansen kapslar immunförsvaret genast in den med ett geléliknade material som effektivt fångar bakterien. Gelén kallas för Limulus Amebocyte Lysate (LAL).

En dolksvans undersida.

Ekologi

Dolksvansen kan både andas i vatten och i korta perioder på land, så länge deras gälar hålls fuktiga. Varje senvår kommer dolksvansarna upp till vattenbrynet för att leka. De första som anländer är hannarna och efter dem kommer honorna som gräver ett cirka 20 centimeter djupt hål i sanden på så grunt vatten de kan. I hålet lägger honan sedan mellan 15000 och 64000 ägg som hannen befruktar. I takt med att de unga dolksvansarna växer drar de sig till allt djupare vatten. En dolksvans blir könsmogen vid cirka 11 års ålder och de kan leva i upp till 25 år. Dolksvansen lever av olika slags blötdjur och maskar.

Dolksvansen och människan

Medicinsk forskning

Dolksvansen är värdefull inom medicinsk forskning, främst på grund av dess unika immunförsvar och deras Limulus Amebocyte Lysate (LAL). Det pågår försök med dolksvansarnas blod för att hitta botemedel för sjukdomar som är resistenta mot penicillin och andra mediciner. Dolksvansarna fångas och ett blodprov tas men efter provtagningen släpps de fria igen. Det gör att forskningen inte hotar artens fortlevnad.

Hot och status

Dolksvansen är inte en hotad djurart men den minskar sakta i antal. Det har gjort att ett flertal naturorganisationer drivit kampanjer för att skydda de fyra arterna av dolksvans. Till exempel ordnade Ecological Resource and Development Group en kampanj, med sloganen "Just Flip 'Em" (svenska: "Vänd dem") som uppmanade människor att vända dolksvansar om man fann dem uppochned på land. Cirka 10 procent av de vuxna djuren dör under parningssäsongen för att de fastnar på land utan möjlighet att ta sig tillbaka till vattnet.

Se även

• Havsspindlar
• Spindeldjur

Referenser

Den här artikeln är helt eller delvis baserad på material från engelskspråkiga Wikipedia

Noter

1. ^ World Conservation Monitoring Centre 1996 Limulus polyphemus . Från: IUCN 2011. IUCN Red List of Threatened Species. Version 2011.2. Läst 2011-12-04.
2. ^ Hästskokrabba, Främmande arter i svenska hav

Källor

• ”Threats to Horseshoe Crabs (with emphasis on Massachusetts, United States)”. Delaware Center for Educational Technology. Arkiverad från originalet den 3 juli 2007. https://web.archive.org/web/20070703164633/http://www.warner.redclay.k12.de.us/threat.html. Läst 14 maj 2006.

Externa länkar

• http://www.horseshoecrab.org/ Horseshoe Crabs.org
• https://web.archive.org/web/20101104120131/http://www.saltwater-fish-tanks.com/fish/horseshoe-crab-conservation.php The Alarming Decrease in Population.

Sam Mỹ (Danh pháp khoa học: Limulus polyphemus), là một loài sam biển thuộc họ Limulidae. Mặc dù tên tiếng Anh là Atlantic horseshoe crab (cua móng ngựa Atlantic), sam lại có quan hệ gần gũi với nhện, bọ ve và bọ cạp hơn là cua.^[3]

Mô tả

Sam Mỹ thường được tìm thấy ở vịnh Mexico và dọc theo bờ Đại Tây Dương của Bắc Mỹ. Một nơi chúng thường di cư tới là vịnh Delaware, dù một số cá thể thỉnh thoảng có thể tìm thấy ở châu Âu.^[4]

Sam Mỹ hiện không phải là một loài bị đe dọa nhưng sự săn bắt và phá hủy môi trường sống đã làm giảm số lượng loài này ở vài khu vực và gây ra quan ngại cho tương lai của chúng. Loài rùa Đại Tây Dương ăn loài sam này cũng chịu ảnh hưởng do sự suy giảm của Sam Mỹ.^[5]

Hình ảnh

Chú thích

Tham khảo

• Dữ liệu liên quan tới Limulus polyphemus tại Wikispecies

• “The Horseshoe Crab: Natural History, Anatomy, Conservation and Current Research”. Ecological Research and Development Group. 2003. Bản gốc lưu trữ ngày 24 tháng 4 năm 2006. Truy cập ngày 14 tháng 5 năm 2006.
• The Alarming Decrease in Population.
• Biomedical Eye Research
• Timeless Traveller - The Horseshoe Crab
• All about the horseshoe crab.
• Horseshoe crab anatomy
• The Crabs, The Birds, The Bay. Short film about migrating shorebirds feasting on horseshoe crab eggs in Delaware Bay.
• Blue Blood 3min clip from Nature (TV series)
• Return of the Sandpiper, Abigail Tucker, with photographs by Doug Gritzmacher, Smithsonian magazine, October 2009.

по данным WoRMS^[1]:

• Limulus albus Bosc, 1802
• Limulus americanus Leach, 1819
• Limulus cyclops Fabricius, 1793
• Limulus occidentalis Lamarck, 1801
• Limulus sowerbii Leach, 1815
• Monoculus polyphemus
  Linnaeus, 1758

• 1 Анатомия и физиология
  • 1.1 Зрение
• 2 Образ жизни и поведение
• 3 Эволюция
• 4 Хозяйственное значение
• 5 Охрана
  • 5.1 В США
  • 5.2 В Мексике
• 6 Примечания
• 7 Библиография
• 8 Ссылки

美洲鱟標本，55 cm，美國東海岸

美洲鲎（學名：Limulus polyphemus）又名大西洋鲎，是鱟科美洲鲎属的一种，也是美洲鲎属的唯一的现存物种。

美洲鲎主要分佈在墨西哥灣和北美洲東岸。主要的活动区域是特拉华湾。漂流的個體偶爾在欧洲发现^[1]。很多渔场也会养殖鲎作为海鲜对外销售。它們可以生長至51厘米長。

食物

主要以軟體動物和環節動物和其他水底無脊椎動物為食，也有時包括蝦和烏賊 (Foster and Smith, 2004)。 牠的口部位於頭胸部之下，兩邊各有一條用作抓緊獵物的螫肢。

身體結構

鱟有五對鰓，位於其五對附肢後面。可幫助鱟於水底呼吸，只要保持濕潤，也可讓牠在地面呼吸一會。 它的外殼分為三部分。 最前的部分稱為甲殼，形狀圓滑，保護其大部分的身體，包裹其眼、附肢、螫肢、吻部、腦部和心臟。其腹部位於中間，容納其鰓和生殖器。其尾節是堅硬的尾刺，當鱟被反轉，可以靠其尾刺來翻身。 鱟的血液呈灰藍色，因為其血色素與其他動物不同。例如人類的血液，具有含鐵的血紅蛋白。但是鱟卻相當原始，與很多節肢動物一樣，其血液含有血藍蛋白，而藍血蛋白含有銅，因而呈藍色。其血中只有一種細胞，當遇上入侵的細菌或病毒時，這種細胞便會分解，使血液迅速凝固，此時鱟便會死亡。^{[來源請求]}科學家以此研製「鱟試劑」，用以測試無菌程度。鱟在過去的一段長時間中都幾乎沒有變異過。今天的鱟與3至4億年前的鱟相比，差別不多，因此鱟被稱為活化石。這也是鱟與現今的許多生物都大為不同的原因。

鱟有20-25年的壽命。每一年的晚春，雄性會先到達岸邊。雌性隨後到達，便會在沙中製造深15-20 cm的巢穴。

研究

鱟具有珍貴的醫學用途。 有研究發現鱟可能可以治療癌症。鱟的免疫系統非常簡單原始，但卻十分有效。一些研究發現，只要有外來細菌入侵，鱟的藍血便會立即凝固成膠狀，包裹異物。現在，鱟血提取物被造成快速含菌量試劑——鲎试剂，用於醫藥用途，只要一有細菌，提取物便會凝結。^[2]

參考文獻

1. ^ NEAT Chelicerata and Uniramia Checklist (PDF). [2006-10-24].
2. ^ Juliet Eilperin. Horseshoe Crabs' Decline Further Imperils Shorebirds (subtitle: Mid-Atlantic States Searching for Ways to Reverse Trend). The Washington Post. June 10, 2005: A03 [2006-05-14].

Monoculus polyphemus Linnaeus, 1758

アメリカカブトガニ（学名:Limulus polyphemus）は、カブトガニ科に属する海生節足動物。メキシコ湾を含む北西大西洋沿岸に分布する。ヨーロッパでも迷入個体が発見されている^[2]。

分類[編集]

デラウェア湾の浜辺で。付着生物（Crepidula 属の巻貝）が確認できる。

米国では horsefoot ・ saucepan などとも呼ばれる。"helmet crab" と呼ばれる場合もあるが、これは本来クリガニを意味する。"king crab" という名も本来はタラバガニ科の総称である。

Limulus は「少し横目の」という意味であり^[3]、種小名polyphemusはギリシア神話のキュクロープスの一人であるポリュペーモスに由来する^[4]。これは、かつて本種の眼は1つしかないと考えられていたためである。

化石記録[編集]

生きている化石と呼ばれ、4億4500万年前の地層から近縁種が見つかっている^[5]。三畳紀（2億3000万年前）からはほぼ同じ形態の化石が出土する。本種自体の化石記録はないが、アメリカカブトガニ属 (Limulus) は2000万年前から記録がある^[6]。

形態・生理[編集]

体は大きく頭胸部・腹部・尾剣に分けることができる。背甲は馬蹄形で灰緑色から暗褐色。雌雄は似ているが、雌は雄より 25-30 % 大きくなり、最大で60 cm程度になる^[5]。脚は脱皮により再生することがある^[7]。

腹面のイラスト。口は脚の間にあり、その下に書鰓がある。

背甲には藻類・ヒラムシ・貝・フジツボ・コケムシなど様々な付着生物が付くため、'living museums' などと呼ばれることもある。本種の豊富な地域ではよく背甲や脱皮殻、またはその破片などが波打ち際で見られる。

雌の腹面。脚と書鰓が見える。

雄の腹面。鉤爪状の触肢(PdP)が確認できる。

腹面には6対の脚があるが、最も前方の鋏角は頭胸部中央の口に餌を渡すために用いられる。 その次の脚は触肢であり、歩脚として用いられるが、雄では交尾中に雌を押さえるために鉤爪状となっている。残り4対は歩脚である。脚先は鋏となるが、最も後方の脚は体を押し出すために葉状になっている^[8]。

脚の後方にはさらに6対の付属肢がある。最初の1対は蓋板と呼ばれ、生殖孔がある。蓋板の形状はアジア産のカブトガニと異なっており、容易に区別できる^[9]。残る5対は書鰓と呼ばれる平板状の構造となっている。これは水中での呼吸に用いられるが、陸上でもこの部分が湿っていれば短時間は呼吸することができる。

他の感覚器官として、腹面の単眼の近くに小さな化学受容器がある^[10]。

視覚[編集]

視覚の研究に広く用いられてきた。1967年のノーベル生理学・医学賞は視覚の化学的、生理学的基礎過程に関する発見に対して与えられたものだが、その実験材料には本種も含まれている。

頭胸部の両側には単色性視覚を与える1対の大きな複眼があり^{[note 1]}、背甲に5個、腹面の口前方に2個の単眼がある^[11]。単眼は胚・幼生期に重要で^[11]、卵の中でも光を感じ取ることができる^[12]。複眼や中央単眼はそれより感度が劣るが、成体では主要な視覚器官となる^[11]。これらとは別に、尾剣には光受容器が並んでいる。

複眼は約1000個の個眼で構成されており^[8]、個々の個眼には300個以上の細胞がある^[12]。個眼の並びは乱雑で、昆虫のような正六角形のパターンにはなっていない^[11]。各個眼には1本の神経がつながっているが、この神経は太くて扱いやすいため、光刺激に対する神経の電気生理学的応答を観察するために用いられた。また、細胞レベルでの側方抑制現象も観察されている。最近では光知覚に関する研究も進んでおり、形状や明るさの情報を用いて雄が雌を認識していることが、光刺激による馴化や古典的条件づけによって示されている。

個眼には光を感覚するための円柱状構造がある。これは扇形の断面を持つ、縦に長い小網膜細胞が環状に並んで構成されている。扇形の根元付近の辺上からは感桿分体という突起が突き出し、周囲の小網膜細胞と結合している。感桿分体は早朝など、急激に強い光を受けた時には崩壊するが、1時間ほどで元の構造を回復する。円柱状構造の中心には偏心視細胞の樹状突起があり、小網膜細胞から入力を受けている^[13]。

他に偏心視細胞を持つ種としては、カイコが知られている。感桿分体は微絨毛から構成されているが、微絨毛は二重膜で包まれており、その厚さは1枚あたり7 nm、間には3.5 nmの電子不足領域がある。微絨毛が接触する場所では外層が融合し、厚さは15 nm程度になる。微絨毛の直径は約40-150nmである^[14]。

血液[編集]

血液中の酸素は軟体動物・環形動物にも見られる呼吸色素、ヘモシアニンによって運搬される。これは血中に 50 g/l ほど含まれている^[15]。ヘモシアニンは無酸素状態では無色であるが、酸素と結合すると濃青色となる。通常血液中の酸素濃度は低く、その色は灰白色から淡黄色であるが^[15]、外気に触れることで青色に変化する^[15]。赤血球のような細胞はなく、ヘモシアニンは血漿に直接溶解している^[15]。

血球は1種のみで変形細胞と呼ばれ、病原体からの防御に重要である。変形細胞はコアギュローゲン (coagulogen) として知られる凝固因子を含んでおり、細菌のエンドトキシンの刺激でこれが放出される。結果として凝固が起こり、循環系から病原体を隔離することができる^[16]。

生態[編集]

貝類・多毛類などの無脊椎動物に加え、僅かに魚も食べる。顎はないため、食物は脚の基部に生えた剛毛や砂嚢で砕くことになる^[5]。

冬場は大陸棚におり、晩春に繁殖のため浜辺に移動する。雄が先に到着し、雌を発見すると触肢の鉤爪で捕まえる。雌雄の包接は1か月にも及び、複数の雄が1個体の雌を包接することもある^[17]。雌は満潮時に波打ち際に近付き^[17]、15–20cmの穴を掘って産卵し、その後雄が受精させる^[17]。産卵数は体長によって変わるが、約 15,000-64,000 個である^[18]。

孵化した幼生は5-7日遊泳した後着底し、最初の脱皮を始める。成長とともにより深い場所へと移動していく。9歳頃性成熟するまでに、およそ17回脱皮する^[5]。寿命の測定は難しいが、平均20-40年だと考えられる^[19]。

本種の概日リズムに関して幾つかの研究が行われている。眼の感度を制御する概日リズムが存在することは知られていたが、これとは別に、潮汐サイクルに影響されて行動を制御する概日リズムが調べられた^[20]。実験個体を人工的な潮汐下に置いたところ、このリズムに同調した行動が観察された。さらに、明暗サイクルよりも潮汐サイクルが行動に与える影響のほうが大きいことも示された^[21]。

医療[編集]

医療分野では、感染症や新薬のテストにおいて、細菌のエンドトキシンを検出するためにリムルス変形細胞溶解物 (LAL) を用いた凝固反応が利用されている^[4]。血液は1リットルあたり約15000ドルで取引され、市場規模は米国だけでも年間5000万ドルに上る。年間の捕獲数は25万匹ほどで、全血液量の約30%が採取される。生産者は死亡率を3%程度と推定しているが、近年の調査では10-15%が死亡しているという結果が得られている^[22]。血液量は1週間後には元に戻り、細胞数は2-3か月で回復することが示されている^[23]。

血中の酵素は国際宇宙ステーションで細菌のコンタミネーションを防ぐために用いられている^[24]。血液中のタンパクから新たな抗生物質を開発する研究も進められている^[17]。

保護[編集]

アカウミガメは本種を餌の一つとしている。

現在絶滅危惧種とはされていないが、捕獲や生息地破壊によってその生存が脅かされている。幾つかの地域では、ウナギや巻貝漁に用いる餌として用いられる、などの原因により、本種の個体数は1970年代から減少を続けている。

コオバシギなどの渉禽類には、春の渡りのエネルギーを本種の卵に依存しているものがいる。デラウェア湾などの場所では、本種の現象の影響を受け、渡り鳥の個体数の急激な減少が観察されている^[25]。本種を捕食するアカウミガメも間接的な影響を受けている^[26]。

1995年、nonprofit Ecological Research and Development Group [1] (ERDG) が、現生カブトガニ4種の保護を目的として設立され、様々な貢献を行なってきた。上記のように巻貝漁の餌として本種が用いられているが、ERDGの設立者Glenn Gauvryは、他の生物による餌の流出を防ぐことができる、巻貝漁のためのトラップを発明した。これにより餌の必要量が約50%減少するため、バージニア州では巻貝漁でこのトラップを用いることが義務化されている。

2006年、大西洋州海洋漁業委員会は、デラウェア州・ニュージャージー州のデラウェア湾岸での本種の捕獲を2年間禁じる措置を含む、幾つかの保護措置を検討した^[27]。2007年6月、デラウェア州高等裁判所判事 Richard Stokes は、雄の捕獲を100,000個体までに制限した。これは、本種の個体数は1998年までの乱獲によって大きく減少したが、その後は安定しており、限られた数の捕獲ならば本種や渡り鳥への影響は少ないと判断されたことによるものである。逆に、デラウェア州環境長官 John Hughes は、コオバシギの減少は確定的であり、カブトガニの卵を確保するためには極端な措置も必要であるとしている^[28][29] Harvesting of the crabs was banned in New Jersey March 25, 2008.^[30]。

毎年、繁殖するカブトガニの10%が裏返しになって元に戻れずに死亡している。そのため、ERDGは"Just Flip 'Em"と題した、裏返しのカブトガニを見つけたら元に戻すというキャンペーンを展開している。

2008年の春-夏に、projectlimulus.orgと題した大規模な標識調査が行われた^[5]。効率的な保護を行うために、さらなる個体群データが必要とされている^[31]。

脚注[編集]

1. ^ 吸収波長のピークは525 nm。

出典[編集]

参考文献[編集]

• リチャード・オーウェン (1873). Anatomy of the king crab (Limulus polyphemus, Latr.). London: Taylor and Francis. http://www.archive.org/details/anatomyofkingcra00owen.

外部リンク[編集]

• “The Horseshoe Crab: Natural History, Anatomy, Conservation and Current Research”. Ecological Research and Development Group (2006年4月24日時点のオリジナルよりアーカイブ。2006年5月14日閲覧。
• The Alarming Decrease in Population. - ウェイバックマシン（2008年2月16日アーカイブ分）
• Biomedical Eye Research
• All about the horseshoe crab.
• Blue Blood 3min clip - YouTube(from Nature (TV series))