| Cane toad | |
|---|---|
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| Adult male | |
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| Adult female | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Amphibia |
| Order: | Anura |
| Family: | Bufonidae |
| Genus: | Rhinella |
| Species: | R. marina |
| Binomial name | |
| Rhinella marina | |
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| Distribution of the cane toad, native distribution in blue, introduced in red | |
| Synonyms | |
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Thecane toad (Rhinella marina), also known as thegiant neotropical toad ormarine toad, is a large,terrestrialtrue toad native toSouth and mainlandCentral America, but which has beenintroduced to various islands throughoutOceania and theCaribbean, as well asNorthern Australia. It is a member of the genusRhinella, which includes many true toadspecies found throughout Central and South America, but it was formerly assigned to the genusBufo.
A fossil toad (specimen UCMP 41159) from theLa Venta fauna of the lateMiocene inColombia is morphologically indistinguishable from modern cane toads from northern South America. It was discovered in a floodplain deposit, which suggests theR. marina habitat preferences have long been for open areas. The cane toad is a prolific breeder; females lay single-clumpspawns with thousands ofeggs. Its reproductive success is partly because of opportunistic feeding: it has a diet, unusual amonganurans, of both dead and living matter. Adults average 10–15 cm (4–6 in) in length; the largest recorded specimen had asnout-vent length of 24 cm (9.4 in).
The cane toad has poisonglands, and thetadpoles are highly toxic to most animals if ingested. Its toxic skin can kill many animals, both wild and domesticated, and cane toads are particularly dangerous to dogs. Because of its voracious appetite, the cane toad has been introduced to many regions of the Pacific and the Caribbean islands as a method of agriculturalpest control. The common name of the species is derived from its use against thecane beetle (Dermolepida albohirtum), which damagessugar cane. The cane toad is now considered a pest and aninvasive species in many of its introduced regions. The 1988 filmCane Toads: An Unnatural History documented the trials and tribulations of the introduction ofcane toads in Australia.
Historically, the cane toad was used to eradicate pests fromsugarcane, giving rise to its common name. The cane toad has many other common names, including "giant toad" and "marine toad"; the former refers to its size, and the latter to thebinomial name,R. marina. It was one of many species described byCarl Linnaeus in his 18th-century workSystema Naturae (1758).[5] Linnaeus based thespecific epithetmarina on an illustration by Dutch zoologistAlbertus Seba, who mistakenly believed the cane toad to inhabit both terrestrial and marine environments.[6] Other common names include "giant neotropical toad",[7] "Dominican toad",[8] "giant marine toad",[9] and "South American cane toad".[10] InTrinidadian English, they are commonly calledcrapaud, the French word for toad.[11]
The genusRhinella is considered to constitute a distinctgenus of its own, thus changing thescientific name of the cane toad. In this case, thespecific namemarinus (masculine) changes tomarina (feminine) to conform with the rules of gender agreement as set out by theInternational Code of Zoological Nomenclature, changing the binomial name fromBufo marinus toRhinella marina; the binomialRhinella marinus was subsequently introduced as a synonym through misspelling by Pramuk, Robertson, Sites, and Noonan (2008).[2][3] Though controversial (with many traditional herpetologists still usingBufo marinus) the binomialRhinella marina is gaining in acceptance with such bodies as theIUCN,[1]Encyclopaedia of Life,[12] Amphibian Species of the World[2] and increasing numbers of scientific publications adopting its usage.
Since 2016, cane toad populations native toMesoamerica and northwestern South America are sometimes considered to be a separate species,Rhinella horribilis.[13]
In Australia, the adults may be confused with large native frogs from the generaLimnodynastes,Cyclorana, andMixophyes. These species can be distinguished from the cane toad by the absence of largeparotoid glands behind their eyes and the lack of a ridge between the nostril and the eye.[14] Cane toads have been confused with thegiant burrowing frog (Heleioporus australiacus), because both are large and warty in appearance; however, the latter can be readily distinguished from the former by its vertical pupils and its silver-grey (as opposed to gold)irises.[15] Juvenile cane toads may be confused with species of the genusUperoleia, but their adult colleagues can be distinguished by the lack of bright colouring on the groin and thighs.[16]
In the United States, the cane toad closely resembles many bufonid species. In particular, it could be confused with thesouthern toad (Anaxyrus terrestris), which can be distinguished by the presence of two bulbs in front of the parotoid glands.[17]
The cane toad genome has been sequenced and certain Australian academics believe this will help in understanding how the toad can quickly evolve to adapt to new environments, the workings of its infamous toxin, and hopefully provide new options for halting this species' march across Australia and other places it has spread as an invasive pest.[18]
Studies of the genome confirm its evolutionary origins in northern part of South America and its close genetic relation toRhinella diptycha and other similar species of the genus.[19] Recent studies suggest thatR. marina diverged between 2.75 and 9.40 million years ago.[20]
A recent split in the species into further subspecies may have occurred approximately 2.7 million years ago following the isolation of population groups by the risingVenezuelan Andes.[21]
Considered the largest species in the Bufonidae,[22] the cane toad is very large;[23] the females are significantly longer than males,[24] reaching a typical length of 10–15 cm (4–6 in),[23] with a maximum of 24 cm (9.4 in).[25] Larger toads tend to be found in areas of lower population density.[26] They have a life expectancy of 10 to 15 years in the wild,[27] and can live considerably longer in captivity, with one specimen reportedly surviving for 35 years.[28]
The skin of the cane toad is dry and warty.[23] Distinct ridges above the eyes run down the snout.[14] Individual cane toads can be grey, yellowish, red-brown, or olive-brown, with varying patterns.[29] A large parotoid gland lies behind each eye.[23] The ventral surface is cream-coloured and may have blotches in shades of black or brown. The pupils are horizontal and the irises golden.[15] The toes have a fleshy webbing at their base,[23] and the fingers are free of webbing.[29]
Typically, juvenile cane toads have smooth, dark skin, although some specimens have a red wash. Juveniles lack the adults' large parotoid glands, so they are usually less poisonous.[26] Thetadpoles are small and uniformly black, and are bottom-dwellers, tending to formschools.[30] Tadpoles range from 10 to 25 mm (0.4 to 1.0 in) in length.[31]
The common name "marine toad" and the scientific nameRhinella marina suggest a link tomarine life,[32] but cane toads do not live in the sea. However, laboratory experiments suggest thattadpoles can tolerate salt concentrations equivalent to 15% ofseawater (~5.4‰),[33] and recent field observations found living tadpoles and toadlets at salinities of 27.5‰ onCoiba Island,Panama.[34] The cane toad inhabits open grassland and woodland, and has displayed a "distinct preference" for areas modified by humans, such as gardens and drainage ditches.[35] In their native habitats, the toads can be found in subtropical forests,[31] although dense foliage tends to limit their dispersal.[36]
The cane toad begins life as an egg, which is laid as part of long strings of jelly in water. A female lays 8,000–25,000 eggs at once and the strings can stretch up to 20 m (66 ft) in length.[32] The black eggs are covered by a membrane and their diameter is about 1.7–2.0 mm (0.067–0.079 in).[32] The rate at which an egg grows into a tadpole increases with temperature. Tadpoles typically hatch within 48 hours, but the period can vary from 14 hours to almost a week.[32] This process usually involves thousands of tadpoles—which are small, black, and have short tails—forming into groups. Between 12 and 60 days are needed for the tadpoles to develop into juveniles, with four weeks being typical.[32] Similarly to their adult counterparts, eggs and tadpoles are toxic to many animals.[23]
When they emerge, toadlets typically are about 10–11 mm (0.39–0.43 in) in length, and grow rapidly. While the rate of growth varies by region, time of year, and sex, an average initial growth rate of 0.647 mm (0.0255 in) per day is seen, followed by an average rate of 0.373 mm (0.0147 in) per day. Growth typically slows once the toads reach sexual maturity.[37] This rapid growth is important for their survival; in the period between metamorphosis and subadulthood, the young toads lose the toxicity that protected them as eggs and tadpoles, but have yet to fully develop the parotoid glands that producebufotoxin.[38] Only an estimated 0.5% of cane toads reach adulthood, in part because they lack this key defense[26][39]—but also due to tadpole cannibalism. Although cannibalism does occur in the native population in South America, therapid evolution occurring in the unnaturally large population in Australia has produced tadpoles 30x more likely to beinterested in cannibalising their siblings, and 2.6x more likely to actuallydo so. They have also evolved to shorten their tadpole phase in response to the presence of older tadpoles. These changes are likely genetic, although no genetic basis has been determined.[40]
As with rates of growth, the point at which the toads become sexually mature varies across different regions. In New Guinea, sexual maturity is reached by female toads with a snout–vent length between 70 and 80 mm (2.8 and 3.1 in), while toads inPanama achieve maturity when they are between 90 and 100 mm (3.5 and 3.9 in) in length.[41] In tropical regions, such as their native habitats, breeding occurs throughout the year, but in subtropical areas, breeding occurs only during warmer periods that coincide with the onset of thewet season.[42]
The cane toad is estimated to have a critical thermal maximum of 40–42 °C (104–108 °F) and a minimum of around 10–15 °C (50–59 °F).[43] The ranges can change due to adaptation to the local environment.[44] Cane toads from some populations can adjust their thermal tolerance within a few hours of encountering low temperatures.[45] The toad is able to rapidly acclimate to the cold using physiological plasticity, though there is also evidence that more northerly populations of cane toads in the United States are better cold-adapted than more southerly populations.[46] These adaptations have allowed the cane toad to establish invasive populations across the world. The toad's ability to rapidly acclimate to thermal changes suggests that current models may underestimate the potential range of habitats that the toad can populate.[45] The cane toad has a high tolerance to water loss; some can withstand a 52.6% loss of body water, allowing them to survive outside tropical environments.[44]
Most frogs identify prey by movement, and vision appears to be the primary method by which the cane toad detects prey; however, it can also locate food using its sense of smell.[47] They eat a wide range of material; in addition to the normal prey of smallrodents, other smallmammals,[22]reptiles, otheramphibians,birds, and evenbats and a range ofinvertebrates (such asants,beetles,earwigs,dragonflies,grasshoppers,true bugs,crustaceans, andgastropods),[48] they also eat plants, dog food, cat food,[48] feces,[22] and household refuse.[49][50]
The skin of the adult cane toad is toxic, as well as the enlarged parotoid glands behind the eyes, and other glands across its back. When the toad is threatened, its glands secrete a milky-white fluid known asbufotoxin.[51] Components of bufotoxin are toxic to many animals;[52] even human deaths have been recorded due to the consumption of cane toads.[31] Dogs are especially prone to be poisoned by licking or biting toads. Pets showing excessive drooling, extremely red gums, head-shaking, crying, loss of coordination, and/or convulsions require immediate veterinary attention.[25]
Bufotenin, one of the chemicals excreted by the cane toad, is classified as a schedule 9 drug under Australian law, alongsideheroin andLSD.[53] The effects of bufotenin are thought to be similar to those of mild poisoning; the stimulation, which includes mildhallucinations, lasts less than an hour.[54] As the cane toad excretes bufotenin in small amounts, and other toxins in relatively large quantities,toad licking could result in serious illness or death.[55]
In addition to releasing toxin, the cane toad is capable of inflating its lungs, puffing up, and lifting its body off the ground to appear taller and larger to a potential predator.[51]
Since 2011, experimenters in the Kimberley region of Western Australia have used poisonous sausages containing toad meat in an attempt to protect native animals from cane toads' deadly impact. The Western Australian Department of Environment and Conservation, along with the University of Sydney, developed these sausage-shaped baits as a tool in order to train native animals not to eat the toads. By blending bits of toad with a nausea-inducing chemical, the baits train the animals to stay away from the amphibians.[56][57][58]
Young cane toads that aren't lethal upon ingestion have also been used to teach native predators avoidance, namelyyellow-spotted monitors. 200,000 metamorphs, tadpoles, and eggs in total were released in areas ahead of inevitable invasion fronts. Following invasion by wild cane toads, yellow-spotted monitors in control areas bereft of the "teacher toads" were virtually wiped out, but experimental areas still contained substantial populations of yellow-spotted monitors.[59]
Many species prey on the cane toad and its tadpoles in its native habitat, including thebroad-snouted caiman (Caiman latirostris), the banded cat-eyed snake (Leptodeira annulata), eels (familyAnguillidae), various species ofkillifish,[60] andParaponera clavata (bullet ants).[61]
Predators outside the cane toad's native range include the rock flagtail (Kuhlia rupestris), some species ofcatfish (order Siluriformes), some species ofibis (subfamily Threskiornithinae),[60] thewhistling kite (Haliastur sphenurus), therakali (Hydromys chrysogaster), theblack rat (Rattus rattus) and thewater monitor (Varanus salvator). Thetawny frogmouth (Podargus strigoides) and thePapuan frogmouth (Podargus papuensis)[62] have been reported as feeding on cane toads; some Australian crows (Corvus spp.) have also learned strategies allowing them to feed on cane toads, such as using their beak to flip toads onto their backs.[63][64]Kookaburras also prey on the amphibians.[65]
Opossums of the genusDidelphis likely can eat cane toads with impunity.[66]Meat ants are unaffected by the cane toads' toxins, so are able to kill them without reaction.[67] The cane toad's normal response to attack is to stand still and let its toxin kill or repel the attacker, which allows the ants to attack and eat the toad.[68]Saw-shelled turtles have also been seen successfully and safely eating cane toads.
In Australia,rakali (Australian water rats) learnt in two years how to eat cane toads safely. They select the largest toads, turn them over, remove the poisonous gallbladder, and eat the heart and other organs with "surgical precision". They remove the toxic skin and eat the thigh muscle. Other animals such as crows and kites turn cane toads inside out and eat non-poisonous organs, also thus avoiding the skin.[69]
The cane toad is native to the Americas, and its range stretches from theRio Grande Valley inSouth Texas to the centralAmazon and southeasternPeru, and some of the continental islands nearVenezuela (such asTrinidad andTobago).[70][71] This area encompasses bothtropical andsemiarid environments. The density of the cane toad is significantly lower within its native distribution than in places where it has been introduced. In South America, the density was recorded to be 20 adults per 100 m (110 yd) of shoreline, 1 to 2% of the density in Australia.[72]
The cane toad has been introduced to many regions of the world—particularly the Pacific—for the biological control of agricultural pests.[70] These introductions have generally been well documented, and the cane toad may be one of the most studied of anyintroduced species.[73]
Before the early 1840s, the cane toad had been introduced intoMartinique andBarbados, fromFrench Guiana andGuyana.[74] An introduction toJamaica was made in 1844 in an attempt to reduce the rat population.[75] Despite its failure to control the rodents, the cane toad was introduced toPuerto Rico in the early 20th century in the hope that it would counter a beetle infestation ravaging the sugarcane plantations. The Puerto Rican scheme was successful and halted the economic damage caused by the beetles, prompting scientists in the 1930s to promote it as an ideal solution to agricultural pests.[76]
As a result, many countries in the Pacific region emulated the lead of Puerto Rico and introduced the toad in the 1930s.[77] Introduced populations are inAustralia;Florida;[78]Papua New Guinea;[79] thePhilippines;[80]Ishigaki Island, theOgasawara, and theDaitō Islands of Japan;[81]Taiwan'sNantouCaotun;[82] most Caribbean islands;[77] andFiji and many other Pacific islands,[77] includingHawaiʻi.[83][84] Since then, the cane toad has become a pest in many host countries, and poses a serious threat to native animals.[85]
Following the apparent success of the cane toad in eating the beetles threatening the sugarcane plantations of Puerto Rico, and the fruitful introductions into Hawaiʻi and the Philippines, a strong push was made for the cane toad to be released in Australia to negate the pests ravaging the Queensland cane fields.[86] As a result, 102 toads were collected from Hawaiʻi and brought to Australia.[87] Queensland's sugar scientists released the toad into cane fields in August 1935.[88] After this initial release, the Commonwealth Department of Health decided to ban future introductions until a study was conducted into the feeding habits of the toad. The study was completed in 1936 and the ban lifted, and large-scale releases were undertaken; by March 1937, 62,000 toadlets had been released into the wild.[87][89] The toads became firmly established in Queensland, increasing exponentially in number and extending their range into theNorthern Territory andNew South Wales.[29][87] In 2010, one was found on the far western coast inBroome, Western Australia.[90]
However, the toad was generally unsuccessful in reducing the targeted grey-backed cane beetles (Dermolepida albohirtum), in part because the cane fields provided insufficient shelter for the predators during the day,[91] and in part because the beetles live at the tops of sugar cane—and cane toads are not good climbers.[86] Since its original introduction, the cane toad has had a particularly marked effect on Australianbiodiversity. The population of a number of native predatory reptiles has declined, such as thevaranid lizardsVaranus mertensi,V. mitchelli, andV. panoptes; the land snakesPseudechis australis andAcanthophis antarcticus; and the freshwater crocodile speciesCrocodylus johnstoni. In contrast, the population of theagamid lizardLophognathus gilberti—known to be a prey item ofV. panoptes—has increased.[92]Meat ants, however, are able to kill cane toads.[93] The cane toad has also been linked to decreases innorthern quolls in the southern region ofKakadu National Park and even theirlocal extinction.[94]
The cane toad was introduced to various Caribbean islands to counter a number of pests infesting local crops.[95] While it was able to establish itself on some islands, such asBarbados,Jamaica,Hispaniola, andPuerto Rico, other introductions, such as inCuba before 1900 and in 1946, and on the islands ofDominica andGrand Cayman, were unsuccessful.[96]
The earliest recorded introductions were to Barbados andMartinique. The Barbados introductions were focused on the biological control of pests damaging the sugarcane crops,[97] and while the toads became abundant, they have done even less to control the pests than in Australia.[98] The toad was introduced to Martinique fromFrench Guiana before 1944 and became established. Today, they reduce themosquito andmole cricket populations.[99] A third introduction to the region occurred in 1884, when toads appeared in Jamaica, reportedly imported from Barbados to help control the rodent population. While they had no significant effect on the rats, they nevertheless became well established.[100] Other introductions include the release onAntigua—possibly before 1916, although this initial population may have died out by 1934 and been reintroduced at a later date[101]—andMontserrat, which had an introduction before 1879 that led to the establishment of a solid population, which was apparently sufficient to survive theSoufrière Hills volcano eruption in 1995.[102]
In 1920, the cane toad was introduced into Puerto Rico to control the populations ofwhite grub (Phyllophaga spp.), a sugarcane pest.[103] Before this, the pests were manually collected by humans, so the introduction of the toad eliminated labor costs.[103] A second group of toads was imported in 1923, and by 1932, the cane toad was well established.[104] The population of white grubs dramatically decreased,[103] and this was attributed to the cane toad at the annual meeting of the International Sugar Cane Technologists in Puerto Rico.[85] However, there may have been other factors.[85] The six-year period after 1931—when the cane toad was most prolific, and the white grub had a dramatic decline—had the highest-ever rainfall for Puerto Rico.[105] Nevertheless, the cane toad was assumed to have controlled the white grub; this view was reinforced by aNature article titled "Toads save sugar crop",[85] and this led to large-scale introductions throughout many parts of the Pacific.[106]
The cane toad has been spotted inCarriacou andDominica, the latter appearance occurring in spite of the failure of the earlier introductions.[107] On September 8, 2013, the cane toad was also discovered on the island ofNew Providence in the Bahamas.[108]
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The cane toad was first introduced deliberately into thePhilippines in 1930 as a biological control agent of pests in sugarcane plantations, after the success of the experimental introductions into Puerto Rico.[110][111] It subsequently became the most ubiquitous amphibian in the islands. It still retains the common name ofbakî orkamprag in theVisayan languages, acorruption of 'American frog', referring to its origins.[109] It is also commonly known as "bullfrog" in Philippine English.[112]
The cane toad was introduced intoFiji to combat insects that infested sugarcane plantations. The introduction of the cane toad to the region was first suggested in 1933, following the successes in Puerto Rico and Hawaiʻi. After considering the possible side effects, the national government of Fiji decided to release the toad in 1953, and 67 specimens were subsequently imported from Hawaiʻi.[113] Once the toads were established, a 1963 study concluded, as the toad's diet included both harmful and beneficial invertebrates, it was considered "economically neutral".[84] Today, the cane toad can be found on all major islands in Fiji, although they tend to be smaller than their counterparts in other regions.[114]
The cane toad was introduced intoNew Guinea to control thehawk moth larvae eatingsweet potato crops.[79] The first release occurred in 1937 using toads imported from Hawaiʻi, with a second release the same year using specimens from the Australian mainland. Evidence suggests a third release in 1938, consisting of toads being used for humanpregnancy tests—many species of toad were found to be effective for this task, and were employed for about 20 years after the discovery was announced in 1948.[115][116] Initial reports argued the toads were effective in reducing the levels ofcutworms and sweet potato yields were thought to be improving.[117] As a result, these first releases were followed by further distributions across much of the region,[117] although their effectiveness on other crops, such as cabbages, has been questioned; when the toads were released atWau, the cabbages provided insufficient shelter and the toads rapidly left the immediate area for the superior shelter offered by the forest.[118] A similar situation had previously arisen in the Australian cane fields, but this experience was either unknown or ignored in New Guinea.[118] The cane toad has since become abundant in rural and urban areas.[119]
The cane toad naturally exists inSouth Texas, but attempts (both deliberate and accidental) have been made to introduce the species to other parts of the country. These include introductions toFlorida and to Hawaiʻi, as well as largely unsuccessful introductions toLouisiana.[120]
Initial releases into Florida failed. Attempted introductions before 1936 and 1944, intended to control sugarcane pests, were unsuccessful as the toads failed to proliferate. Later attempts failed in the same way.[121][122] However, the toad gained a foothold in the state after an accidental release by an importer atMiami International Airport in 1957, and deliberate releases by animal dealers in 1963 and 1964 established the toad in other parts of Florida.[122][123] Today, the cane toad is well established in the state, from theKeys to north ofTampa, and they are gradually extending further northward.[124] In Florida, the toad is a regarded as a threat to native species[125] and pets;[126] so much so, theFlorida Fish and Wildlife Conservation Commission recommends residents to kill them.[25]
Around 150 cane toads were introduced toOʻahu in Hawaiʻi in 1932, and the population swelled to 105,517 after 17 months.[77] The toads were sent to the other islands, and more than 100,000 toads were distributed by July 1934;[127] eventually over 600,000 were transported.[128]
Other than the use as abiological control for pests, the cane toad has been employed in a number of commercial and noncommercial applications. Traditionally, within the toad's natural range in South America, theEmbera-Wounaan would "milk" the toads for their toxin, which was then employed as anarrow poison. The toxins may have been used as anentheogen by theOlmec people. The toad has been hunted as a food source in parts ofPeru, and eaten after the careful removal of the skin andparotoid glands.[129] When properly prepared, the meat of the toad is considered healthy and as a source ofomega-3 fatty acids.[130] More recently, the toad's toxins have been used in a number of new ways:bufotenin has been used in Japan as anaphrodisiac and ahair restorer, and incardiac surgery in China to lower the heart rates of patients.[31] New research has suggested that the cane toad's poison may have some applications in treatingprostate cancer.[131]
Other modern applications of the cane toad include pregnancy testing,[129] as pets,[132] laboratory research,[133] and the production ofleather goods.Pregnancy testing was conducted in the mid-20th century by injecting urine from a woman into a male toad'slymph sacs, and ifspermatozoa appeared in the toad's urine, the patient was deemed to be pregnant.[129] The tests using toads were faster thanthose employing mammals; the toads were easier to raise, and, although the initial 1948 discovery employedBufo arenarum for the tests, it soon became clear that a variety ofanuran species were suitable, including the cane toad. As a result, toads were employed in this task for around 20 years.[116] As alaboratory animal, the cane toad has numerous advantages: they are plentiful, and easy and inexpensive to maintain and handle. The use of the cane toad in experiments started in the 1950s, and by the end of the 1960s, large numbers were being collected and exported to high schools and universities.[133] Since then, a number of Australian states have introduced or tightened importation regulations.[134]
There are several commercial uses for dead cane toads. Cane toad skin is made into leather and novelty items.[135][136] Stuffed cane toads, posed and accessorised, are merchandised at souvenir shops for tourists.[137] Attempts have been made to producefertiliser from toad carcasses.[138]
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