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Scorpions are predatory arachnids of the order Scorpiones. They have eight legs and are easily recognized by a pair of grasping pincers and a narrow, segmented tail, often carried in a characteristic forward curve over the back and always ending with a stinger. The evolutionary history of scorpions goes back 435 million years. They mainly live in deserts but have adapted to a wide range of environmental conditions, and can be found on all continents except Antarctica. There are over 2,500 described species, with 22 extant (living) families recognized to date. Their taxonomy is being revised to account for 21st-century genomic studies.

Scorpions primarily prey on insects and other invertebrates, but some species hunt vertebrates. They use their pincers to restrain and kill prey, or to prevent their own predation. The venomous sting is used for offense and defense. During courtship, the male and female grasp each other’s pincers and dance while he tries to move her onto his sperm packet. All known species give live birth and the female cares for the young as their exoskeletons harden, transporting them on her back. The exoskeleton contains fluorescent chemicals and glows under ultraviolet light.

The vast majority of species do not seriously threaten humans, and healthy adults usually do not need medical treatment after a sting. About 25 species (fewer than one percent) have venom capable of killing a human, which happens frequently in the parts of the world where they live, primarily where access to medical treatment is unlikely.

Scorpions appear in art, folklore, mythology, and commercial brands. Scorpion motifs are woven into kilim carpets for protection from their sting. Scorpius is the name of a constellation; the corresponding astrological sign is Scorpio. A classical myth about Scorpius tells how the giant scorpion and its enemy Orion became constellations on opposite sides of the sky.

Etymology

The word scorpion originated in Middle English between 1175 and 1225 AD from Old French scorpion,^[1] or from Italian scorpione, both derived from the Latin scorpio, equivalent to scorpius,^[2] which is the romanization of the Greek σκορπίος – skorpíos,^[3] with no native IE etymology (cfr. Arabic ʕaqrab ‘scorpion’, Proto-Germanic *krabbô ‘crab’).

Evolution

Fossil record

Scorpion fossils have been found in many strata, including marine Silurian and estuarine Devonian deposits, coal deposits from the Carboniferous Period and in amber. Whether the early scorpions were marine or terrestrial has been debated, and while they had book lungs like modern terrestrial species,^[4][5][6][7] the most basal such as Eramoscorpius were originally considered as still aquatic,^[8] until it was found that Eramoscorpius had book lungs.^[9] Over 100 fossil species of scorpion have been described.^[10] The oldest found as of 2021 is Dolichophonus loudonensis, which lived during the Silurian, in present-day Scotland.^[11] Gondwanascorpio from the Devonian is among the earliest-known terrestrial animals on the Gondwana supercontinent.^[12] Some Palaeozoic scorpions possessed compound eyes similar to those of eurypterids.^[13] The Triassic fossils Protochactas and Protobuthus belong to the modern clades Chactoidea and Buthoidea respectively, indicating that the crown group of modern scorpions had emerged by this time.^[14]

Phylogeny

The Scorpiones are a clade within the pulmonate Arachnida (those with book lungs). Arachnida is placed within the Chelicerata, a subphylum of Arthropoda that contains sea spiders and horseshoe crabs, alongside terrestrial animals without book lungs such as ticks and harvestmen.^[4] The extinct Eurypterida, sometimes called sea scorpions, though they were not all marine, are not scorpions; their grasping pincers were chelicerae, not homologous with the pincers (second appendages) of scorpions.^[15] Scorpiones is sister to the Tetrapulmonata, a terrestrial group of pulmonates containing the spiders and whip scorpions. This 2019 cladogram summarizes:^[4]

Chelicerata	Pycnogonida (sea spiders) ProsomapodaXiphosura (horseshoe crabs) †Eurypterida (sea scorpions) ArachnidaNon‑pulmonates(ticks, harvestmen, etc) pulmonatesScorpionesTetrapulmonataAraneae (spiders) Pedipalpi (whip scorpions, etc)

Recent studies place pseudoscorpions as the sister group of scorpions in the clade Panscorpiones, which together with Tetrapulmonata makes up the clade Arachnopulmonata.^[16]

The internal phylogeny of the scorpions has been debated,^[4] but genomic analysis consistently places the Bothriuridae as sister to a clade consisting of Scorpionoidea and Chactoidea. The scorpions diversified between the Devonian and the early Carboniferous. The main division is into the clades Buthida and Iurida. The Bothriuridae diverged starting before temperate Gondwana broke up into separate land masses, completed by the Jurassic. The Iuroidea and Chactoidea are both seen not to be single clades, and are shown as “paraphyletic” (with quotation marks) in this 2018 cladogram.^[17]

Scorpiones	ButhidaChaeriloidea Pseudochactoidea Buthoidea Iurida”Iuroidea” (part)Bothriuroidea “Chactoidea” (part)”Iuroidea” (part)”Chactoidea” (part)Scorpionoidea

Taxonomy

Main article: Taxonomy of scorpions

Carl Linnaeus described six species of scorpion in his genus Scorpio in 1758 and 1767; three of these are now considered valid and are called Scorpio maurus, Androctonus australis, and Euscorpius carpathicus; the other three are dubious names. He placed the scorpions among his “Insecta aptera” (wingless insects), a group that included Crustacea, Arachnida and Myriapoda.^[18] In 1801, Jean-Baptiste Lamarck divided up the “Insecta aptera”, creating the taxon Arachnides for spiders, scorpions, and acari (mites and ticks), though it also contained the Thysanura, Myriapoda and parasites such as lice.^[19] German arachnologist Carl Ludwig Koch created the order Scorpiones in 1837. He divided it into four families, the six-eyed scorpions “Scorpionides”, the eight-eyed scorpions “Buthides”, the ten-eyed scorpions “Centrurides”, and the twelve-eyed scorpions “Androctonides”.^[20]

More recently, some twenty-two families containing over 2,500 species of scorpions have been described, with many additions and much reorganization of taxa in the 21st century.^[21][4][22] There are over 100 described taxa of fossil scorpions.^[10] This classification is based on Soleglad and Fet (2003),^[23] which replaced Stockwell’s older, unpublished classification.^[24] Further taxonomic changes are from papers by Soleglad et al. (2005).^[25][26]

The extant taxa to the rank of family (numbers of species in parentheses^[21]) are:Order Scorpiones

• Parvorder PseudochactidaSoleglad & Fet, 2003
  • Superfamily PseudochactoideaGromov, 1998
    • Family Pseudochactidae Gromov, 1998 (1 sp.) (Central Asian scorpions of semi-savanna habitats)
• Parvorder ButhidaSoleglad & Fet, 2003
  • Superfamily ButhoideaC. L. Koch, 1837
    • Family Buthidae C. L. Koch, 1837 (1209 spp.) (thick-tailed scorpions, including the most dangerous species)
    • Family Microcharmidae Lourenço, 1996, 2019 (17 spp.) (African scorpions of humid forest leaf litter)
• Parvorder ChaerilidaSoleglad & Fet, 2003
  • Superfamily ChaeriloideaPocock, 1893
    • Family Chaerilidae Pocock, 1893 (51 spp.) (South and Southeast Asian scorpions of non-arid places)
• Parvorder IuridaSoleglad & Fet, 2003
  • Superfamily ChactoideaPocock, 1893
    • Family Akravidae Levy, 2007 (1 sp.) (cave-dwelling scorpions of Israel)
    • Family Belisariidae Lourenço, 1998 (3 spp.) (cave-related scorpions of Southern Europe)
    • Family Chactidae Pocock, 1893 (209 spp.) (New World scorpions, membership under revision)
    • Family Euscorpiidae Laurie, 1896 (170 spp.) (harmless scorpions of the Americas, Eurasia, and North Africa)
    • Family Superstitioniidae Stahnke, 1940 (1 sp.) (cave scorpions of Mexico and Southwestern United States)
    • Family Troglotayosicidae Lourenço, 1998 (4 spp.) (cave-related scorpions of South America)
    • Family Typhlochactidae Mitchell, 1971 (11 spp.) (cave-related scorpions of Eastern Mexico)
    • Family Vaejovidae Thorell, 1876 (222 spp.) (New World scorpions)
  • Superfamily IuroideaThorell, 1876
    • Family Caraboctonidae Kraepelin, 1905 (23 spp.) (hairy scorpions)
    • Family Hadruridae Stahnke, 1974 (9 spp.) (large North American scorpions)
    • Family Iuridae Thorell, 1876 (21 spp.) (scorpions with a large tooth on inner side of moveable claw)
  • Superfamily ScorpionoideaLatreille, 1802
    • Family Bothriuridae Simon, 1880 (158 spp.) (Southern hemisphere tropical and temperate scorpions)
    • Family Hemiscorpiidae Pocock, 1893 (16 spp.) (rock, creeping, or tree scorpions of the Middle East)
    • Family Hormuridae Laurie, 1896 (92 spp.) (flattened, crevice-living scorpions of Southeast Asia and Australia)
    • Family Rugodentidae Bastawade et al., 2005 (1 sp.) (burrowing scorpions of India)
    • Family Scorpionidae Latreille, 1802 (183 spp.) (burrowing or pale-legged scorpions)
    • Family Diplocentridae Karsch, 1880 (134 spp.) (closely related to and sometimes placed in Scorpionidae, but have spine on telson)
    • Family Heteroscorpionidae Kraepelin, 1905 (6 spp.) (scorpions of Madagascar)

Geographical distribution

Scorpions are found on all continents except Antarctica. The diversity of scorpions is greatest in subtropical areas; it decreases toward the poles and equator, though scorpions are found in the tropics. Scorpions did not occur naturally in Great Britain but were accidentally introduced by humans, and have now established a population.^[27][28] New Zealand, and some of the islands in Oceania, have in the past had small populations of introduced scorpions, but they were exterminated.^[29][27] Five colonies of Euscorpius flavicaudis have established themselves since the late 19th century in Sheerness in England at 51°N,^[30][31][32] while Paruroctonus boreus lives as far north as Red Deer, Alberta, at 52°N.^[33] A few species are on the IUCN Red List; Afrolychas braueri is classed as critically endangered (2012), Isometrus deharvengi as endangered (2016) and Chiromachus ochropus as vulnerable (2014).^[34][35][36]

Scorpions are xerocoles, meaning they primarily live in deserts, but they can be found in virtually every terrestrial habitat including high-elevation mountains, caves, and intertidal zones. They are largely absent from boreal ecosystems such as the tundra, high-altitude taiga, and mountain tops.^[37][4] The highest altitude reached by a scorpion is 5,500 meters (18,000 ft) in the Andes, for Orobothriurus crassimanus.^[38]

As regards microhabitats, scorpions may be ground-dwelling, tree-loving, rock-loving or sand-loving. Some species, such as Vaejovis janssi, are versatile and are found in all habitats on Socorro Island, Baja California, while others such as Euscorpius carpathicus, endemic to the littoral zone of rivers in Romania, occupy specialized niches.^[39][40]

Morphology

Scorpions range in size from the 8.5 mm (0.33 in) Typhlochactas mitchelli of Typhlochactidae,^[39] to the 23 cm (9.1 in) Heterometrus swammerdami of Scorpionidae.^[41] The body of a scorpion is divided into two parts or tagmata: the cephalothorax or prosoma, and the abdomen or opisthosoma.^[a] The opisthosoma is subdivided into a broad anterior portion, the mesosoma or pre-abdomen, and a narrow tail-like posterior, the metasoma or post-abdomen.^[43] External differences between the sexes are not obvious in most species. In some, the metasoma is more elongated in males than females.^[44]

Cephalothorax

The cephalothorax comprises the carapace, eyes, chelicerae (mouth parts), pedipalps (which have chelae, commonly called claws or pincers) and four pairs of walking legs. Scorpions have two eyes on the top of the cephalothorax, and usually two to five pairs of eyes along the front corners of the cephalothorax. While unable to form sharp images, their central eyes are amongst the most light sensitive in the animal kingdom, especially in dim light, which makes it possible for nocturnal species to use starlight to navigate at night.^[45] The chelicerae are at the front and underneath the carapace. They are pincer-like and have three segments and sharp “teeth”.^[46][47] The brain of a scorpion is in the back of the cephalothorax, just above the esophagus.^[48] As in other arachnids, the nervous system is highly concentrated in the cephalothorax, but has a long ventral nerve cord with segmented ganglia which may be a primitive trait.^[49]

The pedipalp is a segmented, clawed appendage used for prey immobilization, defense and sensory purposes. The segments of the pedipalp (from closest to the body outward) are coxa, trochanter, femur, patella, tibia (including the fixed claw and the manus) and tarsus (moveable claw). A scorpion has darkened or granular raised linear ridges, called “keels” or “carinae” on the pedipalp segments and on other parts of the body; these are useful as taxonomic characters.^[50] Unlike those of some other arachnids, the legs have not been modified for other purposes, though they may occasionally be used for digging, and females may use them to catch emerging young. The legs are covered in proprioceptors, bristles and sensory setae.^[51] Depending on the species, the legs may have spines and spurs.^[52]

Mesosoma

The mesosoma or preabdomen is the broad part of the opisthosoma.^[43] In the early stages of embryonic development the mesosoma consist of eight segments, but the first segment disappear before birth, so the mesosoma in scorpions actually consist of segments 2-8.^[53][54][55] These anterior seven somites (segments) of the opisthosoma are each covered dorsally by a sclerotized plate called the tergite. Ventrally, somites 3 to 7 are armored with matching plates called sternites. The ventral side of somite 1 has a pair of genital opercula covering the gonopore. Sternite 2 forms the basal plate bearing the pectines,^[56] which function as sensory organs.^[57]

The next four somites, 3 to 6, all bear pairs of spiracles. They serve as openings for the scorpion’s respiratory organs, known as book lungs. The spiracle openings may be slits, circular, elliptical or oval according to the species.^[58][59] There are thus four pairs of book lungs; each consists of some 140 to 150 thin lamellae filled with air inside a pulmonary chamber, connected on the ventral side to an atrial chamber which opens into a spiracle. Bristles hold the lamellae apart. A muscle opens the spiracle and widens the atrial chamber; dorsoventral muscles contract to compress the pulmonary chamber, forcing air out, and relax to allow the chamber to refill.^[60] The 7th and last somite does not bear appendages or any other significant external structures.^[58]

The mesosoma contains the heart or “dorsal vessel” which is the center of the scorpion’s open circulatory system. The heart is continuous with a deep arterial system which spreads throughout the body. Sinuses return deoxygenated blood (hemolymph) to the heart; the blood is re-oxygenated by cardiac pores. The mesosoma also contains the reproductive system. The female gonads are made of three or four tubes that run parallel to each other and are connected by two to four transverse anastomoses. These tubes are the sites for both oocyte formation and embryonic development. They connect to two oviducts which connect to a single atrium leading to the genital orifice.^[61] Males have two gonads made of two cylindrical tubes with a ladder-like configuration; they contain cysts which produce spermatozoa. Both tubes end in a spermiduct, one on each side of the mesosoma. They connect to glandular symmetrical structures called paraxial organs, which end at the genital orifice. These secrete chitin-based structures which come together to form the spermatophore.^[62][63]

Metasoma

The “tail” or metasoma consists of five segments and the telson, which is not strictly a segment. The five segments are merely body rings; they lack apparent sterna or terga, and become larger distally. These segments have keels, setae and bristles which may be used for taxonomic classification. The anus is at the distal and ventral end of the last segment, and is encircled by four anal papillae and the anal arch.^[58] The tails of some species contain light receptors.^[45]

The telson includes the vesicle, which contains a symmetrical pair of venom glands. Externally it bears the curved stinger, the hypodermic aculeus, equipped with sensory hairs. Each of the venom glands has its own duct to convey its secretion along the aculeus from the bulb of the gland to immediately near of the tip, where each of the paired ducts has its own venom pore.^[64] An extrinsic muscle system in the tail moves it forward and propels and penetrates with the aculeus, while an intrinsic muscle system attached to the glands pumps venom through the stinger into the intended victim.^[65] The stinger contains metalloproteins with zinc, hardening the tip.^[66] The optimal stinging angle is around 30 degrees relative to the tip.^[67]

Biology

Most scorpion species are nocturnal or crepuscular, finding shelter during the day in burrows, cracks in rocks and tree bark.^[68] Many species dig a shelter underneath stones a few centimeters long. Some may use burrows made by other animals including spiders, reptiles and small mammals. Other species dig their own burrows which vary in complexity and depth. Hadrurus species dig burrows as over 2 m (6 ft 7 in) deep. Digging is done using the mouth parts, claws and legs. In several species, particularly of the family Buthidae, individuals may gather in the same shelter; bark scorpions may aggregate up to 30 individuals. In some species, families of females and young sometimes aggregate.^[69]

Scorpions prefer areas where the temperature remains in the range of 11–40 °C (52–104 °F), but may survive temperatures from well below freezing to desert heat.^[70][71] Scorpions can withstand intense heat: Leiurus quinquestriatus, Scorpio maurus and Hadrurus arizonensis can live in temperatures of 45–50 °C (113–122 °F) if they are sufficiently hydrated. Desert species must deal with the extreme changes in temperature from day to night or between seasons; Pectinibuthus birulai lives in a temperature range of −30–50 °C (−22–122 °F). Scorpions that live outside deserts prefer lower temperatures. The ability to resist cold may be related to the increase in the sugar trehalose when the temperature drops. Some species hibernate.^[72] Scorpions are resistant to ionizing radiation, surviving nuclear tests in Algeria^[73] and Nevada.^[74]

Desert scorpions have several adaptations for water conservation. They excrete insoluble compounds such as xanthine, guanine, and uric acid, not requiring water for their removal from the body. Guanine is the main component and maximizes the amount of nitrogen excreted. A scorpion’s cuticle holds in moisture via lipids and waxes from epidermal glands, and protects against ultraviolet radiation. Even when dehydrated, a scorpion can tolerate high osmotic pressure in its blood.^[75] Desert scorpions get most of their moisture from the food they eat but some can absorb water from the humid soil. Species that live in denser vegetation and in more moderate temperatures will drink water on plants and in puddles.^[76]

A scorpion uses its stinger both for killing prey and defense. Some species make direct, quick strikes with their tails while others make slower, more circular strikes which can more easily return the stinger to a position where it can strike again. Leiurus quinquestriatus can whip its tail at a speed of up to 128 cm/s (50 in/s) in a defensive strike.^[77]

Mortality and defense

Scorpions may be attacked by other arthropods like ants, spiders, solifugids and centipedes. Major predators include frogs, lizards, snakes, birds, and mammals.^[78] Meerkats are somewhat specialized in preying on scorpions, biting off their stingers and being immune to their venom.^[79][80] Other predators adapted for hunting scorpions include the grasshopper mouse and desert long-eared bat, which are also immune to their venom.^[81][82] In one study, 70% of the latter’s droppings contained scorpion fragments.^[82] Scorpions host parasites including mites, scuttle flies, nematodes and some bacteria. The immune system of scorpions gives them resistance to infection by many types of bacteria.^[83]

When threatened, a scorpion raises its claws and tail in a defensive posture. Some species stridulate to warn off predators by rubbing certain hairs, the stinger or the claws.^[78] Certain species have a preference for using either the claws or stinger as defense, depending on the size of the appendages.^[84] A few scorpions, such as Parabuthus, Centruroides margaritatus, and Hadrurus arizonensis, squirt venom in a narrow jet as far as 1 meter (3.3 ft) to warn off potential predators, possibly injuring them in the eyes.^[85] Some Ananteris species can shed parts of their tail to escape predators. The parts do not grow back, leaving them unable to sting and defecate, but they can still catch small prey and reproduce for at least eight months afterward.^[86]

Diet and feeding

Scorpions generally prey on insects, particularly grasshoppers, crickets, termites, beetles and wasps. Other prey include spiders, solifugids, woodlice and even small vertebrates including lizards, snakes and mammals. Species with large claws may prey on earthworms and mollusks. The majority of species are opportunistic and consume a variety of prey though some may be highly specialized; Isometroides vescus specializes on burrowing spiders. Prey size depends on the size of the species. Several scorpion species are sit-and-wait predators, which involves them waiting for prey at or near the entrance to their burrow. Others actively seek them out. Scorpions detect their prey with mechanoreceptive and chemoreceptive hairs on their bodies and capture them with their claws. Small animals are merely killed with the claws, particularly by large-clawed species. Larger and more aggressive prey is given a sting.^[87][88]

Scorpions, like other arachnids, digest their food externally. The chelicerae, which are very sharp, are used to pull small amounts of food off the prey item into a pre-oral cavity below the chelicerae and carapace. The digestive juices from the gut are egested onto the food, and the digested food is then sucked into the gut in liquid form. Any solid indigestible matter (such as exoskeleton fragments) is trapped by setae in the pre-oral cavity and ejected. The sucked-in food is pumped into the midgut by the pharynx, where it is further digested. The waste passes through the hindgut and out of the anus. Scorpions can consume large amounts of food during one meal. They have an efficient food storage organ and a very low metabolic rate, and a relatively inactive lifestyle. This enables some to survive six to twelve months of starvation.^[89]

Mating

Most scorpions reproduce sexually, with male and female individuals; species in some genera, such as Hottentotta and Tityus, and the species Centruroides gracilis, Liocheles australasiae, and Ananteris coineaui have been reported, not necessarily reliably, to reproduce through parthenogenesis, in which unfertilized eggs develop into living embryos.^[90] Receptive females produce pheromones which are picked up by wandering males using their pectines to comb the substrate. Males begin courtship by moving their bodies back and forth, without moving the legs, a behavior known as juddering. This appears to produce ground vibrations that are picked up by the female.^[62]

The pair then make contact using their pedipalps, and perform a dance called the promenade à deux (French for “a walk for two”). In this dance, the male and female move back and forth while facing each other, as the male searches for a suitable place to deposit his spermatophore. The courtship ritual can involve several other behaviors such as a cheliceral kiss, in which the male and female grasp each other’s mouth-parts, arbre droit (“upright tree”) where the partners elevate their posteriors and rub their tails together, and sexual stinging, in which the male stings the female in the chelae or mesosoma to subdue her. The dance can last from a few minutes to several hours.^[91][92]

When the male has located a suitably stable substrate, such as hard ground, agglomerated sand, rock, or tree bark, he deposits the spermatophore and guides the female over it. This allows the spermatophore to enter her genital opercula, which triggers release of the sperm, thus fertilizing the female. A mating plug then forms in the female to prevent her from mating again before the young are born. The male and female then abruptly separate.^[93][94] Sexual cannibalism after mating has only been reported anecdotally in scorpions.^[95]

Birth and development

Gestation in scorpions can last for over a year in some species.^[96] They have two types of embryonic development; apoikogenic and katoikogenic. In the apoikogenic system, which is mainly found in the Buthidae, embryos develop in yolk-rich eggs inside follicles. The katoikogenic system is documented in Hemiscorpiidae, Scorpionidae and Diplocentridae, and involves the embryos developing in a diverticulum which has a teat-like structure for them to feed through.^[97] Unlike the majority of arachnids, which are oviparous, hatching from eggs, scorpions seem to be universally viviparous, with live births.^[98] They are unusual among terrestrial arthropods in the amount of care a female gives to her offspring.^[99] The size of a brood varies by species, from 3 to over 100.^[100] The body size of scorpions is not correlated either with brood size or with life cycle length.^[101]

Before giving birth, the female elevates the front of her body and positions her pedipalps and front legs under her to catch the young (“birth basket”). The young emerge one by one from the genital opercula, expel the embryonic membrane, if any, and are placed on the mother’s back where they remain until they have gone through at least one molt. The period before the first molt is called the pro-juvenile stage; the young are unable to feed or sting, but have suckers on their tarsi, used to hold on to their mother. This period lasts 5 to 25 days, depending on the species. The brood molt for the first time simultaneously in a process that lasts 6 to 8 hours, marking the beginning of the juvenile stage.^[100]

Juvenile stages or instars generally resemble smaller versions of adults, with fully developed pincers, hairs and stingers. They are still soft and lack pigments, and thus continue to ride on their mother’s back for protection. They become harder and more pigmented over the next couple of days. They may leave their mother temporarily, returning when they sense potential danger. Once the exoskeleton is fully hardened, the young can hunt prey on their own and may soon leave their mother.^[102] A scorpion may molt six times on average before reaching maturity, which may not occur until it is 6 to 83 months old, depending on the species. Some species may live up to 25 years.^[96]

Fluorescence

This black emperor scorpion (Pandinus imperator) fluoresces light blue.

The mother glows bright teal, the young a dull grey.

Scorpions glow a vibrant blue-green when exposed to certain wavelengths of ultraviolet light, such as that produced by a black light, due to fluorescent chemicals such as beta-carboline in the cuticle. Accordingly, a hand-held ultraviolet lamp has long been a standard tool for nocturnal field surveys of these animals. Fluorescence occurs as a result of sclerotization and increases in intensity with each successive instar.^[103] This fluorescence may have an active role in the scorpion’s ability to detect light.^[104]

Relationship with humans

Further information: Human uses of scorpions

Stings

Main article: Scorpionism

Scorpion venom serves to kill or paralyze prey rapidly. The stings of many species are uncomfortable, but only 25 species have venom that is deadly to humans. Those species belong to the family Buthidae, including Leiurus quinquestriatus, Hottentotta spp., Centruroides spp., and Androctonus spp.^[39] People with allergies are especially at risk;^[105] otherwise, first aid is symptomatic, with analgesia. Cases of very high blood pressure are treated with medications that relieve anxiety and relax the blood vessels.^[106][107] Scorpion envenomation with high morbidity and mortality is usually due to either excessive autonomic activity and cardiovascular toxic effects, or neuromuscular toxic effects. Antivenom is the specific treatment for scorpion envenomation combined with supportive measures including vasodilators in patients with cardiovascular toxic effects, and benzodiazepines when there is neuromuscular involvement. Although rare, severe hypersensitivity reactions including anaphylaxis to scorpion antivenin are possible.^[108]

Scorpion stings are a public health problem, particularly in the tropical and subtropical regions of the Americas, North Africa, the Middle East and India. Around 1.5 million scorpion envenomations occur each year, with around 2,600 deaths.^{[109][110][111]} Mexico is one of the most affected countries, with the highest biodiversity of scorpions in the world, some 200,000 envenomations per year and at least 300 deaths.^[112][113]

Efforts are made to prevent envenomation and to control scorpion populations. Prevention encompasses personal activities such as checking shoes and clothes before putting them on, not walking in bare feet or sandals, and filling in holes and cracks where scorpions might nest. Street lighting reduces scorpion activity. Control may involve the use of insecticides such as pyrethroids, or gathering scorpions manually with the help of ultraviolet lights. Domestic predators of scorpions, such as chickens and turkeys, can help to reduce the risk to a household.^[109][110]

Potential medical use

Scorpion venom is a mixture of neurotoxins; most of these are peptides, chains of amino acids.^[115] Many of them interfere with membrane channels that transport sodium, potassium, calcium, or chloride ions. These channels are essential for nerve conduction, muscle contraction and many other biological processes. Some of these molecules may be useful in medical research and might lead to the development of new disease treatments. Among their potential therapeutic uses are as analgesic, anti-cancer, antibacterial, antifungal, antiviral, antiparasitic, bradykinin-potentiating, and immunosuppressive drugs. As of 2020, no scorpion toxin-based drug is for sale, though chlorotoxin is being trialled for use against glioma, a brain cancer.^[114]

Consumption

Scorpions are eaten by people in West Africa, Myanmar^[116] and East Asia. Fried scorpion is traditionally eaten in Shandong, China.^[117] There, scorpions can be cooked and eaten in a variety of ways, including roasting, frying, grilling, raw, or alive. The stingers are typically not removed, since direct and sustained heat negates the harmful effects of the venom.^[118] In Thailand, scorpions are not eaten as often as other arthropods, such as grasshoppers, but they are sometimes fried as street food.^[119] They are used in Vietnam to make snake wine (scorpion wine).^[120]

Pets

Scorpions are often kept as pets. They are relatively simple to keep, the main requirements being a secure enclosure such as a glass terrarium with a lockable lid and the appropriate temperature and humidity for the chosen species, which typically means installing a heating mat and spraying regularly with a little water. The substrate needs to resemble that of the species’ natural environment, such as peat for forest species, or lateritic sand for burrowing desert species. Scorpions in the genera Pandinus and Heterometrus are docile enough to handle. A large Pandinus may consume up to three crickets each week. Cannibalism is more common in captivity than in the wild and can be minimized by providing many small shelters within the enclosure and ensuring there is plenty of prey.^[121][122] The pet trade has threatened wild populations of some scorpion species, particularly Androctonus australis and Pandinus imperator.^[123]

Culture

• Late period bronze figure of Isis-Serket
• “Scorpion and snake fighting”, Anglo-Saxon Herbal, c. 1050
• The constellation Scorpius, depicted in Urania’s Mirror as “Scorpio”, London, c. 1825
• A scorpion motif (two types shown) was often woven into Turkish kilim flatweave carpets, for protection from their sting.^[124]

The scorpion is a culturally significant animal, appearing as a motif in art, especially in Islamic art in the Middle East.^[125] A scorpion motif is often woven into Turkish kilim flatweave carpets, for protection from their sting.^[124] The scorpion is perceived both as an embodiment of evil and as a protective force such as a dervish‘s powers to combat evil.^[125] In Muslim folklore, the scorpion portrays human sexuality.^[125] Scorpions are used in folk medicine in South Asia, especially in antidotes for scorpion stings.^[125]

One of the earliest occurrences of the scorpion in culture is its inclusion, as Scorpio, in the 12 signs of the Zodiac by Babylonian astronomers during the Chaldean period. This was then taken up by western astrology; in astronomy the corresponding constellation is named Scorpius.^[126] In ancient Egypt, the goddess Serket, who protected the Pharaoh, was often depicted as a scorpion.^[127] In ancient Greece, a warrior’s shield sometimes carried a scorpion device, as seen in red-figure pottery from the 5th century BC.^[128] In Greek mythology, Artemis or Gaia sent a giant scorpion to kill the hunter Orion, who had said he would kill all the world’s animals. Orion and the scorpion both became constellations; as enemies they were placed on opposite sides of the world, so when one rises in the sky, the other sets.^[129][130] Scorpions are mentioned in the Bible and the Talmud as symbols of danger and maliciousness.^[130]

The fable of The Scorpion and the Frog has been interpreted as showing that vicious people cannot resist hurting others, even when it is not in their interests.^[131] More recently, the action in John Steinbeck‘s 1947 novella The Pearl centers on a poor pearl fisherman’s attempts to save his infant son from a scorpion sting, only to lose him to human violence.^[132] Scorpions have equally appeared in western artforms including film and poetry: the surrealist filmmaker Luis Buñuel made symbolic use of scorpions in his 1930 classic L’Age d’or (The Golden Age),^[133] while Stevie Smith‘s last collection of poems was entitled Scorpion and other Poems.^[134] A variety of martial arts films and video games have been entitled Scorpion King.^{[135][136][137]}

Since classical times, the scorpion with its powerful stinger has been used to provide a name for weapons. In the Roman army, the scorpio was a torsion siege engine used to shoot a projectile.^[139] The British Army’s FV101 Scorpion was an armored reconnaissance vehicle or light tank in service from 1972 to 1994.^[140] A version of the Matilda II tank, fitted with a flail to clear mines, was named the Matilda Scorpion.^[141] Several ships of the Royal Navy and of the US Navy have been named Scorpion including an 18-gun sloop in 1803,^[142] a turret ship in 1863,^[143] a patrol yacht in 1898,^[144] a destroyer in 1910,^[145] and a nuclear submarine in 1960.^[146]

The scorpion has served as the name or symbol of products and brands including Italy’s Abarth racing cars^[147] and a Montesa scrambler motorcycle.^[148] A hand- or forearm-balancing asana in modern yoga as exercise with the back arched and one or both legs pointing forward over the head in the manner of the scorpion’s tail is called Scorpion pose.^[

Starfish or sea stars are star-shaped echinoderms belonging to the class Asteroidea (/ˌæstəˈrɔɪdiə/). Common usage frequently finds these names being also applied to ophiuroids, which are correctly referred to as brittle stars or basket stars. Starfish are also known as asteroids due to being in the class Asteroidea. About 1,900 species of starfish live on the seabed in all the world’s oceans, from warm, tropical zones to frigid, polar regions. They are found from the intertidal zone down to abyssal depths, at 6,000 m (20,000 ft) below the surface.

Starfish are marine invertebrates. They typically have a central disc and usually five arms, though some species have a larger number of arms. The aboral or upper surface may be smooth, granular or spiny, and is covered with overlapping plates. Many species are brightly coloured in various shades of red or orange, while others are blue, grey or brown. Starfish have tube feet operated by a hydraulic system and a mouth at the centre of the oral or lower surface. They are opportunistic feeders and are mostly predators on benthic invertebrates. Several species have specialized feeding behaviours including eversion of their stomachs and suspension feeding. They have complex life cycles and can reproduce both sexually and asexually. Most can regenerate damaged parts or lost arms and they can shed arms as a means of defense. The Asteroidea occupy several significant ecological roles. Starfish, such as the ochre sea star (Pisaster ochraceus) and the reef sea star (Stichaster australis), have become widely known as examples of the keystone species concept in ecology. The tropical crown-of-thorns starfish (Acanthaster planci) is a voracious predator of coral throughout the Indo-Pacific region, and the Northern Pacific seastar is on the list of the World’s 100 Worst Invasive Alien Species.

The fossil record for starfish is ancient, dating back to the Ordovician around 450 million years ago, but it is rather sparse, as starfish tend to disintegrate after death. Only the ossicles and spines of the animal are likely to be preserved, making remains hard to locate. With their appealing symmetrical shape, starfish have played a part in literature, legend, design and popular culture. They are sometimes collected as curios, used in design or as logos, and in some cultures, despite possible toxicity, they are eaten.

Anatomy

Most starfish have five arms that radiate from a central disc, but the number varies with the group. Some species have six or seven arms and others have 10–15 arms.^[3] The Antarctic Labidiaster annulatus can have over fifty.^[4]

Mapping the expression patterns of genes that express differently across the body axes suggests that one could think of the body of a starfish as a disembodied head walking about the sea floor on its lips. The known markers for trunk structures are expressed only in internal tissues rather than on the surface. Only the front part of the axis, which specifies head-related structures, is represented on the body surface.^[5]

Body wall

The body wall consists of a thin cuticle, an epidermis consisting of a single layer of cells, a thick dermis formed of connective tissue and a thin coelomic myoepithelial layer, which provides the longitudinal and circular musculature. The dermis contains an endoskeleton of calcium carbonate components known as ossicles. These are honeycombed structures composed of calcite microcrystals arranged in a lattice.^[6] They vary in form, with some bearing external granules, tubercles and spines, but most are tabular plates that fit neatly together in a tessellated manner and form the main covering of the aboral surface.^[7] Some are specialised structures such as the madreporite (the entrance to the water vascular system), pedicellariae and paxillae.^[6] Pedicellariae are compound ossicles with forceps-like jaws. They remove debris from the body surface and wave around on flexible stalks in response to physical or chemical stimuli while continually making biting movements. They often form clusters surrounding spines.^[8][9] Paxillae are umbrella-like structures found on starfish that live buried in sediment. The edges of adjacent paxillae meet to form a false cuticle with a water cavity beneath in which the madreporite and delicate gill structures are protected. All the ossicles, including those projecting externally, are covered by the epidermal layer.^[6]

Several groups of starfish, including Valvatida and Forcipulatida, possess pedicellariae.^[8] In Forcipulatida, such as Asterias and Pisaster, they occur in pompom-like tufts at the base of each spine, whereas in the Goniasteridae, such as Hippasteria phrygiana, the pedicellariae are scattered over the body surface. Some are thought to assist in defence, while others aid in feeding or in the removal of organisms attempting to settle on the starfish’s surface.^[10] Some species like Labidiaster annulatus, Rathbunaster californicus and Novodinia antillensis use their large pedicellariae to capture small fish and crustaceans.^[11]

There may also be papulae, thin-walled protrusions of the body cavity that reach through the body wall and extend into the surrounding water. These serve a respiratory function.^[12] The structures are supported by collagen fibres set at right angles to each other and arranged in a three-dimensional web with the ossicles and papulae in the interstices. This arrangement enables both easy flexion of the arms by the starfish and the rapid onset of stiffness and rigidity required for actions performed under stress.^[13]

Water vascular system

The water vascular system of the starfish is a hydraulic system made up of a network of fluid-filled canals and is concerned with locomotion, adhesion, food manipulation and gas exchange. Water enters the system through the madreporite, a porous, often conspicuous, sieve-like ossicle on the aboral surface. It is linked through a stone canal, often lined with calcareous material, to a ring canal around the mouth opening. A set of radial canals leads off this; one radial canal runs along the ambulacral groove in each arm. There are short lateral canals branching off alternately to either side of the radial canal, each ending in an ampulla. These bulb-shaped organs are joined to tube feet (podia) on the exterior of the animal by short linking canals that pass through ossicles in the ambulacral groove. There are usually two rows of tube feet but in some species, the lateral canals are alternately long and short and there appear to be four rows. The interior of the whole canal system is lined with cilia.^[14]

When longitudinal muscles in the ampullae contract, valves in the lateral canals close and water is forced into the tube feet. These extend to contact the substrate. Although the tube feet resemble suction cups in appearance, the gripping action is a function of adhesive chemicals rather than suction.^[15] Other chemicals and relaxation of the ampullae allow for release from the substrate. The tube feet latch on to surfaces and move in a wave, with one arm section attaching to the surface as another releases.^[16][17] Some starfish turn up the tips of their arms while moving which gives maximum exposure of the sensory tube feet and the eyespot to external stimuli.^[18]

Having descended from bilateral organisms, starfish may move in a bilateral fashion, particularly when hunting or in danger. When crawling, certain arms act as the leading arms, while others trail behind.^[3][19][9] Most starfish cannot move quickly, a typical speed being that of the leather star (Dermasterias imbricata), which can manage just 15 cm (6 in) in a minute.^[20] Some burrowing species from the genera Astropecten and Luidia have points rather than suckers on their long tube feet and are capable of much more rapid motion, “gliding” across the ocean floor. The sand star (Luidia foliolata) can travel at a speed of 2.8 m (9 ft 2 in) per minute.^[21] When a starfish finds itself upside down, two adjacent arms are bent backwards to provide support, the opposite arm is used to stamp the ground while the two remaining arms are raised on either side; finally the stamping arm is released as the starfish turns itself over and recovers its normal stance.^[19]

Apart from their function in locomotion, the tube feet act as accessory gills. The water vascular system serves to transport oxygen from, and carbon dioxide to, the tube feet and also nutrients from the gut to the muscles involved in locomotion. Fluid movement is bidirectional and initiated by cilia.^[14] Gas exchange also takes place through other gills known as papulae, which are thin-walled bulges on the aboral surface of the disc and arms. Oxygen is transferred from these to the coelomic fluid, which acts as the transport medium for gasses. Oxygen dissolved in the water is distributed through the body mainly by the fluid in the main body cavity; the circulatory system may also play a minor role.^[22]

Digestive system and excretion

The gut of a starfish occupies most of the disc and extends into the arms. The mouth is located in the centre of the oral surface, where it is surrounded by a tough peristomial membrane and closed with a sphincter. The mouth opens through a short oesophagus into a stomach divided by a constriction into a larger, eversible cardiac portion and a smaller pyloric portion. The cardiac stomach is glandular and pouched, and is supported by ligaments attached to ossicles in the arms so it can be pulled back into position after it has been everted. The pyloric stomach has two extensions into each arm: the pyloric caeca. These are elongated, branched hollow tubes that are lined by a series of glands, which secrete digestive enzymes and absorb nutrients from the food. A short intestine and rectum run from the pyloric stomach to open at a small anus at the apex of the aboral surface of the disc.^[23]

Primitive starfish, such as Astropecten and Luidia, swallow their prey whole, and start to digest it in their cardiac stomachs. Shell valves and other inedible materials are ejected through their mouths. The semi-digested fluid is passed into their pyloric stomachs and caeca where digestion continues and absorption ensues.^[23] In more advanced species of starfish, the cardiac stomach can be everted from the organism’s body to engulf and digest food. When the prey is a clam or other bivalve, the starfish pulls with its tube feet to separate the two valves slightly, and inserts a small section of its stomach, which releases enzymes to digest the prey. The stomach and the partially digested prey are later retracted into the disc. Here the food is passed on to the pyloric stomach, which always remains inside the disc.^[24] The retraction and contraction of the cardiac stomach is activated by a neuropeptide known as NGFFYamide.^[25]

Because of this ability to digest food outside the body, starfish can hunt prey much larger than their mouths. Their diets include clams and oysters, arthropods, small fish and gastropod molluscs. Some starfish are not pure carnivores, supplementing their diets with algae or organic detritus. Some of these species are grazers, but others trap food particles from the water in sticky mucus strands that are swept towards the mouth along ciliated grooves.^[23]

The main nitrogenous waste product is ammonia. Starfish have no distinct excretory organs; waste ammonia is removed by diffusion through the tube feet and papulae.^[22] The body fluid contains phagocytic cells called coelomocytes, which are also found within the hemal and water vascular systems. These cells engulf waste material, and eventually migrate to the tips of the papulae, where a portion of body wall is nipped off and ejected into the surrounding water. Some waste may also be excreted by the pyloric glands and voided with the faeces.^[22]

Starfish do not appear to have any mechanisms for osmoregulation, and keep their body fluids at the same salt concentration as the surrounding water. Although some species can tolerate relatively low salinity, the lack of an osmoregulation system probably explains why starfish are not found in fresh water or even in many estuarine environments.^[22]

Sensory and nervous systems

Although starfish do not have many well-defined sense organs, they are sensitive to touch, light, temperature, orientation and the status of the water around them. The tube feet, spines and pedicellariae are sensitive to touch. The tube feet, especially those at the tips of the rays, are also sensitive to chemicals, enabling the starfish to detect odour sources such as food.^[24] There are eyespots at the ends of the arms, each one made of 80–200 simple ocelli. These are composed of pigmented epithelial cells that respond to light and are covered by a thick, transparent cuticle that both protects the ocelli and acts to focus light. Many starfish also possess individual photoreceptor cells in other parts of their bodies and respond to light even when their eyespots are covered. Whether they advance or retreat depends on the species.^[26]

While a starfish lacks a centralized brain, it has a complex nervous system with a nerve ring around the mouth and a radial nerve running along the ambulacral region of each arm parallel to the radial canal. The peripheral nerve system consists of two nerve nets: a sensory system in the epidermis and a motor system in the lining of the coelomic cavity. Neurons passing through the dermis connect the two.^[26] The ring nerves and radial nerves have sensory and motor components and coordinate the starfish’s balance and directional systems.^[12] The sensory component receives input from the sensory organs while the motor nerves control the tube feet and musculature. The starfish does not have the capacity to plan its actions. If one arm detects an attractive odour, it becomes dominant and temporarily over-rides the other arms to initiate movement towards the prey. The mechanism for this is not fully understood.^[26]

Circulatory system

The body cavity contains the circulatory or haemal system. The vessels form three rings: one around the mouth (the hyponeural haemal ring), another around the digestive system (the gastric ring) and the third near the aboral surface (the genital ring). The heart beats about six times a minute and is at the apex of a vertical channel (the axial vessel) that connects the three rings. At the base of each arm are paired gonads; a lateral vessel extends from the genital ring past the gonads to the tip of the arm. This vessel has a blind end and there is no continuous circulation of the fluid within it. This liquid does not contain a pigment and has little or no respiratory function but is probably used to transport nutrients around the body.^[27]

Secondary metabolites

Starfish produce a large number of secondary metabolites in the form of lipids, including steroidal derivatives of cholesterol, and fatty acid amides of sphingosine. The steroids are mostly saponins, known as asterosaponins, and their sulphated derivatives. They vary between species and are typically formed from up to six sugar molecules (usually glucose and galactose) connected by up to three glycosidic chains. Long-chain fatty acid amides of sphingosine occur frequently and some of them have known pharmacological activity. Various ceramides are also known from starfish and a small number of alkaloids have also been identified. The functions of these chemicals in the starfish have not been fully investigated but most have roles in defence and communication. Some are feeding deterrents used by the starfish to discourage predation. Others are antifoulants and supplement the pedicellariae to prevent other organisms from settling on the starfish’s aboral surface. Some are alarm pheromones and escape-eliciting chemicals, the release of which trigger responses in conspecific starfish but often produce escape responses in potential prey.^[28] Research into the efficacy of these compounds for possible pharmacological or industrial use occurs worldwide.^[29]

Life cycle

Sexual reproduction

Most species of starfish are gonochorous, there being separate male and female individuals. These are usually not distinguishable externally as the gonads cannot be seen, but their sex is apparent when they spawn.^[30] Some species are simultaneous hermaphrodites, producing eggs and sperm at the same time, and in a few of these the same gonad, called an ovotestis, produces both eggs and sperm.^[31] Other starfish are sequential hermaphrodites. Protandrous individuals of species like Asterina gibbosa start life as males before changing sex into females as they grow older. In some species such as Nepanthia belcheri, a large female can split in half and the resulting offspring are males. When these grow large enough they change back into females.^[32]

Each starfish arm contains two gonads that release gametes through openings called gonoducts, located on the central disc between the arms. Fertilization is generally external but in a few species, internal fertilization takes place. In most species, the buoyant eggs and sperm are simply released into the water (free spawning) and the resulting embryos and larvae live as part of the plankton. In others, the eggs may be stuck to the undersides of rocks.^[33] In certain species of starfish, the females brood their eggs – either by simply enveloping them^[33] or by holding them in specialised structures. Brooding may be done in pockets on the starfish’s aboral surface,^[34][30] inside the pyloric stomach (Leptasterias tenera)^[35] or even in the interior of the gonads themselves.^[31] Those starfish that brood their eggs by “sitting” on them usually assume a humped posture with their discs raised off the substrate.^[36] Pteraster militaris broods a few of its young and disperses the remaining eggs, that are too numerous to fit into its pouch.^[34] In these brooding species, the eggs are relatively large, and supplied with yolk, and they generally develop directly into miniature starfish without an intervening larval stage.^[31] The developing young are called lecithotrophic because they obtain their nutrition from the yolk as opposed to “planktotrophic” larvae that feed in the water column. In Parvulastra parvivipara, an intragonadal brooder, the young starfish obtain nutrients by eating other eggs and embryos in the brood pouch.^[37] Brooding is especially common in polar and deep-sea species that live in environments unfavourable for larval development^[30] and in smaller species that produce just a few eggs.^[38][39]

In the tropics, a plentiful supply of phytoplankton is continuously available for starfish larvae to feed on. Spawning takes place at any time of year, each species having its own characteristic breeding season.^[40] In temperate regions, the spring and summer brings an increase in food supplies. The first individual of a species to spawn may release a pheromone that serves to attract other starfish to aggregate and to release their gametes synchronously.^[41] In other species, a male and female may come together and form a pair.^[42][43] This behaviour is called pseudocopulation^[44] and the male climbs on top, placing his arms between those of the female. When she releases eggs into the water, he is induced to spawn.^[41] Starfish may use environmental signals to coordinate the time of spawning (day length to indicate the correct time of the year,^[42] dawn or dusk to indicate the correct time of day), and chemical signals to indicate their readiness to breed. In some species, mature females produce chemicals to attract sperm in the sea water.^[45]

Larval development

Most starfish embryos hatch at the blastula stage. The original ball of cells develops a lateral pouch, the archenteron. The entrance to this is known as the blastopore and it will later develop into the anus—together with chordates, echinoderms are deuterostomes, meaning the second (deutero) invagination becomes the mouth (stome); members of all other phyla are protostomes, and their first invagination becomes the mouth. Another invagination of the surface will fuse with the tip of the archenteron as the mouth while the interior section will become the gut. At the same time, a band of cilia develops on the exterior. This enlarges and extends around the surface and eventually onto two developing arm-like outgrowths. At this stage the larva is known as a bipinnaria. The cilia are used for locomotion and feeding, their rhythmic beat wafting phytoplankton towards the mouth.^[8]

The next stage in development is a brachiolaria larva and involves the growth of three short, additional arms. These are at the anterior end, surround a sucker and have adhesive cells at their tips. Both bipinnaria and brachiolaria larvae are bilaterally symmetrical. When fully developed, the brachiolaria settles on the seabed and attaches itself with a short stalk formed from the ventral arms and sucker. Metamorphosis now takes place with a radical rearrangement of tissues. The left side of the larval body becomes the oral surface of the juvenile and the right side the aboral surface. Part of the gut is retained, but the mouth and anus move to new positions. Some of the body cavities degenerate but others become the water vascular system and the visceral coelom. The starfish is now pentaradially symmetrical. It casts off its stalk and becomes a free-living juvenile starfish about 1 mm (0.04 in) in diameter. Starfish of the order Paxillosida have no brachiolaria stage, with the bipinnaria larvae settling on the seabed and developing directly into juveniles.^[8]

Asexual reproduction

Main article: Asexual reproduction in starfish

Some species of starfish in the three families Asterinidae, Asteriidae and Solasteridae are able to reproduce asexually as adults either by fission of their central discs^[46] or by autotomy of one or more of their arms.^[47] Which of these processes occurs depends on the genus. Among starfish that are able to regenerate their whole body from a single arm, some can do so even from fragments just 1 cm (0.4 in) long.^[48] Single arms that regenerate a whole individual are called comet forms. The division of the starfish, either across its disc or at the base of the arm, is usually accompanied by a weakness in the structure that provides a fracture zone.^[49]

The larvae of several species of starfish can reproduce asexually before they reach maturity.^[50] They do this by autotomising some parts of their bodies or by budding.^[51] When such a larva senses that food is plentiful, it takes the path of asexual reproduction rather than normal development.^[52] Though this costs it time and energy and delays maturity, it allows a single larva to give rise to multiple adults when the conditions are appropriate.^[51]

Regeneration

Main article: Starfish regeneration

Some species of starfish have the ability to regenerate lost arms and can regrow an entire new limb given time.^[48] A few can regrow a complete new disc from a single arm, while others need at least part of the central disc to be attached to the detached part.^[22] Regrowth can take several months or years,^[48] and starfish are vulnerable to infections during the early stages after the loss of an arm. A separated limb lives off stored nutrients until it regrows a disc and mouth and is able to feed again.^[48] Other than fragmentation carried out for the purpose of reproduction, the division of the body may happen inadvertently due to part being detached by a predator, or part may be actively shed by the starfish in an escape response.^[22] The loss of parts of the body is achieved by the rapid softening of a special type of connective tissue in response to nervous signals. This type of tissue is called catch connective tissue and is found in most echinoderms.^[53] An autotomy-promoting factor has been identified which, when injected into another starfish, causes rapid shedding of arms.^[54]

Lifespan

The lifespan of a starfish varies considerably between species, generally being longer in larger forms and in those with planktonic larvae. For example, Leptasterias hexactis broods a small number of large-yolked eggs. It has an adult weight of 20 g (0.7 oz), reaches sexual maturity in two years and lives for about ten years.^[8] Pisaster ochraceus releases a large number of eggs into the sea each year and has an adult weight of up to 800 g (28 oz). It reaches maturity in five years and has a maximum recorded lifespan of 34 years.^[8] The average lifespan of a starfish is 35 years, and larger starfish species typically live longer than their smaller counterparts.^[55]

Ecology

Distribution and habitat

Echinoderms, including starfish, maintain a delicate internal electrolyte balance that is in equilibrium with sea water, making it impossible for them to live in a freshwater habitat.^[16] Starfish species inhabit all of the world’s oceans. Habitats range from tropical coral reefs, rocky shores, tidal pools, mud, and sand to kelp forests, seagrass meadows^[56] and the deep-sea floor down to at least 6,000 m (20,000 ft).^[57] The greatest diversity of species occurs in coastal areas.^[56]

Diet

Most species are generalist predators, eating microalgae, sponges, bivalves, snails and other small animals.^[24][58] The crown-of-thorns starfish consumes coral polyps,^[59] while other species are detritivores, feeding on decomposing organic material and faecal matter.^[58][60] A few are suspension feeders, gathering in phytoplankton; Henricia and Echinaster often occur in association with sponges, benefiting from the water current they produce.^[61] Various species have been shown to be able to absorb organic nutrients from the surrounding water, and this may form a significant portion of their diet.^[61]

The processes of feeding and capture may be aided by special parts; Pisaster brevispinus, the short-spined pisaster from the West Coast of America, can use a set of specialized tube feet to dig itself deep into the soft substrate to extract prey (usually clams).^[62] Grasping the shellfish, the starfish slowly pries open the prey’s shell by wearing out its adductor muscle, and then inserts its everted stomach into the crack to digest the soft tissues. The gap between the valves need only be a fraction of a millimetre wide for the stomach to gain entry.^[16] Cannibalism has been observed in juvenile sea stars as early as four days after metamorphosis.^[63]

Ecological impact

Starfish are keystone species in their respective marine communities. Their relatively large sizes, diverse diets and ability to adapt to different environments makes them ecologically important.^[64] The term “keystone species” was in fact first used by Robert Paine in 1966 to describe a starfish, Pisaster ochraceus.^[65] When studying the low intertidal coasts of Washington state, Paine found that predation by P. ochraceus was a major factor in the diversity of species. Experimental removals of this top predator from a stretch of shoreline resulted in lower species diversity and the eventual domination of Mytilus mussels, which were able to outcompete other organisms for space and resources.^[66] Similar results were found in a 1971 study of Stichaster australis on the intertidal coast of the South Island of New Zealand. S. australis was found to have removed most of a batch of transplanted mussels within two or three months of their placement, while in an area from which S. australis had been removed, the mussels increased in number dramatically, overwhelming the area and threatening biodiversity.^[67]

The feeding activity of the omnivorous starfish Oreaster reticulatus on sandy and seagrass bottoms in the Virgin Islands appears to regulate the diversity, distribution and abundance of microorganisms. These starfish engulf piles of sediment removing the surface films and algae adhering to the particles.^[68] Organisms that dislike this disturbance are replaced by others better able to rapidly recolonise “clean” sediment. In addition, foraging by these migratory starfish creates diverse patches of organic matter, which may play a role in the distribution and abundance of organisms such as fish, crabs and sea urchins that feed on the sediment.^[69]

Starfish sometimes have negative effects on ecosystems. Outbreaks of crown-of-thorns starfish have caused damage to coral reefs in Northeast Australia and French Polynesia.^[59][70] A study in Polynesia found that coral cover declined drastically with the arrival of migratory starfish in 2006, dropping from 50% to under 5% in three years. This had a cascading effect on the whole benthic community and reef-feeding fish.^[59] Asterias amurensis is one of a few echinoderm invasive species. Its larvae likely arrived in Tasmania from central Japan via water discharged from ships in the 1980s. The species has since grown in numbers to the point where they threaten commercially important bivalve populations. As such, they are considered pests,^[71] and are on the Invasive Species Specialist Group’s list of the world’s 100 worst invasive species.^[72]

Sea Stars (starfish) are the main predators of kelp-eating sea urchins. Satellite imagery shows that sea urchin populations have exploded due to starfish mass deaths, and that by 2021, sea urchins have destroyed 95% of California’s kelp forests.^[73]

Threats

Starfish may be preyed on by conspecifics, sea anemones,^[74] other starfish species, tritons, crabs, fish, gulls and sea otters.^{[38][71][75][76]} Their first lines of defence are the saponins present in their body walls, which have unpleasant flavours.^[77] Some starfish such as Astropecten polyacanthus also include powerful toxins such as tetrodotoxin among their chemical armoury, and the slime star can ooze out large quantities of repellent mucus. They also have body armour in the form of hard plates and spines.^[78] The crown-of-thorns starfish is particularly unattractive to potential predators, being heavily defended by sharp spines, laced with toxins and sometimes with bright warning colours.^[79] Other species protect their vulnerable tube feet and arm tips by lining their ambulacral grooves with spines and heavily plating their extremities.^[78]

Several species sometimes suffer from a wasting condition caused by bacteria in the genus Vibrio;^[75] however, a more widespread wasting disease, causing mass mortalities among starfish, appears sporadically. A paper published in November 2014 revealed the most likely cause of this disease to be a densovirus the authors named sea star-associated densovirus (SSaDV).^[80] The protozoan Orchitophrya stellarum is known to infect the gonads of starfish and damage tissue.^[75] Starfish are vulnerable to high temperatures. Experiments have shown that the feeding and growth rates of P. ochraceus reduce greatly when their body temperatures rise above 23 °C (73 °F) and that they die when their temperature rises to 30 °C (86 °F).^[81][82] This species has a unique ability to absorb seawater to keep itself cool when it is exposed to sunlight by a receding tide.^[83] It also appears to rely on its arms to absorb heat, so as to protect the central disc and vital organs like the stomach.^[84]

Starfish and other echinoderms are sensitive to marine pollution.^[85] The common starfish is considered to be a bioindicator for marine ecosystems.^[86] A 2009 study found that P. ochraceus is unlikely to be affected by ocean acidification as severely as other marine animals with calcareous skeletons. In other groups, structures made of calcium carbonate are vulnerable to dissolution when the pH is lowered. Researchers found that when P. ochraceus were exposed to 21 °C (70 °F) and 770 ppm carbon dioxide (beyond rises expected in the next century), they were relatively unaffected. Their survival is likely due to the nodular nature of their skeletons, which are able to compensate for a shortage of carbonate by growing more fleshy tissue.^[87]

Evolution

Fossil record

Echinoderms first appeared in the fossil record in the Cambrian. The first known asterozoans were the Somasteroidea, which exhibit characteristics of both groups.^[88] Starfish are infrequently found as fossils, possibly because their hard skeletal components separate as the animal decays. Despite this, there are a few places where accumulations of complete skeletal structures occur, fossilized in place in Lagerstätten – so-called “starfish beds”.^[89]

By the late Paleozoic, the crinoids and blastoids were the predominant echinoderms, and some limestones from this period are made almost entirely from fragments from these groups. In the two major extinction events that occurred during the late Devonian and late Permian, the blastoids were wiped out and only a few species of crinoids survived.^[88] Many starfish species also became extinct in these events, but afterwards the surviving few species diversified rapidly within about sixty million years during the Early Jurassic and the beginning of the Middle Jurassic.^[90][91] A 2012 study found that speciation in starfish can occur rapidly. During the last 6,000 years, divergence in the larval development of Cryptasterina hystera and Cryptasterina pentagona has taken place, the former adopting internal fertilization and brooding and the latter remaining a broadcast spawner.^[92]

Diversity

See also: List of echinodermata ordersVideo showing the tube feet movement of a starfish

The scientific name Asteroidea was given to starfish by the French zoologist de Blainville in 1830.^[93] It is derived from the Greek aster, ἀστήρ (a star) and the Greek eidos, εἶδος (form, likeness, appearance).^[94] The class Asteroidea belongs to the phylum Echinodermata. As well as the starfish, the echinoderms include sea urchins, sand dollars, brittle and basket stars, sea cucumbers and crinoids. The larvae of echinoderms have bilateral symmetry, but during metamorphosis this is replaced with radial symmetry, typically pentameric.^[12] Adult echinoderms are characterized by having a water vascular system with external tube feet and a calcareous endoskeleton consisting of ossicles connected by a mesh of collagen fibres.^[95] Starfish are included in the subphylum Asterozoa, the characteristics of which include a flattened, star-shaped body as adults consisting of a central disc and multiple radiating arms. The subphylum includes the two classes of Asteroidea, the starfish, and Ophiuroidea, the brittle stars and basket stars. Asteroids have broad-based arms with skeletal support provided by calcareous plates in the body wall^[90] while ophiuroids have clearly demarcated slender arms strengthened by paired fused ossicles forming jointed “vertebrae”.^[96]

The starfish are a large and diverse class with over 1,900 living species. There are seven extant orders, Brisingida, Forcipulatida, Notomyotida, Paxillosida, Spinulosida, Valvatida and Velatida^[1] and two extinct ones, Calliasterellidae and Trichasteropsida.^[2] Living asteroids, the Neoasteroidea, are morphologically distinct from their forerunners in the Paleozoic. The taxonomy of the group is relatively stable but there is ongoing debate about the status of the Paxillosida, and the deep-water sea daisies, though clearly Asteroidea and currently included in Velatida, do not fit easily in any accepted lineage. Phylogenetic data suggests that they may be a sister group, the Concentricycloidea, to the Neoasteroidea, or that the Velatida themselves may be a sister group.^[91]

Living groups

Brisingida (2 families, 17 genera, 111 species)^[97]Species in this order have a small, inflexible disc and 6–20 long, thin arms, which they use for suspension feeding. They have a single series of marginal plates, a fused ring of disc plates, a reduced number of aboral plates, crossed pedicellariae, and several series of long spines on the arms. They live almost exclusively in deep-sea habitats, although a few live in shallow waters in the Antarctic.^[98][99] In some species, the tube feet have rounded tips and lack suckers.^[100]

Forcipulatida (6 families, 63 genera, 269 species)^[101]Species in this order have distinctive pedicellariae, consisting of a short stalk with three skeletal ossicles. They tend to have robust bodies^[102] and have tube feet with flat-tipped suckers usually arranged in four rows.^[100] The order includes well-known species from temperate regions, including the common starfish of North Atlantic coasts and rock pools, as well as cold-water and abyssal species.^[103]Notomyotida (1 family, 8 genera, 75 species)^[104]These starfish are deep-sea dwelling and have particularly flexible arms. The inner dorso-lateral surfaces of the arms contain characteristic longitudinal muscle bands.^[1] In some species, the tube feet lack suckers.^[100]

Paxillosida (7 families, 48 genera, 372 species)^[105]This is a primitive order and members do not extrude their stomach when feeding, lack an anus and have no suckers on their tube feet. Papulae are plentiful on their aboral surface and they possess marginal plates and paxillae. They mostly inhabit soft-bottomed areas of sand or mud.^[8] There is no brachiolaria stage in their larval development.^[106] The comb starfish (Astropecten polyacanthus) is a member of this order.^[107]

Spinulosida (1 family, 8 genera, 121 species)^[108]Most species in this order lack pedicellariae and all have a delicate skeletal arrangement with small or no marginal plates on the disc and arms. They have numerous groups of short spines on the aboral surface.^[109][110] This group includes the red starfish Echinaster sepositus.^[111]Valvatida (16 families, 172 genera, 695 species)^[112]Most species in this order have five arms and two rows of tube feet with suckers. There are conspicuous marginal plates on the arms and disc. Some species have paxillae and in some, the main pedicellariae are clamp-like and recessed into the skeletal plates.^[110] This group includes the cushion stars,^[113] the leather star^[114] and the sea daisies.^[115]Velatida (4 families, 16 genera, 138 species)^[116]This order of starfish consists mostly of deep-sea and other cold-water starfish often with a global distribution. The shape is pentagonal or star-shaped with five to fifteen arms. They mostly have poorly developed skeletons with papulae widely distributed on the aboral surface and often spiny pedicellariae.^[117] This group includes the slime star.^[118]

Extinct groups

Extinct groups within the Asteroidea include:^[2]

• † Calliasterellidae, with the type genus Calliasterella from the Devonian and Carboniferous^[119]
• † Palastericus, a Devonian genus^[120]
• † Trichasteropsida, with the Triassic genus Trichasteropsis (at least 2 species)^[2]

Phylogeny

External

Starfish are deuterostome animals, like the chordates. A 2014 analysis of 219 genes from all classes of echinoderms gives the following phylogenetic tree.^[121] The times at which the clades diverged are shown under the labels in millions of years ago (mya).

Bilateria	Xenacoelomorpha NephrozoaDeuterostomiaChordata EchinodermataEleutherozoaEchinozoaHolothuroidea Echinoidea AsterozoaOphiuroidea Asteroidea Crinoidea c. 500 mya>540 myaProtostomiaEcdysozoa Spiralia 610 mya650 mya

Internal

The phylogeny of the Asteroidea has been difficult to resolve, with visible (morphological) features proving inadequate, and the question of whether traditional taxa are clades in doubt.^[2] The phylogeny proposed by Gale in 1987 is:^[2][122]

	† Palaeozoic AsteroidsPaxillosidaValvatida, including Velatida, Spinulosida (not a clade)[2]Forcipulatida, including Brisingida

The phylogeny proposed by Blake in 1987 is:^[2][123]

	† Palaeozoic Asteroids† Calliasterellidae† Compasteridae† TrichasteropsidaBrisingidaForcipulatidaSpinulosidaVelatidaNotomyotidaValvatidaPaxillosida

Later work making use of molecular evidence, with or without the use of morphological evidence, had by 2000 failed to resolve the argument.^[2] In 2011, on further molecular evidence, Janies and colleagues noted that the phylogeny of the echinoderms “has proven difficult”, and that “the overall phylogeny of extant echinoderms remains sensitive to the choice of analytical methods”. They presented a phylogenetic tree for the living Asteroidea only; using the traditional names of starfish orders where possible, and indicating “part of” otherwise, the phylogeny is shown below. The Solasteridae are split from the Velatida, and the old Spinulosida is broken up.^[124]

	Solasteridae and part of Spinulosida, e.g. Stegnaster and part of Valvatida, e.g. AsterinaOdontasteridae, which was a part of ValvatidaPaxillosidapart of Spinulosida, e.g. Echinaster, part of Valvatida, e.g. ArchasterForcipulatidaBrisingida with part of Velatida, e.g. Caymanostella and part of Forcipulatida, e.g. StichasterVelatida except for Solasteridae

	Notomyotida (not analysed)

Human relations

In research

Starfish are deuterostomes, closely related, together with all other echinoderms, to chordates, and are used in reproductive and developmental studies. Female starfish produce large numbers of oocytes that are easily isolated; these can be stored in a pre-meiosis phase and stimulated to complete division by the use of 1-methyladenine.^[125] Starfish oocytes are well suited for this research as they are large and easy to handle, transparent, simple to maintain in sea water at room temperature, and they develop rapidly.^[126] Asterina pectinifera, used as a model organism for this purpose, is resilient and easy to breed and maintain in the laboratory.^[127]

Another area of research is the ability of starfish to regenerate lost body parts. The stem cells of adult humans are incapable of much differentiation and understanding the regrowth, repair and cloning processes in starfish may have implications for human medicine.^[128]

Starfish also have an unusual ability to expel foreign objects from their bodies, which makes them difficult to tag for research tracking purposes.^[129]

In legend and culture

An aboriginal Australian fable retold by the Welsh school headmaster William Jenkyn Thomas (1870–1959)^[130] tells how some animals needed a canoe to cross the ocean. Whale had one but refused to lend it, so Starfish kept him busy, telling him stories and grooming him to remove parasites, while the others stole the canoe. When Whale realized the trick he beat Starfish ragged, which is how Starfish still is today.^[131]

In 1900, the scholar Edward Tregear documented The Creation Song, which he describes as “an ancient prayer for the dedication of a high chief” of Hawaii. Among the “uncreated gods” described early in the song are the male Kumulipo (“Creation”) and the female Poele, both born in the night, a coral insect, the earthworm, and the starfish.^[132]

Georg Eberhard Rumpf‘s 1705 The Ambonese Curiosity Cabinet describes the tropical varieties of Stella Marina or Bintang Laut, “Sea Star”, in Latin and Malay respectively, known in the waters around Ambon. He writes that the Histoire des Antilles reports that when the sea stars “see thunder storms approaching, [they] grab hold of many small stones with their little legs, looking to … hold themselves down as if with anchors”.^[133]

Starfish is the title of novels by Peter Watts^[134] and Jennie Orbell,^[135] and in 2012, Alice Addison wrote a non-fiction book titled Starfish: A Year in the Life of Bereavement and Depression.^[136] The Starfish and the Spider is a 2006 business management book by Ori Brafman and Rod Beckstrom; its title alludes to the ability of the starfish to regenerate itself because of its decentralized nervous system, and the book suggests ways that a decentralized organisation may flourish.^[137]

In the Nickelodeon animated television series SpongeBob SquarePants, the eponymous character’s best friend is a dim-witted starfish, Patrick Star.^[138]

As food

Starfish are widespread in the oceans, but are only occasionally used as food. There may be good reason for this: the bodies of numerous species are dominated by bony ossicles, and the body wall of many species contains saponins, which have an unpleasant taste,^[77] and others contain tetrodotoxins which are poisonous.^[139] Some species that prey on bivalve molluscs can transmit paralytic shellfish poisoning.^[140] Georg Eberhard Rumpf found few starfish being used for food in the Indonesian archipelago, other than as bait in fish traps, but on the island of “Huamobel” [sic] the people cut them up, squeeze out the “black blood” and cook them with sour tamarind leaves; after resting the pieces for a day or two, they remove the outer skin and cook them in coconut milk.^[133] Starfish are sometimes eaten in China,^[141] Japan^[142][143] and in Micronesia.^[144]

As collectables

Starfish are in some cases taken from their habitat and sold to tourists as souvenirs, ornaments, curios or for display in aquariums. In particular, Oreaster reticulatus, with its easily accessed habitat and conspicuous coloration, is widely collected in the Caribbean. In the early to mid 20th century, this species was common along the coasts of the West Indies, but collection and trade have severely reduced its numbers. In the State of Florida, O. reticulatus is listed as endangered and its collection is illegal. Nevertheless, it is still sold throughout its range and beyond.^[76] A similar phenomenon exists in the Indo-Pacific for species such as Protoreaster nodosus.^[145]

In industry and military history

With its multiple arms, the starfish provides a popular metaphor for computer networks,^[146] companies^[147][148] and software tools.^[149] It is also the name of a seabed imaging system and company.^[150]

Starfish has repeatedly been chosen as a name in military history. Three ships of the Royal Navy have borne the name HMS Starfish: an A-class destroyer launched in 1894;^[151] an R-class destroyer launched in 1916;^[152] and an S-class submarine launched in 1933 and lost in 1940.^[153] In World War II, Starfish sites were large-scale night-time decoys created during The Blitz to simulate burning British cities.^[154] Starfish Prime was a high-altitude nuclear test conducted by the United States on 9 July 1962.