Croaking Science

Recent publication: Virus found in a strain of Batrachochytrium dendrobatis, Bd;

June 1, 2025 by Admin

Written by Andrew Smart, Head of Science & Research

A recent publication by Rebecca Clemons and colleagues has found a DNA virus in strains of Bd, the fungus that causes chytridiomycosis.

Bd is one of the fungal pathogens that cause the disease chytridiomycosis in amphibians, resulting in skin infections that can led to mortality. Bd is associated with declines and some extinctions in over 500 amphibian species.

One of the factors contributing to panzootic (global infectious disease outbreak) expansion is ‘escape’ from fungal enemies such as viruses. Many fungi are reduced in virulence by mycoviruses (viruses infecting fungi) which are ‘hyperparasites’ (parasites of parasites), some of which are RNA viruses and some single-stranded DNA viruses. The research team screened various strains of Bd to look for evidence of DNA viruses and discovered they occurred in all of the five strains examined but at high, medium and low frequencies; the lowest by far being the global panzootic strain.

Further experimental work with live fungal hosts found that there was significantly higher mortality rate in frogs (dwarf clawed frogs Hymenochirus boettgeri were used) infected by the Bd strain containing the mycovirus.

The mycovirus was named BdDV-1 and experiments found that strains with the mycovirus had reduced fungal growth rates in laboratory culture conditions. However, further experimental work with live fungal hosts found that there was significantly higher mortality rate in frogs (dwarf clawed frogs Hymenochirus boettgeri were used) infected by the Bd strain containing the mycovirus. Conversely, animals that died from the Bd mycovirus positive strain shed a lower number of zoospores (infective fungal life stages) than those infected by Bd without the mycovirus.

This variation in results based on laboratory growth and virulence in live animals warrants further investigation, but this first identification of a mycovirus in Bd may lead to long-term developments that could influence the impact of Bd on amphibians. However, there is also a risk that hybridization between Bd strains could increase the virulence of existing global panzootic strains. Initial work by Clemons and colleagues also found that Bsal, another cause of chytridiomycosis, did not show evidence of these DNA viruses, so we must continue to be vigilant and do our best to maintain biosecurity and avoid spreading infection where we can.

You can read the full paper here.

Climbing salamanders: perspectives from great heights

January 1, 2025 by Admin

Written by Laurence Jarvis (external contributor)

Climbing salamanders within the genus Aneides are a fascinating group with varied behaviour and ecology. There are currently 10 recognised species, found predominantly across North America and Canada, notably California and British Columbia. The genus belongs to the Plethodontidae, which is the largest salamander family, comprising approximately 518 species. Most plethodontid salamanders are endemic to North America and, unlike many European salamanders, lack lungs and instead completely rely on gaseous exchange through their skin. They have evolved to live in a large range of habitats including streams, trees, on land and underground. Most are direct developers, laying eggs on land which hatch into live juveniles which resemble miniature adults.

The arboreal salamander (Aneides lugubris) is the largest of the genus, growing up to 100 mm with a rounded, robust body. As its name suggests, it is excellent at climbing and may ascend trees up to 18 metres, using specially adapted toes with expanded tips¹. Uniquely amongst salamanders, the tail is prehensile enabling it to cling to the branches of trees. Arboreal salamanders are highly aggressive, possessing rows of sharp teeth. Both males and females have been reported with scars on their bodies as a result of aggressive combat¹. Despite these tendencies, arboreal salamanders perform parental care and either the male or female will guard their clutch of around 5-26 eggs whilst they develop. Like other members of the genus, the eggs are laid in damp areas of rotting wood such as under tree bark. After hatching the small juveniles remain with their parents in family groups.

Figure 1. The Arboreal salamander (Aneides lugubris) is a highly mobile species living within Californian forests.

Wandering salamanders (A. vagrans) are as equally mobile and arboreal as A. lugubris and will climb trees very readily. They have been reported to ascent up to 87 metres in Douglas fir forests of British Colombia². If disturbed individuals exhibit an unusual behaviour of jumping from the tree canopy and performing an aerial skydive to land safely. Unlike other species which may jump and glide from a height, such as flying squirrels or frogs, wandering salamanders do not possess obvious adaptations such as flaps of skin to slow their fall. Therefore, this behaviour seems unexpected. Recent research by Brown et al. (2022)³ examined the morphology and aerodynamics of individuals as they fell under experimental conditions in a wind tunnel. They found that, compared to other species within the genus, wandering salamanders have more flattened bodies and slightly wider toes, which they spread during their fall. In addition, individuals stretch out their limbs in a skydive posture and undulate their bodies. The researchers showed that these combined behaviours slowed their fall and prevented damage on impact with the ground.

Climbing salamanders are unusual amongst amphibians since the adults of several species are monomorphic, that is, both males and females are of equal size. The vast majority of amphibians (over 90%) show a degree of sexual dimorphism, with females often being larger than males. However, in plethodontid salamanders, males are often larger than females. It is thought this is because they are highly territorial and use their size to defend territories. Many other frog and toad species possess other dimorphic traits which may include differences in skin colour or development of secondary sex characters such as nuptial pads. The Sacramento Mountains Salamander (A. hardii) is one of the Aneides salamanders that is sexually dimorphic with males being larger and possessing wider heads than females. Other members of the Aneides genus are monomorphic, with females also having large bodies and wider heads like males.

Since many other plethodontid salamanders are sexually dimorphic it is thought that increased male size over females is the ancestral state⁴. Research by Staub (2021)⁴ examining the evolution of plethodontid salamanders suggests that the monomorphism observed in species of the Aneides genus has evolved later. These monomorphic species are therefore described as having ‘derived monomorphism’, since they have evolved from sexually dimorphic ancestors⁴. The exact reason for the similar size in both males and females is uncertain but Staub et al. (2024)⁵ postulated that higher levels of androgen hormone in females may explain their equal size to males. However, their experimental work did not confirm this. Therefore, further research is required to determine whether there are differences in steroid sensitivity or signalling within females, rather than different levels of hormone⁵.

Figure 2. The wandering salamander (Aneides vagrans) may jump from treetops and perform a skydive to land safely.

Climbing salamanders are a unique group and whilst several species are common and widespread, six species within the genus are listed as either Near Threatened or Vulnerable. Habitat loss is a continual problem with unsustainable logging causing the declines in several species. The Shasta Black Salamander (A. iecanus) is now classed as Vulnerable, and has suffered rapid declines due to the construction of the Shasta Reservoir in California. Like other plethodontids, climbing salamanders are also vulnerable to climate change and several species have suffered due to longer and drier summers across their range. Conserving these threatened species is a challenge and combined conservation efforts are required to protect these fascinating species.

Click for References

1AmphibiaWeb (2024). <https://amphibiaweb.org> Aneides lugubris. University of California, Berkeley, CA, USA. Accessed 12 Dec 2024.

²AmphibiaWeb (2024). <https://amphibiaweb.org> Aneides vagrans. University of California, Berkeley, CA, USA. Accessed 12 Dec 2024.

³Brown C.E., Sathe E.A., Dudley R. and Deban S.M. (2022). Gliding and parachuting by arboreal salamanders. Current Biology, 32 (10): R453-R454.

⁴Staub N.L. (2021). The evolution of derived monomorphism from sexual dimorphism: a case study on salamanders. Integrative Organismal Biology, 3 (1): p.obaa044.

⁵Staub N.L., Hayes S.G. and Mendonca M.T. (2024). Levels of Sex Steroids in Plethodontid Salamanders: A Comparative Study Within the Genus Aneides. Ecology and Evolution, 14 (11): p.e70550.

Croaking Science: Alternative strategies for amphibians – neoteny and overwintering

December 1, 2024 by Admin

Written by Dr Andrew Smart, Head of Science and Research at Froglife

People often wonder why they occasionally find tadpoles in their ponds over the winter. This is an example of delayed metamorphosis, which can be advantageous to some individuals, particularly in northern latitudes. In newts we find overwintering larvae and also examples of neoteny when the larval form is retained and, in some cases, mature and reproduce.

The classic case of neoteny in amphibians is the axolotl, which retain their feathery gills into adulthood, becoming enlarged larvae that develop reproductive organs and can produce large numbers of fertile eggs. Axolotls evolved to live in two lakes in Mexico where the water levels were stable and food levels high, enabling animals, lacking the thyroid hormone that triggers metamorphosis, to survive and evolve a life-strategy that was more effective than having a terrestrial stage.

In frogs and toads, there are a range of reasons why tadpoles might be found in a pond in the winter months. The first to rule out is the risk of the invasive species Lithobates catesbeianus, the American bullfrog, which has a larval stage than can be anything from 1 to 4 years. Overwintering of this large, predatory species has been found to have a neutral or negative impact on other amphibians in introduced populations in the USA[1]. Thankfully, the bullfrog appears to have been eradicated from the UK since 2015[2].

If you have other ‘normal sized’ tadpoles in your pond, the chances are that a number of things may have happened: in some northern latitudes, frog and toad tadpoles have been shown to adopt different strategies[3], with some animals metamorphosing early while others delay metamorphosis, remaining in the pond to be able to exploit resources early in the spring , outcompeting a new cohort of smaller tadpoles[4]. As well as food availability and temperature, other unknown factors lead to this strategy[5]. This enables them to produce larger metamorphosed froglets or toadlets which have a better chance of survival through the winter.

So why don’t all anurans follow this strategy? It can be high risk; extreme temperatures could kill animals during hibernation or ponds could dry out before the end of the late summer period and kill all the ‘delaying’ tadpoles. Most Anurans have developmental cues that trigger metamorphosis, linked to the proximity of other tadpoles and to reduction in water level; [6] in the case of ‘drying ponds’ reducing water depth[7]^,[8] leads to faster metamorphosis and smaller froglets.

Anurans that develop into ‘large’ tadpoles do have the capacity to develop gonads but in the case of Xenopus (the African clawed toad) ‘giant tadpoles’ that occur develop gonads but struggle to release eggs and sperm[9] and in any case, a ‘giant tadpole’ would be an unlikely animal to participate in amplexus. A similar situation occurs in Pseudis paradoxa, the paradoxical frog, which has evolved a ‘giant tadpole’ that metamorphoses into an adult rather than a froglet.[10] The tadpoles have developed gonads that are close to maturity but the authors point out that life stages in Anurans are very different and that the opportunity for full neoteny is unlikely because of the need for behavioural changes to enable successful reproduction.

African Clawed Frog - Xenopus laevis — In the case of Xenopus (the African clawed toad) ‘giant tadpoles’ that occur develop gonads but struggle to release eggs and sperm.

What about Caudata? There is evidence that some individuals, usually young adults, may move into ponds in the autumn and overwinter[11]. Examples of palmate newts overwintering in ponds in Scotland are well recorded[12]^,[13], again a strategy to enable a more robust metamorph emerging in the summer to face colder winters.

There are now several well recorded cases of neoteny in smooth newts – in a Norfolk swimming pool, numbers of neotenous newts were found, resembling female smooth newts but with feathery gills[14]. Two of these animals were removed for examination and very quickly metamorphosed to adult newts. The authors of the study suggested a number of possible outcomes suggesting that a change in water quality (increase in iodine) or change in water depth could have triggered the change.

Examples of delayed metamorphosis and also total neoteny have also been recorded at a site in Cambridgeshire, where newt larvae have been seen in large numbers in autumn and early spring, after presumably hibernating in the pond. One of the newts was taken to captivity and laid fertile eggs[15]. This animal did rise to the surface for air, a behaviour not normally linked with neotenous newts, which have fully functioning gills. As in many cases of neoteny, the records were incidental and unable to be investigated in detail.

So overwintering tadpoles and newt larvae may be a way of amphibians having two different life strategies within a population, some animals taking the risk in the pond, some taking the risk overwintering as a smaller sized individual. Neotenous individuals may be taking advantage of a stable environment with significant food sources and low levels of competition as an alternative strategy, but to really investigate this further we need more information on the habitats where these animals are found. If you know of a site with neotenous animals, please let us know by emailing: info@froglife.org

References

[1] Boone, M.D., Little, E.E. and Semlitsch, R.D., 2004. Overwintered bullfrog tadpoles negatively affect salamanders and anurans in native amphibian communities. Copeia, 2004(3), pp.683-690

[2] New strategy launched to protect biodiversity and economy from non-native species – GOV.UK (www.gov.uk)

[3] Tattersall, G.J. and Ultsch, G.R., 2008. Physiological ecology of aquatic overwintering in ranid frogs. Biological Reviews, 83(2), pp.119-140.

[4] McNeill, D.C. and Downie, J.R., 2017. Overwintering of smooth and palmate newt larvae in the Gartcosh Nature Reserve, Scotland. The Glasgow Naturalist, 26.

[5] Walsh, P.T., Downie, J.R. and Monaghan, P., 2008. Larval over‐wintering: plasticity in the timing of life‐history events in the common frog. Journal of Zoology, 276(4), pp.394-401..

[6] Walsh, P.T., Downie, J.R. and Monaghan, P., 2008. Larval over‐wintering: plasticity in the timing of life‐history events in the common frog. Journal of Zoology, 276(4), pp.394-401.

[7] Merilä, J., Laurila, A., Pahkala, M., Räsänen, K. and Timenes Laugen, A., 2000. Adaptive phenotypic plasticity in timing of metamorphosis in the common frog Rana temporaria. Ecoscience, 7(1), pp.18-24.

[8] Laurila, A. and Kujasalo, J., 1999. Habitat duration, predation risk and phenotypic plasticity in common frog (Rana temporaria) tadpoles. Journal of Animal Ecology, 68(6), pp.1123-1132.

[9] Rot-Nikcevic, I. and Wassersug, R.J., 2004. Arrested development in Xenopus laevis tadpoles: how size constrains metamorphosis. Journal of Experimental Biology, 207(12), pp.2133-2145.

[10] Downie, J.R., Sams, K. and Walsh, P.T., 2009. The paradoxical frog Pseudis paradoxa: larval anatomical characteristics, including gonadal maturation. The Herpetological Journal, 19(1), pp.1-10.

[11] Beebee, T. & Griffiths, R. , 2000, Amphibians and Reptiles: A Natural History of the British Herpetofauna: Book 87 (Collins New Naturalist Library), Harper Collins, London

[12] McNeill, D.C. and Downie, J.R., 2017. Overwintering of smooth and palmate newt larvae in the Gartcosh Nature Reserve, Scotland. The Glasgow Naturalist, 26.

[13] Walker, G., Fairclough, B. and Paterson, E., 2019. Winter presence of adult male palmate newts (Lissotriton helveticus) in a pond in Scotland. Herpetological Bulletin, 149, pp.24-27.

[14] Allain, S.J. and Phillips, N., 2023. Observations of a neotenous population of Smooth NewtsLissotriton vulgaris from Norfolk. Trans. Norfolk Norwich Nat. Soc, 56(1).

[15] Leeke, C. 1990, An occurrence of neotenous sooth newts (Triturus vulgaris) in Cambridgeshire. Bulletin of the British Herpetological Society 31,

Croaking Science – a ‘new’ species of leech predating amphibians

November 1, 2024 by Admin

Written by Andrew Smart, Head of Science & Research

Lawson, B. et al , 2024. Predation of anurans in southern England by Batracobdella algira, a leech previously unknown in the UK. Herpetological Journal. 34 221-227

Leeches are often identified as potential predators of the various life stages of amphibians and in the case of frogs and toads, four native species of leech have been identified as feeding on blood and one leech, Haempois sanguisuga, (the horse leech) are know to be predatory on froglets and toadlets.

Reports of leech infestation on a common toad in 2020 led the authors of this recently published paper to explore the occurrence of leech predation on anurans more widely. A call for Citizen Scientists to report sightings led to 41 new records between 2020 and 2023. The authors undertook a detailed review of the species involved, either through morphological or genetic identification, and found that in 2 of the 9 cases identified, the species was Placobdella costata, while in the remaining 7 of the 9 records where leeches were identified to species, the species was Batracobdella algira, a species previously unrecorded in the UK.

Dorsal and ventral view of Placobdella costata

The genotype of the animals collected corresponds with that of individuals from Tunisia and, along with a localised distribution in the south and south west, this suggests the possibility that it is a non-native invasive species. The authors suggest that possible routes of ‘invasion’ include release through animal or plant trade or natural long distance dispersal by birds.

Two other publications from 2016[1] and 2018[2] highlight the link between this leech species and cave salamanders in Sardinia, which suggest that the leech could parasitise newt species. The leech has been recorded on painted frog (Discoglossus pictus) and marsh frog ( Pelopphylax ridibundus) [3] in Morocco and has also be found to parasitize Bufo spiniosus in Tunisia[4] and Algeria[5], a possible pre-adaptation for parasitism of Bufo bufo in the UK.

There is no evidence of the leech being a problem for amphibians within its natural range but it will certainly be worth toad patrols and amphibian surveyors looking out for any new records of leech predation this coming breeding season and notifying the Garden Wildlife Health Project at ZSL of any new records.

References

Click to read more...

[1] Manenti, R., Lunghi, E., Canedoli, C., Bonaccorsi, M. and Ficetola, G., 2016. Parasitism of the leech, Batracobdella algira (Moquin-Tandon, 1846), on Sardinian cave salamanders (genus Hydromantes)(Caudata: plethodontidae). Herpetozoa, 29(1-2), pp.27-35.

[2] Lunghi, E., Ficetola, G.F., Mulargia, M., Cogoni, R., Veith, M., Corti, C. and Manenti, R., 2018. Batracobdella leeches, environmental features and Hydromantes salamanders. International Journal for Parasitology: Parasites and Wildlife, 7(1), pp.48-53.

[3] Mabrouki, Y., Ahmed, R.B., Taybi, A.F. and Rueda, J., 2019. An annotated checklist of the leech (Annelida: Hirudinida) species of the Moulouya River basin, Morocco, with several new distribution records and a historical overview. African Zoology, 54(4), pp.199-214.

[4] Hassine, J.B. and Escoriza, D.., 2014. Bufo spinosus in Tunisia: new data on occurrence, parasitism and tadpole morphology. Herpetol. Bull, 127, p.22.

[5] Merabet, K.. and Karar, M., 2021. First cases of predation of Bufo spinosus by two leech species in Algeria. The Herpetological Bulletin, 156(156), pp.38-39.

Croaking Science- Horned frogs: specialised predators

September 1, 2024 by Admin

Written by Dr. Laurence Jarvis (external contributor)

Horned frogs within the genus Ceratophrys are highly specialised frogs with particularly interesting ecology and behaviour. The eight recognised species are endemic to South America and inhabit grassland and rainforest habitats.

Their appearance is somewhat unusual with extremely wide heads and a relatively large gape. This allows adults to feed on a large range of prey including insects, small mammals and in many cases other frog species¹. They are sit-and-wait predators spending many hours motionless, often concealed under leaf litter waiting for prey to move past.

For the majority of the year the adults burrow underground, often concealed in dry soil, unlike that of many other hibernating or aestivating amphibians which burrow into damp habitats². During this period adult horned frogs develop a cocoon of dead skin around themselves to prevent desiccation. After the first seasonal rains, the adults emerge and breed with a highly explosive breeding strategy laying several thousand eggs in temporary water bodies. Unusually, the tadpoles are voracious carnivores, feeding on a range of other invertebrates, even members of its own species. This enables them to grow and metamorphose within a few short weeks before the water bodies dry.

Figure 1. Horned frogs, such as the Suriname Horned Frog (C. cornuta), have an unusual appearance with a large gape and head.

Horned frogs have evolved a range of unique adaptations to enable them to catch fast moving and difficult to seize prey. Research by Lappin et al. (2017)³ showed that the bite force of Cranwell’s Horned Frog (C. cranwelli) can reach 500 N which is exceptional and rivals that of many medium-sized reptiles or mammals³. This enables them to subdue many species of frogs, lizards, snakes, birds and rodents. In addition, the teeth of horned frogs are sharp, pointed and curved and resemble those of predatory reptiles³. They also have a pair of long fangs enabling them to catch their prey.

Figure 2. The unusual teeth of horned frogs with sharp tips and curved fangs.

In addition to strong jaws, horned frogs possess highly sticky tongues which allows them to latch onto rough substrates such as mammal fur and bird feathers. These allow the tongues of horned frogs to have extremely high adhesive forces which are beyond their own body weight and also outweigh any potential prey item⁴.

It appears that the high adhesive properties are due to a combination of adaptations. First, their tongues are covered in mucus, which enhances adhesion. However, this alone is not enough to explain the high adhesive properties. It appears that the mucus spreads across the surface of the tongue as contact is made with the prey which results in stronger adhesion⁴. Second, a high contact pressure, due to the fast movement of the tongue, allows an additional adhesive property. This is quite unlike the method of attachment of many reptiles such as geckos, which rely on small hair-like projections to adhere to the substrate’s surface⁴.

Furthermore, two species of horned frogs have also been observed carrying out complex pedal luring behaviour to capture their prey⁵. This involves the adult sitting partly concealed in the leaf litter and waving its back legs to attract moving insect or vertebrate prey.

Figure 3. Bell’s Horned Frog (C. ornata) burrow into soil to wait for passing prey.

In 2011 a novel discovery on the tadpoles of Bell’s Horned Frog (C. ornata) in Brazil was made by Natale et al. (2011)⁶. The researchers provide the first evidence for sound production by anuran larvae. Prior to 2011, sound production by anuran larvae was totally unknown⁶. It appears that the larvae of Bell’s Horned Frog produce underwater sounds only with physical contact with others of the same species. The reason for this is unclear but it may be that sound production reduces the chances of larval cannibalism in aggressive, rapidly developing predators.

Although four of the eight species of horned frog are relatively common across South America, two species are declining and threatened due to a combination of habitat loss, urbanisation and illegal collecting from the wild.

In addition, two species are Data Deficient with very little known of their ecology and distribution. Field-based research studies are beginning to reveal important aspects of their ecology¹ but more information is required to enable effective conservation measures to be implemented. Horned frogs are a group of fascinating species and it would be a great loss to see them decline further throughout their range.

References:

¹da Silva Jorge J., Sales R.F.D., de Carvalho Kokubum M.N. and Freire E.M.X. (2015). On the natural history of the Caatinga horned frog, Ceratophrys joazeirensis (Anura: Ceratophryidae), a poorly known species of northeastern Brazil. Phyllomedusa, 14 (2): 147-156.

² Faivovich J.L., Nicoli B.L., Blotto M.O., Pereyra D., Baldo J.S., Barrionuevo, M., Fabrezi, E.R. and Haddad, C.V.B. (2014). Big, bad, and beautiful: phylogenetic relationships of the Horned Frogs (Anura: Ceratophryidae). South American Journal of Herpetology, 9: 207–227.

³ Lappin A.K., Wilcox S.C., Moriarty D.J., Stoeppler S.A., Evans S.E. and Jones M.E. (2017). Bite force in the horned frog (Ceratophrys cranwelli) with implications for extinct giant frogs. Nature Scientific Reports, 7 (1): 11963.

⁴ Kleinteich T. and Gorb S.N. (2014). Tongue adhesion in the horned frog Ceratophrys sp. Nature Scientific Reports, 4 (1): 5225.

⁵ Radcliffe C.W., Chiszar D., Estep K., Murphy J.B. and Smith H.M. (1986). Observations on pedal luring and pedal movements in leptodactylid frogs. Journal of Herpetology, 20 (3): 300-306.

⁶ Natale G.S., Alcalde L., Herrera R., Cajade R., Schaefer E.F., Marangoni F. and Trudeau V.L. (2011). Underwater acoustic communication in the macrophagic carnivorous larvae of Ceratophrys ornata (Anura: Ceratophryidae). Acta Zoologica, 92 (1): 46-53.

Croaking Science: who are the salamanders?

August 1, 2024 by Admin

Written by Elsbeth Leighton, Coalface to Wildspace Project Assistant

The salamanders, or Caudata, are one of the three classes of animals that make up the Amphibians, alongside the Gymnophiona; the worm-like caecilians, and the Anura; the frogs and toads. The salamanders are the second largest group, after the Anura, comprised of over 800 known species. These species are split into 9 main families, with diverse traits and life histories (1).

Gaps in the amphibian fossil record make determining the exact evolutionary relationships between groups challenging and debated. However, it is generally agreed that there are three more basal, i.e. more ancient, families and the rest are younger and form a suborder called the Salamandroidea, otherwise called the advanced salamanders (2). A key difference between the ancient families and the younger families is in their reproduction. The ancient families fertilise their eggs externally, and the younger families fertilise them internally (3).

Cryptobranchidae

The Cryptobranchidae, commonly called the Giant Salamanders, is one of the ancient families and has 6 known living species (1). They live fully aquatic lifestyles as paedomorphic adults, meaning that they keep key features into adulthood that are usually seen in larval states. These features include their gill slits and their lack of eyelids (4). These are considered basal traits, along with their feeding behaviour. They are suction feeders, whose specialised jaw anatomy enables them to grab prey from one side of their mouth at a time (5). Well-known examples from this family include the giant Japanese salamander and the giant Chinese salamander, which can both grow over 100cm (6).

Hynobiidae

The Hynobiidae, or the Asian Salamanders, branched from the same common ancestor as the Cryptobranchidae (2). They have 100 known species, distributed across Asia and Russia (1). Unlike the Cryptobranchidae, these salamanders live biphasic life cycles, where they fully metamorphose from aquatic larvae into terrestrial or aquatic adults without gills (4).

Sirenidae

The Sirenidae, or the Sirens, are unique ancient salamanders that lack back legs entirely and only have small front legs. They have long bodies and external gills, suited to their fully aquatic lifestyles (4). All seven known living species are found in the southeastern USA and northeastern Mexico (1). Interestingly, some siren species eat plants and are omnivorous, whereas all other salamanders are generally believed to be carnivorous (7).

Ambystomatidae

The Ambystomatidae are part of the advanced salamanders and consist of 30 living species (1). They are commonly called mole salamanders because many of these species live a fossorial lifestyle in underground terrestrial burrows, only emerging at nighttime to feed. This is not the case for all species in this family, however, as it also includes aquatic paedomorphic species, such as the famous axolotl (4). The 4 species of Dicamptodon or Pacific Giant Salamanders, are also grouped into this family, having previously been considered a separate group (1).

Salamandridae

The Salamandridae, or the true salamanders and newts, includes our UK native newts and is a large group with 144 species (1). They are found across Europe, Asia, northern Africa, and America and are the only family where some species reproduce viviparously. This means that they give birth to live young as aquatic larvae or fully formed terrestrial young, instead of laying eggs (8). The newts are found in the subfamily Pleurodelinae and can have two (biphasic) or three (triphasic) stage life cycles (4). Triphasic life cycles are unique to the newt and are seen in species such as the Eastern Newt, which develops from aquatic larvae into terrestrial juveniles before returning to the water again as aquatic adults (9).

Proteidae

The Proteidae are a small group, with only 9 species (1). They have long bodies and small limbs, living fully aquatic life cycles as paedomorphs who keep their external gills into adulthood. They include the mudpuppies from North America as well as the Olm, a blind cave-dwelling salamander from Europe (4). The Olm is a unique species and is thought to be the longest-lived salamander with a potential lifespan of over 100 years (10). There has been some confusion around how Olms reproduce and if they give birth to live young, partially due to the challenges their habitat presents for study, but the current understanding is that they lay eggs (11).

Rhyacotritonidae

The Rhyacotritonidae or torrent salamanders are another small group with only 4 species (1). They live a biphasic lifestyle and are semi-aquatic as adults, adapted to live alongside the edge of fast-flowing waterbodies such as streams. To adapt to these environments, their larvae have developed smaller gills to suit the fast-flowing water (4). They live in the Northwestern USA and are strongly associated with intact humid forests, as damage to this habitat limits their ability to disperse (12).

Amphiumidae

The Amphiumidae or amphiumas are fully aquatic paedomorphic salamanders with gill slits and absent eyelids as adults. They have very small limbs, but unlike the sirens who only have front limbs, they have all four limbs (4). There are 3 known species living today, all of which are restricted to the Southeastern USA (1).

Plethodontidae

The final family, the Plethodontidae is the biggest group with 520 known species (1). They are known as the lungless salamanders because they evolved to lose their lungs and instead breathe entirely through their skin and a patch at the back of their throat containing a special protein (13). They are a very diverse group, occurring in a range of habitats mostly within the Americas, but some species are also known from Southern Europe and Korea (1). They display very varied reproduction, but the most common and ancestral method is direct development, where the larval stage is skipped entirely to produce fully developed terrestrial young from eggs. However, some species have re-evolved the ability to have aquatic larvae and lead biphasic lifestyles or are paedomorphic (8, 14).