Horseshoe crabs

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Horseshoe crabs
Horseshoe Crab (fished as bycatch in the Bay of Bengal)

Horseshoe Crab (fished as bycatch in the Bay of Bengal)

Over trunk : Molting animals (Ecdysozoa)
Trunk : Arthropod (arthropoda)
Sub-stem : Jawbearers (Chelicerata)
Class : Merostomata
Order : Xiphosura
Family : Horseshoe crabs
Scientific name
Leach , 1819
Underside of a horseshoe crab
Swimming horseshoe crab

The horseshoe crabs (Limulidae) (also Moluccas crabs , horseshoe crabs or king crab ) are the only extant family within the order of the sword tails (Xiphosura). The swordtails are a basal group of the jaw-claw carriers , possibly the sister group of all (more recently) land-living arachnids (all except the woodlice spiders ). Their position within the extinct groups that have only survived in fossil form continues to be controversial. Their association with the (extinct) sea ​​scorpions (Eurypterida) in a taxon called merostomata was considered the standard hypothesis for decades, but is now considered unlikely.


The body of the xiphosuren is divided into the horseshoe-shaped prosoma and the smaller opisthosoma . The latter is in turn subdivided into the mesosoma of seven fused segments and the metasoma consisting of three segments, which ends in the eponymous pointed, flexible tail spine. The first and parts of the second opisthosoma segment are attached to the anterior body. This is connected to the back of the body by a joint, which, in contrast to arachnids, does not correspond to the connection between the pro- and opisthosoma. The cuticle is thick, firm and free of calcium deposits.

The prosoma has five pairs of legs that are used for locomotion. When horseshoe crabs run, their legs are moved alternately. Horseshoe crabs swim with the ventral side up; the legs beat synchronously. The tail spike helps to turn around when the animals are lying on their backs, as well as a rudder. All legs consist of six segments each. The legs of the first four pairs carry scissors , those of the fifth have basal bristles. Only the legs of the fifth pair have a leaf-shaped epipodite , which is known as a flabellum (Latin for "fan"). It is primarily used to direct the flow of water into the gill space, but probably also has a sensory function. The claws of the adult males' first and second pairs of legs are enlarged compared to those of the females and serve as clasps during mating. Behind the fifth pair of legs is a pair of small, movable appendages, the Chilaria (cf. Greek χεῖλος, cheilos, "lip"). These probably go back to another, regressed pair of legs, which many fossil species show in the corresponding places. In front of the legs are the tripartite, claw-like chelicerae that serve as mouthparts . On the coxes of the legs there are inwardly directed, spiked endites (coxal shops) that transport food from the vestibule to the mouth. This lies between the coxes of the pairs of legs and thus in the middle of the ventral side of the front body. When viewed from above, the back of the body shows a large back plate, which is often spiked at the back, a small back plate (tergum or thoracetron), which consists of three segments as a fusion, and the tail spine. Viewed from below, it shows six pairs of split legs that have been redesigned to form a plate . The first of these pairs, called the operculum, carries the paired sex openings. On segments three to seven of the opisthosoma there are a total of five pairs of book gills on the exopodites of the split legs , which consist of up to 150 lamellae lying close together. The tripartite telopodite on these split legs is used to clean the gills.

The two complex or compound eyes sitting on the sides of the back shield are missing the crystal cones ; they are thus built more simply than those of the mandibular animals (Mandibulata). The so-called median eyes lie dorsally in the middle of the prosoma. They each consist of a lens. Not visible from the outside, a second pair of reduced eyes, the endoparietal eyes, are located below the median eyes. At the base of the upper lip, just in front of the brain, there is a third pair of eyes. While this is still well developed in the larvae, it later fuses with the frontal organ, which is probably a chemoreceptor.

Gonads are located in the prosoma and open onto the fused extremities of the second segment of the opisthosoma, the genital operculum. Xiphosur sperm are among the most original of the arthropods.

Horseshoe crabs grow up to 85 cm long. Their color ranges from dark red-brown to black-brown.

Way of life

Fossil with traces

Horseshoe crabs normally live on the ocean floor, but can swim with their belly side up. They feed on mussels and other molluscs as well as carrion that they find in the ground and lead to the vestibule with the chelicere or the legs. All known species can curl up and protect themselves from enemies. By repeatedly rolling up and unfolding, the animals can bury themselves in the soft sand. During the mating season they come close to the shore. The characteristic tracks of horseshoe crabs are often easily identifiable in fossil form, as the end links of the first four pairs of legs differ from those of the fifth pair of legs and the traces of the tail spine can usually be seen. These trace fossils are called kouphichnium .

distribution and habitat

Horseshoe crabs occur on the flat sandy coasts of tropical seas at depths between 10 and 40 meters. The species Limulus polyphemus is widespread on the American Atlantic coast . Carcinoscorpoius rotundicauda and the two species Tachypleus gigas and Tachypleus tridentatus live in Southeast Asia .

Reproduction and development

The sexually mature animals gather in the early summer in the tidal area on the flat coasts of their home seas, where the males cling to the females with the help of their appropriately designed front legs. They lay their 200 to 1000 eggs in a shallow sand trough, where they are then inseminated and covered.

The first free-swimming larva of the horseshoe crabs is known as the trilobite larva due to their shape . It already has all segments, but only 9 pairs of extremities. The remaining pairs of legs and the tail sting are given to them after the first molt, the animals become sexually mature after 9 to 12 years.


Cancer Moluccanum from Exoticorum libri decem by Charles de l'Écluse (1605)

The horseshoe crabs have been known since the Ordovician and had their heyday from the Silurian to the Jura . The family is currently only represented with four species from three genera.

Almost identical forms are known from the Mesozoic Era, which is why the horseshoe crabs are often referred to as living fossils .

In addition to the horseshoe crabs, the extinct sea ​​scorpions (Eurypterida) were also frequently placed among the merostomata. Recent phylogenetic analyzes using the cladistic method make this position seem improbable. Most of the common features of the fossil forms are probably symplesiomorphies (common trunk group features) that resulted from the aquatic way of life. Presumably, the sea scorpions are actually more closely related to the arachnids. Their position on other all extinct groups such as the Chasmataspidida is not completely clear. In addition, in the Cambrian there were probably numerous core group representatives of the jaw-claw bearers with superficially very similar looking physiques, whose exact relationship is unclear; but its inclusion in the Xiphosura would presumably make it paraphyletic . The oldest horseshoe crabs in the narrower sense, such as the genus Lunataspis , probably come from the Ordovician . The summary of the basal forms in a taxon Synziphosurina is also considered obsolete today. The closer relationship of the recent forms, the family Limulidae, could come from the Permian . According to the methodology of the molecular clock , the recent genera may have separated as early as the early Cretaceous . The East Asian genera are more closely related to each other than to the American genus Limulus .

Limulus amebocyte lysate test

In the 1970s, the first in vitro test for the detection of pyrogenic substances was developed. For this test, the horseshoe crab's blood, which is colored blue by the oxygen transporter hemocyanin, is removed. The method is used to detect bacterial endotoxins (lipopolysaccharides) that arise after the sterilization-related disintegration of bacteria from the cell walls in or on the medium (injectables, medical devices). The blood of the horseshoe reacts to these bacterial decay substances ( endotoxin ) and coagulates to form a gel.

The Limulus amebocyte lysate test (LAL test) measures the coagulation of a lysate obtained from blood cells (amebocytes) of the horseshoe crab, triggered by endotoxin . After activation of factors C and B by lipopolysaccharide , a clotting enzyme activates coagulation , which is then assessed turbidimetrically or with the help of a color reaction.

Horseshoe crabs are sometimes killed to obtain lysate , which has met with strong criticism. This is particularly because it is possible and customary in many places to take the blood sample without harming the animals (losses of 15% or less).

Individual evidence

  1. Helmut Mayr: Limulus - The small, cross-eyed Cyclop. Online article accompanying the special exhibition "Forgotten by Evolution? - Living Fossils. Bavarian State Collection for Paleontology and Historical Geology, Ludwig Maximilians University Munich, accessed on November 6, 2013 .
  2. Lexicon of Biology: Xiphosura. Spectrum of Science , 1999, accessed July 6, 2019 .
  3. a b c d e f g h i Wilfried Westheide, Gunde Rieger (ed.): Special Zoology. Part 1: Protozoa and invertebrates . 3. Edition. Springer-Verlag, Berlin Heidelberg 2013, ISBN 978-3-642-34695-8 , pp. 498-500 .
  4. Kōichi Sekiguchi: Biology of Horseshoe Crabs . Science House, 1988, ISBN 978-4-915572-25-8 .
  5. Jeffrey W. Shultz (2007): A phylogenetic analysis of the arachnid orders based on morphological characters. Zoological Journal of the Linnean Society 150: 221-265.
  6. ^ Jason A. Dunlop (2010): Geological history and phylogeny of Chelicerata. Arthropod Structure & Development 39: 124-142. doi : 10.1016 / j.asd.2010.01.003
  7. ^ Russell J. Garwood & Jason Dunlop (2014): Three-dimensional reconstruction and the phylogeny of extinct chelicerate orders. PeerJ 2: e641. doi : 10.7717 / peerj.641
  8. DM Rudkin, GA Young, GS Nowlan (2008): The oldest horseshoe crab: a new xiphosurid from Late Ordovician Konservat-Lagerstatten deposits, Manitoba, Canada. Palaeontology 51: 1-9 doi : 10.1111 / j.1475-4983.2007.00746.x
  9. James C. Lamsdell (2016): Horseshoe crab phylogeny and independent colonizations of fresh water: ecological invasion as a driver for morphological innovation. Palaeontology 59: 181-194. doi : 10.1111 / pala.12220
  10. Matthias Obst, Søren Faurby, Somchai Bussarawit, Peter Funch (2012): Molecular phylogeny of extant horseshoe crabs (Xiphosura, Limulidae) indicates Paleogene diversification of Asian species. Molecular Phylogenetics and Evolution 62: 21-26. doi : 10.1016 / j.ympev.2011.08.025
  11. Alexis C. Madrigal: The Blood Harvest. Each year, half a million horseshoe crabs are captured and bled alive to create an unparalleled biomedical technology. The Atlantic, February 26, 2014
  12. ^ Nature on PBS Public Broadcasting Service: Crash. A Tale of Two Species. The Benefits of Blue Blood. June 10, 2008

Web links

Commons : Limulidae  - album with pictures, videos and audio files