Salamanders

Salamanders

For other uses, see Salamander (disambiguation) and Caudata (disambiguation).
Salamander
Temporal range: Middle Jurassic-Recent, 164–0Ma
Spotted Salamander, Ambystoma maculatum
Suborders

Cryptobranchoidea
Salamandroidea
Sirenoidea

Native distribution of salamanders (in green)

Salamanders are any of approximately 550 extant species of amphibians within the order Caudata.[1] They are typically characterized by a superficially lizard-like appearance, with slender bodies, short noses, and long tails. All known fossil salamanders and all extinct species fall under the order Caudata, while sometimes the extant species are grouped together as the Urodela.[2] Salamanders have never more than four toes on their front legs and five on their rear legs, but some species have fewer. Their moist skin usually makes them reliant on habitats in or near water, or under some protection (e.g., moist ground), often in a wetland. Some salamander species are fully aquatic throughout life, some take to the water intermittently, and some are entirely terrestrial as adults. Unique among vertebrates, they are capable of regenerating lost limbs, as well as other body parts. Many of the members of the family Salamandridae are known as newts.

The earliest known salamander fossils have been found in geological deposits of China and Kazakhstan, which have been dated to the middle Jurassic period, up to 164 million (plus or minus 4 million) years ago.[3]

Morphology

The skin lacks scales and is moist and smooth to the touch, except in newts of the Salamandridae which may have velvety or warty skin that is wet to the touch. The skin may be drab or brightly colored, exhibiting various patterns of stripes, bars, spots, blotches or dots. Male newts become dramatically colored during the breeding season. Cave species dwelling in darkness lack pigmentation and have a translucent pink or pearlescent appearance.[4]

Salamanders range in size from the minute salamanders, with a total length of 2.7 cm (1.1 in), including the tail, to the Chinese giant salamander which reaches 1.8 m (5.9 ft) and weighs up to 65 kg (143 lb). Most, however, are between 10 and 20 cm (3.9 and 7.9 in) in length.

Limbs and tail

Adult salamanders generally resemble small lizards, having a basal tetrapod body form with a cylindrical trunk, four limbs and a long tail. Some aquatic species such as sirens and amphiumas have reduced or no hind limbs, giving them an eel-like appearance but in most species, the front and rear limbs are about the same length and project sidewards, barely raising the trunk off the ground. The feet are broad with short digits, usually four on the front feet and five on the rear. Salamanders do not have claws and the shape of the foot varies according to the animal's habitat, with climbing species having elongate, square-tipped toes and rock-dwellers having larger feet with short blunt toes. The tree climbing salamander (Bolitoglossa spp.) has plate-like webbed feet which adhere to smooth surfaces by suction, while the rock climbing Hydromantes spp. from California has feet with fleshy webs and short digits and uses its tail as an extra limb. When ascending, the tail props up the rear of the body while one hind foot moves forward and then swings to the other side to provide support as the other hind foot advances.[5]

In larvae and aquatic salamanders, the tail is laterally flattened, has dorsal and ventral fins and undulates from side to side to propel the animal through the water. In the families Ambystomatidae and Salamandridae, the male's tail, which is larger than that of the female, is used during the amplexus embrace to propel the mating couple to a secluded location. In terrestrial species, the tail moves to counterbalance the animal as it runs and in the arboreal salamander and other tree-climbing species, it is prehensile. It is also used by certain plethodontid salamanders that can jump to help launch themselves into the air.[5] The tail is used in courtship and as a storage organ for proteins and lipids. It also functions as a defence against predation when it may be lashed at the attacker or autotomised when grabbed. The separated tail continues to wriggle and may distract the predator from persevering with the attack. Unlike frogs, adult salamanders are able to regenerate limbs and tail when these are lost.[5]

Skin

The skin of salamanders in common with other amphibians, is thin, permeable to water, serves as a respiratory membrane and is well-supplied with glands. It has highly cornified outer layers renewed periodically through a skin shedding process controlled by the pituitary and thyroid glands. During moulting, the skin initially breaks around the mouth and the animal moves forwards through the gap. When the arms have been worked clear, a series of body ripples pushes the shed skin towards the rear. The hind limbs are extracted and push it farther back before it is eventually freed by friction as the salamander moves forward with the tail pressed against the ground.[6] They often then eat the resulting slough.[4]

Mucous glands in the skin discharge mucus which keeps the skin moist, an important factor in skin respiration and thermoregulation. The sticky layer helps protect against bacterial infections and moulds, reduces friction when swimming and makes the animal slippery and less easily caught by predators. Granular glands scattered on the upper surface, particularly the head, back and tail, produce repellent or toxic secretions.[6] Some salamander toxins are particularly potent. The rough-skinned newt (Taricha granulosa) contains the neurotoxin tetrodotoxin (TTX), the most toxic non-protein substance known. Handling the newts does no harm but ingestion of even a minute fragment of skin is deadly. In feeding trials, fish, frogs, reptiles, birds and mammals were all found to be susceptible.[7]

Mature adults of some salamander species have "nuptial" glandular tissue in the cloaca, at the base of the tail, on the head or under the chin. Some females release chemical substances, possibly from the ventral cloacal gland, which act to attract males, but males do not seem to use pheromones for this purpose.[8] In some plethodonts, males have conspicuous mental glands on the chin which are pressed against the females' nostrils during the courtship ritual. They may function to speed up the mating process with the consequent reduced risk of its being disrupted by a predator or rival male.[9] The gland at the base of the tail in Plethodon cinereus is used to mark faecal pellets to proclaim territorial ownership.[8]

Senses

Olfaction in salamanders plays a role in territory maintenance, the recognition of predators and courtship rituals, but is probably secondary to sight during prey selection and feeding. Salamanders have two types of sensory area that respond to the chemistry of the environment. Olfactory epithelium in the nasal cavity picks up airborne and aquatic odours while adjoining vomeronasal organs detect non-volatile chemical cues such as tastes in the mouth. In plethodonts, the sensory epithelium of the vomeronasal organs extends to the nasolabial grooves which stretch from the nostrils to the corners of the mouth. These extended areas seems to be associated with the identification of prey items, the recognition of conspecifics and the identification of individuals.[10]

The eyes of most salamanders are adapted primarily for vision at night. In some permanently aquatic species, they are reduced in size and have a simplified retinal structure, and in cave dwellers such as the Georgia blind salamander, they are absent or covered with a layer of skin. In species that spend some time on land and some in the water, the eyes are a compromise and are shortsighted in air and longsighted in water. Fully terrestrial species such as the fire salamander have a flatter lens which can focus over a much wider range of distances.[11] To find their prey, salamanders use trichromatic color vision extending into the ultraviolet range, based on three photoreceptor types that are maximally sensitive around 450 nm, 500 nm and 570 nm.[12] The larvae, and the adults of some highly aquatic species, also have a lateral line organ, similar to that of fish, which can detect changes in water pressure.[4]

Salamanders lack eardrums, but like frogs have an opercularis system in the middle ear. This consists of two ossicles: the columella (equivalent to the stapes of higher vertebrates) which is fused to the skull, and the operculum. An opercularis muscle connects the latter to the pectoral girdle and is kept under tension when the animal is alert.[13] The system seems able to detect low frequency vibrations (500–600 Hz) which may be picked up from the ground by the forelimbs and transmitted to the inner ear. This may serve to warn the animal of an approaching predator.[14]


Salamanders are usually considered to have no voice, but some species can make quiet ticking or popping noises, perhaps by the opening and closing of valves in the nose. The California giant salamander can produce a bark or rattle and a few species can squeak by contracting muscles in the throat. The arboreal salamander can squeak using a different mechanism; it retracts its eyes into its head, forcing air out of its mouth. The ensatina salamander occasionally makes a hissing sound and the sirens sometimes produce quiet clicks and can resort to faint shrieks if attacked.[15]

Respiration

Respiration differs among the different species of salamanders. Species that lack lungs respire through gills. In most cases, these are external gills, visible as tufts on either side of the head, although the amphiumas have internal gills and gill slits. Some terrestrial salamanders have lungs used in respiration, although these are simple and sac-like, unlike the more complex organs found in mammals. Many species, such as the olm, have both lungs and gills as adults.[4]

Some terrestrial species, such as the plethodonts, lack both lungs and gills and perform gas exchange through their skin, a process in which the capillary beds are spread throughout the epidermis and inside the mouth.[16] Even some species with lungs can respire through the skin in this manner.

Feeding and diet

Most species of salamander have small teeth in both the upper and lower jaws. Unlike frogs, even the larvae of salamanders possess these teeth.[4] Although larval teeth are shaped like pointed cones, the teeth of adults are adapted to enable them to readily grasp prey. The crown, which has two cusps (bicupsid), is attached to a pedicel by collagenous fibers. The joint formed between the bicuspid and the pedicel is partially flexible, as it can bend inward but not outward. When struggling prey is advanced into the salamander's mouth, the teeth tips relax and bend in the same direction, encouraging movement toward the throat, and resisting the prey's escape.[17] Many salamanders have patches of teeth attached to the vomer and the palatine bones in the roof of the mouth and these help to retain prey. All types of teeth are resorbed and replaced at intervals throughout the animal's life.[18]

A terrestrial salamander catches its prey by flicking out its sticky tongue in an action that takes only about ten milliseconds. In some species, the tongue is attached anteriorly to the floor of the mouth, while in others, it is mounted on a pedicel. It is rendered sticky by secretions of mucus from glands in its tip and on the roof of the mouth.[19] High speed cinematography shows how the tiger salamander (Ambystoma tigrinum) positions itself with its snout close to its prey. Its mouth then gapes widely, the lower jaw remains stationary and the tongue bulges and changes shape as it shoots forward. The protruded tongue has a central depression and the rim of this collapses inward as the target is struck, trapping the prey in a mucus-laden trough. Here it is held while the animal's neck is flexed, the tongue retracted and jaws closed. Large or resistant prey is retained by the teeth while repeated protrusions and retractions of the tongue draw it in. Swallowing is done by alternate contraction and relaxation of muscles in the throat, assisted by depression of the eyeballs into the roof of the mouth.[20] Bolitoglosids have a more advanced feeding method. Muscles surrounding the hyoid bone contract to create pressure and actually "shoot" the hyoid bone out of the mouth, thus elongating the tongue. Muscles in the pelvic region are used in order to reel the tongue and the hyoid back to its original position.[21]

An aquatic salamander lacks muscles in the tongue and captures its prey in an entirely different manner. It grabs the food item, grasps it with its teeth and adopts a kind of inertial feeding. This involves tossing its head about, drawing water sharply in and out of its mouth and snapping its jaws, all of which tends to tear and macerate the prey which is then swallowed.[21]

Salamanders are opportunistic predators. They are generally not restricted to specific foods, but will feed on almost any organism of a reasonable size.[22] In a study of dusky salamanders (Desmognathus) in the Appalachian Mountains, the diet included earthworms, flies, beetles, beetle larvae, leafhoppers, springtails, moths, spiders, grasshoppers and mites.[22]

Defense

Salamanders have thin skins, soft bodies and move rather slowly and at first sight might appear to be vulnerable to opportunistic predators. However, they have several effective lines of defense. Mucus coating the damp skin makes them difficult to grasp and may have an offensive taste or be toxic. When attacked by a predator, a salamander may position itself in such a way that the main poison glands face the aggressor. Often these are on the tail which may be waggled or turned up and arched over the animal's back. The sacrifice of the tail may seem a worthwhile strategy if the salamander escapes with its life and the predator learns to avoid that species of salamander in future.[23] Skin secretions of the tiger salamander (Ambystoma tigrinum) fed to rats have been shown to produce aversion to the flavor and the avoidance of the presentational medium when it was offered to the rats again.[24] The fire salamander (Salamandra salamandra) has a ridge of large granular glands down its spine which are able to squirt a fine jet of toxic fluid at its molester. By angling its body appropriately, it can accurately direct the spray a distance of up to 80 centimetres (31 in).[25] The Iberian ribbed newt (Pleurodeles waltl) has another method of deterring aggressors. Its skin exudes a poisonous, viscous fluid and at the same time, it rotates its sharply pointed ribs through an angle of between 27° and 92° and adopts an inflated posture. This action causes the ribs to puncture the body wall, often protruding through a lateral row of orange warts which may provide an aposematic signal that makes the spines more visible. When the danger has passed, the ribs retract and the skin heals.[26]

Although many salamanders have cryptic colors so as to be unnoticeable, others signal their toxicity by their vivid coloring. Yellow, orange and red are the colors generally used, often with black for greater contrast. Sometimes the animal will posture if attacked, revealing a flash of warning hue on its underside. The red eft, the brightly-colored terrestrial juvenile form of the eastern newt (Notophthalmus viridescens), is highly poisonous. It is avoided by birds and snakes and can survive for up to thirty minutes after being swallowed (later being regurgitated).[27] The red salamander (Pseudotriton ruber) is a palatable species with a similar coloring to the red eft. Predators that previously fed on it have been shown to avoid it after encountering red efts, an example of Batesian mimicry.[27] Other species exhibit similar mimicry. In California, the palatable yellow-eyed salamander (Ensatina eschscholtzii) closely resembles the toxic California newt (Taricha torosa) and the Rough-skinned newt (Taricha granulosa), whereas in other parts of its range it is cryptically colored.[28]

Some salamander species use tail autotomy to escape predators. The tail will drop off and wriggle around for a little while, and the salamanders will either run away or stay still enough to not be noticed while the predator is distracted. Salamanders routinely regenerate complex tissues, including the lens or retina of the eye. Within only a few weeks of losing a piece of limb, a salamander perfectly reforms the missing structure.[29]

Distribution and habitat

Salamanders split off from the other amphibians during the Mid to Late Permian, and initially were similar to modern members of the Cryptobranchoidea. Their resemblance to lizards is the result of symplesiomorphy, their common retention of the primitive tetrapod body plan, and they are no more closely related to lizards than they are to mammals. Their nearest relatives are the frogs and toads, within Batrachia.

Caudates are found exclusively in the Holarctic and Neotropical regions, not reaching south of the Mediterranean Basin, the Himalayas, or in South America the Amazon Basin. They do not extend north of the Arctic tree line, with the northernmost species, Salamandrella keyserlingii occurring in the Siberian larch forests of Sakha and of the most northerly species in North America, Ambystoma laterale reaching no farther than Labrador and Taricha granulosa not beyond the Alaska Panhandle.[30]

One third of the known salamander species are found in North America. The highest concentration of these is found in the Appalachian Mountains region where the Plethodontidae are thought to have originated in the mountain streams. Here vegetation zones and proximity to water have been shown to be of greater importance than altitude. Only species that adopted a more terrestrial mode of life have been able to disperse to other localities. The northern slimy salamander (Plethodon glutinosus) has a wide range and occupies a habitat similar to that of the southern gray-cheeked salamander (Plethodon metcalfi). The latter is restricted to the slightly cooler and wetter conditions in north-facing cove forests in the Southern Appalachians and to higher elevations above 900 metres (3,000 feet). P. glutinosus is adaptable and would be perfectly able to inhabit these locations, but some unknown form of competition largely prevents the two species from co-existing.[22]

Development

The life history of salamanders is similar to that of other amphibians, such as frogs and toads. Most species fertilize the eggs internally, with the male depositing a sac of sperm in the female's cloaca. The most primitive salamanders – those grouped together as the Cryptobranchoidea – exhibit external fertilization instead. The eggs are laid in a moist environment, often a pond, but sometimes moist soil, or inside bromeliads. Some species are ovoviviparous, with the female retaining the eggs inside her body until they hatch.[4]

A larval stage follows in which the organism is fully aquatic or water-dwelling, and possesses gills. Depending on species, the larval stage may or may not possess legs. The larval stage may last from days to years, depending on the species. Some species (such as Dunn's salamander) exhibit no larval stage at all, with the young hatching as miniature versions of the adult. Neoteny has been observed in all salamander families, in which an individual may retain gills into sexual maturity. This may be universally possible in all salamander species.[31] More commonly, however, metamorphosis continues with the loss of gills, the growth (or increase in size) of legs, and the capability of the animal to function out of the water.

Conservation

A general decline in living amphibian species has been linked with the fungal disease chytridiomycosis. A higher proportion of salamander species than of frogs or caecilians are in one of the at-risk categories established by the IUCN. Salamanders showed a significant diminution in numbers in the last few decades of the 20th century, although no direct link between the fungus and the population decline has yet been found.[32] Researchers also cite deforestation, resulting in fragmentation of suitable habitats, and climate change as possible contributory factors. Species such as Pseudoeurycea brunnata and Pseudoeurycea goebeli that had been abundant in the cloud forests of Guatemala and Mexico during the 1970s were found by 2009 to be rare. [33] However, little data has been gathered on population sizes over the years, and by intensive surveying of historic and suitable new locations, it has been possible to locate individuals of other species such as Parvimolge townsendi, which had been thought to be extinct.[32]

Various conservation initiatives are being attempted around the world. The Chinese giant salamander, at 1.8 metres (6 feet) the largest amphibian in the world, is critically endangered as it is collected for food and for use in traditional Chinese medicine. An environmental education programme is being undertaken to encourage sustainable management of wild populations in the Qinling Mountains and captive breeding programmes have been set up.[34] The hellbender is another large, long-lived species with dwindling numbers and fewer juveniles reaching maturity than previously.[35] Another alarming finding is the increase in abnormalities in up to 90% of the hellbender population in the Spring River (Arkansas) watershed.[36] Habitat loss, silting of streams, pollution and disease have all been implicated in the decline and a captive breeding programme at Saint Louis Zoo has been successfully established.[37] Of the twenty species of minute salamanders in Mexico, half are believed to have become extinct and most of the others are critically endangered. Specific reasons for the decline may include climate change, chytridiomycosis or volcanic activity, but the main threat is habitat destruction with logging, agricultural activities and human settlement impacting their often tiny, fragmented ranges. Survey work is being undertaken to assess the status of these salamanders and to better understand the factors involved in their population declines with a view to taking action.[38]

Research is being done on the environmental cues that have to be replicated before captive animals can be persuaded to breed. Common species such as the tiger salamander and the mudpuppy are being given hormones to stimulate the production of sperm and eggs, and the role of arginine vasotocin in courtship behaviour is being investigated. Another line of research is artificial insemination, either in vitro or by inserting spermatophores into the cloacas of females. It is hoped to use the results of this research in captive breeding programmes for endangered species.[39]

Taxonomy

The 10 families belonging to the order Caudata are divided into three suborders.[2] The clade Neocaudata is often used to separate Cryptobranchoidea and Salamandroidea from the Sirenoidea.

Cryptobranchoidea (Giant salamanders)
Family Common names Example species

Example image

Cryptobranchidae Giant salamanders Hellbender (Cryptobranchus alleganiensis)
Hynobiidae Asiatic salamanders Hida salamander (Hynobius kimurae)
Salamandroidea (Advanced salamanders)
Ambystomatidae Mole salamanders Marbled salamander (Ambystoma opacum)
Amphiumidae Amphiumas or Congo eels Two-toed amphiuma (Amphiuma means)
Dicamptodontidae Pacific giant salamanders Pacific giant salamander (Dicamptodon tenebrosus)
Plethodontidae Lungless salamanders Red back salamander (Plethodon cinereus)
Proteidae Mudpuppies and olms Olm (Proteus anguinus)
Rhyacotritonidae Torrent salamanders Southern torrent salamander (Rhyacotriton variegatus)
Salamandridae Newts and true salamanders Alpine newt (Triturus alpestris)
Sirenoidea (Sirens)
Sirenidae Sirens Greater siren (Siren lacertina)

Phylogeny

Caudata

Bissektia nana



Marmorerpeton



Kiyatriton leshchinskiyi



Hemitrypus jordanianus



Jeholotriton paradoxus



Laccotriton subsolanus



Pangerpeton sinensis



Prosiren elinorae {Prosirenidae}



Sinerpeton fengshanensis



Triassurus sixtelae {Triassuridae}



Vaughniella urodeloides



Karauridae


Urodela

Liaoxitriton


Cryptobranchoidei

Regalerpeton weichangensis



Nesovtriton mynbulakensis



Hynobiidae



Cryptobranchidae



Diadectosalamandroidei

Apricosiren ensomi



Beiyanerpeton jianpingensis



Galverpeton ibericum



Comonecturoides marshi



Hylaeobatrachus croyii



Iridotriton hechti



Valdotriton gracilis



Scapherpetontidae



Batrachosauroididae



Sirenidae





Proteidae


Treptobranchia


Ambystomatidae



Dicamptodontidae




Salamandridae




Plethosalamandroidei

Rhyacotritonidae


Xenosalamandroidei

Amphiumidae



Plethodontidae








Urodela


Hynobiidae



Cryptobranchidae






Rhyacotritonidae




Amphiumidae



Plethodontidae




Hydatinosalamandroidei
Perrenibranchia

Proteidae



Sirenidae





Ambystomatidae



Salamandridae






Urodela

Sirenidae


Neocaudata


Hynobiidae



Cryptobranchidae






Proteidae




Rhyacotritonidae




Amphiumidae



Plethodontidae







Salamandridae




Ambystomatidae



Dicamptodontidae







Cladograms based on the work of Wiens et al. (2005),[40] Marjanovic & Laurin (2007)[41] (A), Frost et al. (2006)[42] (B) and Zhang & Wake (2009) (C).[43]

Mythology and popular culture

Numerous legends have developed around the salamander over the centuries, many related to fire. This connection likely originates from the tendency of many salamanders to dwell inside rotting logs. When placed into a fire, the salamander would attempt to escape from the log, lending to the belief that salamanders were created from flames.[44]

Associations of the salamander with fire appeared first in the writings of Aristotle and Pliny, even mentioned in the Talmud, later writers during and after the Middle Ages like Leonardo da Vinci, Paracelsus, Conrad Lycosthenes, Benvenuto Cellini and Jean-Jacques Rousseau mentioned this association, more recently writers like Victor Hugo, Ray Bradbury, David Weber and J. K. Rowling.

Implications of limb regeneration as applied to humans

Salamanders' limb regeneration has been the focus of significant interest among scientists. Researchers have been trying to find out the conditions required for regeneration. A theory persists in the scientific community that such regeneration could be artificially recreated in humans using stem cells. Axolotls have been highlighted for research.[45] Research published in the National Academy of Sciences researchers from the Australian Regenerative Medicine Institute have found that when macrophages were removed, salamanders lost their ability to regenerate and formed scar tissue instead.[46] The salamander is considered as the epitome of perfect regeneration and besides that, it may also have applications in healing injuries related to the spinal cord or brain and improvement in treatments which are linked to fibrosis or scarring.[46]

Aphrodisiac

In Slovenia a traditional aphrodesiac brandy is created using salamanders.[47][48]

References

Cited texts

External links

  • Tree of Life: Caudata
  • Salamander Gallery
  • Salamanders.nl – The Official Dutch Newt & Salamander Society Site
  • Caudata Culture
  • Salamandridae
  • Department of Transportation
  • ArchéoZooThèque : Urodele skeleton drawing : available in vector, image and PDF formats

Regional lists

  • California Salamanders
  • Salamanders of North Carolina
  • Salamanders of Texas

Media

  • Salamander Video
  • Salamander feeding movies in slow motion
  • Living Underworld salamander images
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