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Croaking Science: Cave Salamanders

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Cave salamanders

Cave salamanders belonging to the amphibian genera Hydromantes and Eurycea occur in southern Europe and widely across the United States. There are at least 11 species of salamander which spend their entire lives in cave environments and many more which complete part of their life-cycle in underground environments e.g. fire salamander (Salamandra salamandra). Many salamanders which live in cave environments exhibit two key characteristics: lack of lungs and direct development (i.e. loss of aquatic larval stage). Having no lungs forces these salamanders to rely on absorbing oxygen across their skin and use bucco-pharangeal breathing, which is obtaining oxygen through the moist lining of the mouth. This type of breathing can only occur effectively in specific conditions of high moisture and cool temperature, which are often found in cave environments. Due to their underground existence, the ecology and behaviour of cave salamanders is poorly understood and recent research is only just beginning to understand the biology of these species.

Figure 1. The Ambrosi Salamander (Hydromantes ambrosii) occurs throughout the French and Italian Alps to altitudes of 2,432 m. It does not rely on water for reproduction and often frequents caves and other underground systems to avoid desiccation.
[Photo credit: Benny Trapp, https://commons.wikimedia.org/wiki/File:BennyTrapp_Ambrosis_H%C3%B6hlensalamander_Speleomantes_ambrosii_Italien.jpg]

Previous research on cave salamanders of the genus Hydromantes in southern Italy has shown that these salamanders may reproduce at any time of year, being unconstrained by seasonal environmental changes that affect most non-cave dwelling species. It appears that in most species the female lays a small number of large eggs and protects them until hatching, guarding them from predators and preventing fungal infections (Figure 2). However, the detail of reproductive behaviours such as the number of breeding attempts and nest site selection remain unknown. Recently, Lunghi et al. (2018) carried out a four year study into cave salamanders within the genus Hydromantes at 150 sites in southern Italy. Lunghi et al. (2018) found that in several Hydromantes species the females may retain eggs for many months before releasing them, confirming that the large eggs require a long time to mature in the female before laying. This is followed by a period of at least 6 months of attending eggs and then parental care post-hatching. Therefore, these species require at least two years to complete one reproductive cycle. Given a life-span of only 11 years and maturity at 3-4 years of age, this restricts the number of available reproductive attempts by each female and limits population growth, which is of concern in threatened or vulnerable species. Lunghi et al. (2018) also found that nest site selection was extremely important and that each cave only had a limited number of suitable sites for nesting. Females would occupy only a small number of sites which would vary depending on the year and environmental conditions. This is supported by Lunghi et al. (2014) who found that the wrong choice of nest site can cause breeding failure. Successful breeding seems to occur in nest sites that have high humidity and low temperature. On a yearly basis, different females may use the same nesting site since suitable nests are highly limited. This further suggests that in some species population growth may be limited by suitable areas to breed.

Figure 2. The Italian cave salamander (Hydromantes italicus) lays between 6 and 14 eggs, which take 10 months to hatch into juveniles which resemble the adults (direct development). [Photo credit: Benny Trapp, https://commons.wikimedia.org/wiki/File:BennyTrapp_Speleomantes_italicus.jpg]

Living in cave environments can be challenging for salamanders as often prey densities are low. Relying on freshwater invertebrates may not always provide enough nutrition for species inhabiting these environments. Recent research by Fenolio et al. (2006) has shown that the Ozark Blind Cave Salamander (Eurycea spelaea) exploits another, less obvious food source. During a study to investigate the community ecology of bat caves in Oklahoma, USA, they found to their surprise that the larval salamanders were consuming bat guano. Initially the researchers thought that the ingestion of guano was incidental since consumption of non-food items in amphibians usually only occurs by accident whilst consuming their normal invertebrate prey. Fenolio et al. (2006) investigated this further and found that the salamanders were actually deliberately ingesting the bat guano as a food source. Nutritional analyses of bat guano revealed that it contained nutrients roughly equivalent to those that would be found in a potential prey item in this ecosystem, amphipods. The researchers also found that numbers of E. spelaea increased significantly in the main parts of the cave system where grey bats deposit fresh guano. This study contradicts the general understanding that salamanders are strictly carnivorous and shows that this adaptation enhances the survival of this species in this harsh environment.

Figure 3. The Ozark Blind Cave Salamander (Eurycea spelaea) of Missouri and southern Kansas, USA lives in caves and feeds on bat guano for nutrition.
[Photo credit: Peter Paplanus, https://commons.wikimedia.org/wiki/File:Eurycea_spelaea,_Izard_County,_Arkansas,_by_Peter_Paplanus.jpg]

Cave salamanders may also exhibit several morphological adaptations to living in dark and humid underground environments. The Ozark Blind Cave Salamander is also unique in that it starts life as a fully sighted larva but then metamorphoses underground into a terrestrial adult that loses its pigment and becomes blind, with the eyelids eventually fusing (AmphibiaWeb, 2016). There are several hypotheses to explain the reason for this morphological change. However, the most favoured is the energy economy hypothesis which states that the cost of developing and maintaining eyes is substantial and a large portion of the brain is needed for visual processing with high oxygen consumption rates. Energy savings from the loss of eyes could reduce the amount of time needed for foraging and allow energy to be re-invested into other physiological processes, including reproduction (AmphibiaWeb, 2016). In addition, eye loss may be linked to the process of skull development which is a pattern also observed in blind cavefish.

Cave salamanders are a unique group of amphibians with highly evolved ecology and life history characteristics. Unfortunately, an increasing number of cave salamanders are now threatened and classified as Vulnerable or Endangered on the IUCN Red List. Increasing our understanding of the ecology and biology of these declining species is critical in developing effective conservation initiatives, especially in cave environments which are prone to human-induced and climatic change.

References

AmphibiaWeb (2016) Eurycea spelaea: Grotto Salamander <http://amphibiaweb.org/species/4220> University of California, Berkeley, CA, USA. Accessed 27 September, 2019.

Fenolio, D.B., Graening, G.O., Collier, B.A. & Stout, J.F. (2006) Coprophagy in a cave-adapted salamander; the importance of bat guano examined through nutritional and stable isotope analyses. Proceedings of the Royal Society, London B, 273: 439–443.

Lunghi, E., Manenti, R., Manca, S., Mulargia, M., Pennati, R. & Ficetola, G.F. (2014). Nesting of cave salamanders (Hydromantes flavus and H. italicus) in natural environments. Salamandra, 50 (2): 105-109.

Lunghi, E., Corti, C., Manenti, R., Barzaghi, B., Buschettu, S., Canedoli, C., Cogoni, R., De Falco, G., Fais, F., Manca, A., Mirimin, V., Mulargia, M., Mulas, C., Muraro, M., Murgia, R., Veith, M. & Ficetola, G.F. (2018) Comparative reproductive biology of European cave salamanders (genus Hydromantes): nesting selection and multiple annual breeding. Salamandra, 54 (2): 101-108.

What our animals are doing this month…

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What our animals are doing this month… August 2019

August is the month that young grass snakes are likely to emerge from their eggs.  Previously in the year, gravid female grass snakes will have been seen basking more intensively than other grass snakes as they developed their eggs. During this time, females will fast for roughly 45 days whilst eggs develop and are laid.

Grass snakes usually just lay one clutch in a breeding year and do not necessarily reproduce on an annual basis. Eggs are laid in warm, secluded areas such as compost heaps, manure piles, old mammal holes, log piles or under rocks. These eggs need to be incubated anywhere from 22 days to 45 days depending on the environmental conditions. Areas of higher temperature have shown beneficial results for the numbers of eggs developing, with temperatures of 210C to 310C being preferred.

An average clutch size of eggs for a grass snake is 30 eggs, however many females may lay communally with others which can result in areas with thousands of eggs being incubated together.

By August hatchlings should emerge, measuring 15-20cm with the same identification features as adult grass snakes. Hatchlings use an ‘egg tooth’ to escape their egg by breaking the shell – this tooth is lost shortly after emerging.

Remember to report your sightings (including egg sightings) with our FREE Dragon Finder app.

Figure 1: Juvenile grass snakes can be easily identified being miniature versions of adults

Croaking Science: Heterospecific attraction: using the sounds of others to find the way

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Croaking Science: Heterospecific attraction: using the sounds of others to find the way

Amphibians may use a variety of cues to exploit new habitats including olfactory, visual and celestial cues. Often, individuals may use conspecific cues, i.e. those from individuals of the same species, to locate suitable habitats. For example, many pond-breeding amphibian of the genus Hyla and Rana, have been found to be attracted to calls of the same species (Buxton et al., 2015). However, the use of heterospecific cues, i.e. those from individuals of a different species, is less wide spread and our knowledge and understanding of how species use such cues is generally poorly understood. The heterospecific attraction hypothesis (HAH) has been proposed to describe how individuals may select habitats based on the presence of other species (Mönkkonen et al., 1997). Although initially described for birds, in recent years both field and laboratory experiments have shown an increasing number of amphibian species that use heterospecific cues to locate suitable habitats.

In the tropical forests of Taiwan a number of amphibian species co-exist, with varying degrees of niche overlap. Males of an oriental tree frog species, Rhacophorus prasinatus, form choruses in bushes and produce foam nests on vegetation near or above ephemeral and permanent water (Figure 1). Within the same habitat also lives Babina adenopleura which usually breeds at permanent water bodies and produces an egg mass in the water. This species is therefore a non-competitor as both species have different ecological requirements. However, Polypedates braueri is a resource competitor of R. prasinatus and uses ephemeral and water during the same time period and also produces a foam nest on vegetation. Research by Chang et al. (2018) has found that R. prasinatus frogs initially locate breeding aggregations of males using conspecific cues i.e. the cues of calling males of the same species. However, when these are lacking, they locate suitable habitat using the calls of the non-competitor B. adenopleura, rather than the competitor, P. braueri. This behaviour indicates that the calls of non-competitors could serve as cues for breeding site allocation (Chang et al., 2018). Since breeding sites are limited in this environment, avoiding choruses with heterospecific competitors, such as P. braueri, may reduce competition.

Figure 1. The tree frog Rhacophorus prasinatus from Taiwan may use the cues of another species, Babina adenopleura, to locate potential breeding sites. (Photo credit: Evan Pickett, licensed under the Creative Commons Attribution-Share Alike 4.0 International license. https://commons.wikimedia.org/wiki/File:Rhacophorus_prasinatus.jpg )

Many pond-breeding amphibians living in temperate areas often undergo dispersal to seek alternative breeding sites. In particular, in European newt genera such as Triturus, adults may switch breeding ponds in alternative years (Griffiths & Williams, 2000). The dispersal behaviour of individuals is often influenced by environmental factors such as food supply, predation risk, parasite load, and habitat disturbance (context-dependent dispersal) (Cayuela et al., 2018). However, individuals may also make decisions over dispersal direction, distance and habitat selection using the cues of other individuals (social-dependent dispersal). In central Europe great crested newts (Triturus cristatus), palmate newts (Lissotriton helveticus) and alpine newts (Ichthyosaura alpestris) may all co-habit the same ponds. Such newt species have a high degree of niche overlap, with great crested newts being competitively dominant, feeding on similar prey to the other species (Griffiths, 1994). Research by Cayuela et al. (2018) found that great crested newts make decisions on whether to leave a pond or occupy a new one based on the density of palmate newt and alpine newts. In this study, great crested newts were less likely to disperse from ponds which had a high heterospecifc density, i.e. a high density of palmate and alpine newts. In addition, the researchers found that when emigrating from ponds to seek other suitable breeding sites, great crested newts were more likely to select ponds which had a high density of palmate and alpine newts. These findings suggest that great crested newt adults do not avoid ponds with a high density of heterospecifics to limit the negative effects of interspecific competition on larval development and survival (Cayuela et al., 2018), but instead actively choose ponds where the density of other newt species is high. The presence of other newt species may indicate high pond quality with abundant food and egg laying resources, and since great crested newts are competitively dominant over the other newt species, they will gain by breeding in these ponds. The exact mechanism by which great crested newts can detect the density of other species within a pond is unknown, but is likely to be due to chemical cues (Cayuela et al., 2018).

Figure 2. Great crested (left), alpine (right) and palmate (below) newts all coexist in the same pond. (Photo credits: 1- Great crested newt: Rainer Theuer, licensed under the Creative Commons Attribution-Share Alike 2.5 Generic license. https://commons.wikimedia.org/wiki/File:Kammmolchmaennchen.jpg. 2- Palmate newt: Froglife. 3- Alpine newt: Anevrisme, Creative Commons Attributio)n-Share Alike 3.0 Unported license. https://commons.wikimedia.org/wiki/File:Triturus_alpestris.jpg. 

In Northern Italy, the smooth newt (L. vulgaris) breeds in similar water bodies to both the common toad (Bufo bufo) and green toad (B. viridis). In this region, the common toad breeds in large permanent water bodies and arrives at ponds in early February, whereas the green toad breeds in ephemeral water bodies and has a variable arrival time between February and March, depending on levels of rainfall (Pupin et al., 2007). Since smooth newts will breed in all types of pond, both toad species are therefore potential cues for water presence in a given area and may signal water quality. Research by Pupin et al. (2007) showed that smooth newts positively orientated towards the calls of both species and were using these to indicate location of potential breeding water bodies. Madden & Jehle (2017) carried out similar experiments on the great crested newt and found that individuals oriented to the calls of the common toad, but not common frog (Rana temporaria). They hypothesised that this was because common toads bred later than common frogs, at a more suitable time of year for great crested newts, so orienting towards ponds with common toads was more ecologically adaptive than those of common frogs.

Heterospecific attraction is a relatively newly studied process in amphibians and research has shown a number of examples when individuals use the cues of other species to locate and orientate towards suitable habitats. Such research has conservation significance since it shows the importance of food webs and community cohesion and demonstrates that different species are highly connected. Community level and landscape scale conservation, which targets the conservation of whole groups of species and widespread habitats, may therefore have high benefit to overall biodiversity.

 

References

Buxton, V.L., Ward, M.P. & Sperry, J.H. (2015) Use of chorus sounds for location of breeding habitat in 2 species of anuran amphibians. Behavioral Ecology, 26: 1111-1118.

Cayuela, H., Grolet, O. & Joly, P. (2018) Context‑dependent dispersal, public information, and heterospecific attraction in newts. Oecologia, 188 (4): 1069 – 1080. https://doi.org/10.1007/s00442-018-4267-3.

Chang, C., Cheng, Y. & Lin, S. (2018) Influence of conspecific and heterospecific cues on phonotaxis behaviour in a polyandrous treefrog. Behavioral Ecology and Sociobiology, 72: 179-190. https://doi.org/10.1007/s00265-018-2593-4.

Griffiths, R.A., Wijer P. de & May, R.T. (1994) Predation and competition within an assemblage of larval newts (Triturus). Ecography, 17 (2): 176-181.

Griffiths, R.A. & Williams, C. (2000) Modelling population dynamics of the great crested newt (Triturus cristatus): a population viability analysis. The Herpetological Journal, 10: 157-163.

Madden, N. & Jehle, R. (2017) Acoustic orientation in the great crested newt (Triturus cristatus). Amphibia-Reptilia, 38 (1): 57-65.

Mönkkonen, M., Helle, P., Niemi, G. & Montgomery, K. (1997) Heterospecific attraction affects community structure and migrant abundances in northern breeding bird communities. Canadian Journal of Zoology, 75: 2077-2083.

Pupin, F., Sacchi, R., Gentilli, A., Galeotti, P. & Fasola, M. (2007) Discrimination of toad calls by smooth newts: support for the heterospecific attraction hypothesis. Animal Behaviour, 74: 1683-1690. doi:10.1016/j.anbehav.2007.03.020.

Croaking Science: Caecilians – unusual reproductive ecology

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Croaking Science: Caecilians – unusual reproductive ecology

Caecilians, or blind snakes, as they are also often called, are limbless, elongate amphibians which inhabit tropical regions, with hotspots in South America and Asia (Kupfer et al., 2016; Gomes et al., 2012). Of all the groups of amphibians, caecilians are the most poorly understood, which is in part due to their secretive, fossorial lifestyle (Figure 1). They belong to the order Gymnophiona, one of three orders which comprises the Amphibia. There are currently 209 recognised species, which represents just 3% of the total 7,950 amphibian species worldwide (AmphibiaWeb, 2018). Caecilians are all ground-dwelling tropical amphibians which exhibit a wide range of reproductive modes, from laying eggs which hatch into aquatic larvae to those which hatch into miniature terrestrial juveniles. Little is known of their diverse reproductive modes but in recent years research has elucidated more information on their unusual breeding ecology.

Figure 1. Caecilians have an unusual appearance, being primarily fossorial and elongate in form. [Photo credit: Will Brown – Brown-snouted Blind Snake (Ramphotyphlops wiedii or nirgrogrescens) 3, CC BY 2.0,https://commons.wikimedia.org/w/index.php?curid=71399151]
Oviparous caecilians lay eggs, which are usually laid in a small terrestrial nest (Figure 2). The females of all these species exhibit extensive parental care, guarding their clutches of eggs against predators (Gomes et al., 2012). Eggs will hatch into either free-swimming larvae which develop in small water bodies, or terrestrial juveniles which are independent of their parents. Ichthyophis kohtaoensis inhabits the tropical forests of northern Thailand and belongs to the second largest caecilian family. In this species, breeding coincides with the rainy season with clutches of eggs laid in damp soil at the onset of the first rains (Kupfer et al., 2004). Females lay their eggs in small chambers near to temporary and permanent ponds, pools and slow flowing brooks and rivers. This allows the free-swimming larvae to readily hatch into the water to complete their aquatic development. Research suggests that females eat little or no food during the three month incubation period since the body condition of females attending young clutches is higher than those with older clutches (Kupfer et al., 2004). This, along with the small clutch size, indicates that there are costs to parental investment and attending eggs. In addition, there is a correlation between female body length and clutch size with older females being longer than young females (Kupfer et al., 2004). This implies that older, more experienced females produce larger clutches and more offspring (Kupfer et al., 2004).

Figure 2. A female caecilian guarding her brood of eggs from predators. [Photo credit: Davidvraju, Creative Commons Attribution-Share Alike 4.0 International, https://commons.wikimedia.org/wiki/File:Caecilian_guarding_its_eggs.jpg]
The ringed caecilian, Siphonops annulatus, is an unusual oviparous species inhabiting South America. The eggs hatch into poorly developed terrestrial young which feed on the skin of the female. The young possess specialised teeth which allow them to scrape skin from their mother’s back immediately after hatching (Wilkinson et al., 2008). The feeding behaviour of the young is described as being “quite frenetic with the young frequently tearing pieces of skin by spinning along their long axes and sometimes struggling over the same piece of skin” (Wilkinson et al., 2008) (Figure 3). This maternal dermatophagy is also known from a second, distantly related African caecilian species, Boulengerula taitana (Kupfer et al., 2008), which suggests that skin feeding is an ancient form of caecilian parental care and may have persisted for more than 100 million years (Wilkinson et al., 2008). In addition, Wilkinson et al. (2008) report a previously undocumented behaviour which involves the female raising herself upward, exposing her vent and releasing a clear fluid, which the young consume. This appears to be a form of feeding the young, but further studies are required to ascertain the function of this behaviour.

Figure 3. A caecilian mother with eggs which will hatch into young which feed off her skin. [Photo credit: Wilkinson M, Sherratt E, Starace F, Gower DJ (2013)]
Viviparous caecilians, which give birth to live young, represent another unusual reproductive mode for amphibians. During development inside the female, the foetuses initially obtain their nutrition from a yolk, similar to egg-developing species. After this, the developing young feed on a uterine milk, secreted from the oviducts of the female. The young obtain this through possessing specialised embryonic teeth which they use to scrape at the oviduct wall (Gomes et al., 2012). The evolution of viviparity in caecilians is poorly understood, but since both viviparous and oviparous species possess specialised teeth for scraping (either skin or oviduct milk), this suggests that all these species evolved from a common, oviparous ancestor (Kupfer et al., 2006). Gegeneophis seshachari is the only known species of viviparous caecilian from the Indo-Seychellean region (Gower et al., 2008). In this area the rainy season is short and unpredictable such that laying eggs which hatch into offspring that rely on the presence of water or a damp environment may be highly risky. Water bodies may dry up rapidly before larvae can fully develop. Therefore, Gower et al. (2008) propose that viviparity may have evolved in this species in response to short, unpredictable rainy seasons and the lack of suitable locations to lay and incubate their eggs.

Caecilians possess interesting and unusual reproductive modes, much of which is poorly understood. In addition, many of these species are threatened or Data Deficient and in regions which are undergoing large habitat loss and fragmentation. More primary data are required to fully understand caecilian reproductive ecology and evolution, which will help inform much needed improvements in conservation assessments for these species.

 

References

AmphibiaWeb (2018) https://amphibiaweb.org. University of California, Berkeley, CA, USA. Accessed 15 Nov 2018.

Gomes, A.D., Moreira, R.G., Navas, C.A., Antoniazzi, M.M. and Jared, C. (2012). Review of the reproductive biology of caecilians (Amphibia, Gymnophiona). South American Journal of Herpetology, 7 (3): 191-202.

Gower, D.J., Giri, V., Dharne, M.S. and Shouche, Y.S. (2008) Frequency of independent origins of viviparity among caecilians (Gymnophiona): evidence from the first ‘live-bearing’ Asian amphibian. Journal of Evolutionary Biology, 21: 1220–1226.

Kupfer, A.K., Nabhitabhata, J. and Himstedt, W. (2004) Reproductive ecology of female caecilian amphibians (genus Ichthyophis): a baseline study. Biological Journal of the Linnean Society, 83: 207-217.

Kupfer, A., Müller, H., Antoniazzi, M.M., Jared, C., Greven, H., Nussbaum, R.A. and Wilkinson, M. (2006) Parental investment by skin feeding in a caecilian amphibian. Nature, 440 (13 April 2006), doi:10.1038/nature04403.

Kupfer, A., Wilkinson, M., Gower, D.J. Müller, H. and Jehle, R. (2008) Care and parentage in a skin-feeding caecilian amphibian. Journal of Experimental Biology, 309A: 460–467.

Kupfer, A., Maxwell, E., Reinhard, S. and Kuehnel, S. (2016) The evolution of parental investment in caecilian amphibians: a comparative approach. Biological Journal of the Linnean Society, 119: 4–14.

Wilkinson, M., Kupfer, A., Marques-Porto, R., Jeffkins, H., Antoniazi, M.M. and Jared, C. (2008) One hundred million years of skin feeding? Extended parental care in a Neotropical caecilian (Amphibia: Gymnophiona). Biological Letters, 4: 358–361.

doi:10.1098/rsbl.2008.0217

Croaking Science: Toxicity in Amphibians

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Croaking Science: Toxicity in amphibians

Frogs, toads and salamanders have long been considered to be noxious and a large range of amphibian species have been found to secrete toxic chemicals from their skin. Over recent decades the type and mode of action of these chemicals has been characterised from many amphibian species. Toads of the genus Bufo, for example the common toad (Bufo bufo), are able to synthesize and store a host of chemicals, some of which are known as steroidal bufadienolides (Daly, 1998). These are a type of cardiac glycoside and specifically block nerve action in the heart, resulting in irregular or slow heartbeat, rapid heartbeat and possibly lethal cardiac arrest in their predators. Leptodactylid frogs, which includes a diverse range of species from the Neotropics, harbour phenolic amines in their skin which are toxic to a range of vertebrate predators. Another class of amphibian toxins are the tetrodotoxins (TTX), which were originally discovered in newts of the family Salamandridae, specifically of the genus Taricha. The rough-skinned newt (Taricha granulosa) of North America is the most toxic salamander in this region and is considered harmful to humans (AmphibiaWeb, 2014) (Figure 1). This species, along with others in the Salamandridae family, such as Triturus newts (e.g. great crested and Italian crested newts) from Europe contain tetrodotoxin in their skin, which is a potent neurotoxin and is a sodium channel blocker. This means that it inhibits the firing of action potentials in neurons and prevents the nervous system from carrying messages and thus muscles from flexing (Bane et al., 2014). Some of the most well documented species of frog containing skin toxins are the poison dart frogs of South America. Unlike Bufo toads or salamanders, which synthesise their own toxins, poison dart frogs accumulate toxic lipophilic alkaloids from their food sources which may be small arthropods, such as mites, ants, springtails, and flies (Daly, 1998).

Figure 1. The rough-skinned newt of North America secretes highly toxic tetrodotoxins from its skin which is harmful to humans. [Photo credit: By Chuck Spidell – Imported from 500px (archived version) by the Archive Team. (detail page), CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=72144165]
The production of skin toxins that have been derived from arthropod prey remains a source of scientific interest and research. The occurrence of alkaloid toxins in different populations and species of poison frogs provides information as to the distribution and availability of the arthropods that provide alkaloids to them. For example, if a species of the bufonid in the genus Melanophryniscus at a particular site do not have alkaloids that are known to be from a certain group of arthropods, then that mite, ant, beetle, or millipede is either not present or not eaten by the toad. Recent research by Daly et al. (2007) investigated the profile of alkaloid skin toxins from different populations of Melanophryniscus toads from Uruguay and Argentina (Figure 2). Daly et al. (2007) found that the nature and availability of dietary sources of alkaloids found in the skin of these toads is strongly dependent on the site of collection and can change with time. This is likely to have a strong impact on the toxicity of the skin toxins produced by individual toads and thus have implications for predator defence. A change or drop in the number of a prey species of alkaloid-producing arthropod is therefore likely to have knock-on impacts for frog chemical defence. Such research highlights the importance of the interconnectedness of species groups and maintaining overall biodiversity in protected areas.

Figure 2. Many species of Melanophryniscus toad secrete alkaloid toxins which they obtain from their diet. [Photo credit: Bornschein MR et al., (2015) Three new species of phytotelm-breeding Melanophryniscus from the Atlantic Rainforest of Southern Brazil (Anura: Bufonidae). PLOS ONE 10(12): e0142791. https://doi.org/10.1371/journal.pone.0142791, CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=60310764]
Toxins, such as those secreted from a frog, toad or salamander are only likely to be effective if they have immediate effects on their predator. Small molecules such as alkaloids or steroids are rapidly absorbed across cell membranes even if the predator’s skin is not punctured. This results in immediate effect in the predator such as distastefulness or pain. However, many frog species secrete large molecule toxins such as peptides or small proteins that makes them unsuitable for fast absorption (Nelson et al., 2014). Therefore, the poisons of many frogs seem to be unsuitable for fast toxin uptake by predators and to have any deterrent effect. However, as well as secreting toxins, the skin of many amphibians also contains antimicrobial peptides (AMPs), which are a defence strategy against a range of bacteria and fungi. Until recently, it was thought that AMPs were used only to fight off bacterial infections which would otherwise infect a frog’s skin. However, recent research by Raaymakers et al. (2017) suggests that these AMPs may actually also be used to facilitate the uptake of toxins into predators and increase the speed of action. They found that in frogs which secrete large molecule peptide toxins, the uptake into predators was increased in the presence of AMPs. This highlights the complex role that frog skin chemicals play in antipredator defence.

Most salamander species lack toxic skin secretions, but contain other active compounds such as norepinephrine, steroids, enzymes, and antimicrobial or antifungal substances (Daly, 1998). Many may also produce adhesives for defence (Evans & Brodie, 1994). For example, once attacked by a predatory snake, the California slender salamander (Batrachoseps attenuates) grasps the snake, loops its tail around the snake’s head and coats the predator with a sticky viscous fluid (Arnold, 1982). The adhesives in the secretion harden within seconds upon exposure to air and immobilize the snake immediately (Arnold, 1982). The salamander then escapes and the snake is unable to free itself for up to 48 hours (Arnold, 1982). There is a great deal of interest in natural adhesives for human engineering and nanotechnology (Von Byern et al., 2017) and in creating mimics which can be produced commercially. In a set of experiments using several salamanders, Von Byern et al. (2017) found that some salamanders also produced toxic chemicals which augmented the effect of the adhesives. In addition, two salamander species, the northern slimy salamander (Plethodon glutinosus) and spotted salamander (Ambystoma maculatum) from North America produced adhesives which have the potential to be mimicked commercially and used in nanotechnology products. These findings highlight the importance of understanding skin chemicals in amphibians and the potential use in pharmacology and nanotechnology.

Figure 3. The California slender salamander secretes adhesive properties to deter predators. [Photo credit: Greg Schechter from San Francisco, USA – California Slender Salamander, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=55625511]
 

References

AmphibiaWeb (2014) Taricha granulosa: Rough-skinned Newt <http://amphibiaweb.org/species/4288> University of California, Berkeley, CA, USA. Accessed Oct 25, 2018.

Arnold, S.J. (1982). A quantitative approach to antipredator performance: salamander defense against snake attack. Copeia, 1982 (2): 247e253.

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Croaking Science: Frogs & toads at the coast

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Frogs and toads at the coast: salt tolerance in amphibians

It is generally understood that amphibians breed and associate with freshwater habitats such as ponds, lakes and other small waterbodies. Many scientific studies have demonstrated that amphibians are particularly vulnerable to saline conditions at embryo, larval and adult stages. The most vulnerable life stage is the embryo which often experience high mortality when in salt environments (e.g. Beebee, 1985; Hua & Pierce, 2013). This is because salt water tends to disrupt the ionic and water exchange across permeable membranes. However, an increasing number of studies have found amphibian species that are able to adapt to and tolerate salt water habitats, especially those in coastal marshes. In his observations of the lack of amphibians on islands and in saline habitats, Charles Darwin in 1872 noted that only “one Indian species” was able to tolerate salt water (Darwin, 1872 in Hopkins & Brodie, 2015). Since then a range of scientific observations and studies have vastly increased our knowledge of the number of amphibians that are able to tolerate salt water in the natural environment. In a recent review of all scientific studies relating to amphibians and their ability to resist salt water Hopkins and Brodie (2015) identified 144 amphibian species from 28 amphibian families across the world from anurans (frogs and toads), salamanders and caecilians that are able to inhabit salt habitats. Salt tolerance in amphibians has been recorded from all continents except Antarctica.

The majority of studies which have found amphibians in salt water habitats have recorded them from coastal areas, which get flooded predictably by sea water. For example, mangrove swamps are flooded daily by tidal fluctuations which results in predictable levels of salt water in surrounding standing water. The most well-known frog to inhabit such environments is the crab-eating frog (Fejervarya cancrivora) of southeast Asia (Figure 1). This unique species can live in freshwater as well as 75% seawater and can adapt from one to the other in a matter of hours (Ren et al., 2010). It is able to achieve this by rapidly changing levels of urea in its body tissues which avoids excessive water loss through the skin in salty conditions (Ren et al., 2010). In addition, it has three glands within its skin: mucous glands, mixed glands, and vacuolated gland which help buffer the additional salt which would otherwise enter its body (Amphibiaweb, 2018). Other studies on amphibians which are able to tolerate salt water environments have also noted similar changes in physiology.

Figure 1. Crab-eating frog (Fejervarya cancrivora) from southeast Asia which can tolerate daily influxes of seawater. By Bernard DUPONT from FRANCE – Crab-eating Frog (Fejervarya cancrivora), CC BY-SA 2.0, https://commons.wikimedia.org/w/index.php?curid=40168133

The exact mechanisms behind the evolution of salt tolerance in amphibians is still under debate but may be due to a number of genetic variables. All amphibians have highly permeable skin, which is prone to desiccation. Most amphibians are efficient at water regulation through their skin and retaining salts through effective transport of sodium and chlorine ions. Therefore, there may be a genetic predisposition to adapting to salt water environments through altering the physiology of ion transfer across the skin. Several amphibian species which are tolerant of highly arid conditions are also able to resist salt environments. The green toad (Bufotes viridis) of Central Europe is able to tolerate extremely high temperatures of 40°C and withstand up to 50% body water loss (Amphibiaweb, 2018) (Figure 2). It aestivates (hibernates) during the hottest summer months and has the same adaptations to avoiding water loss as species which inhabit saline environments, i.e. accumulation of urea in body tissues. The same occurs in the tiger salamander (Ambystoma tigrinum) of North America which uses a similar physiological mechanism to inhabit saline habitats. Some biologists have therefore proposed that the ability to tolerate saline environments has evolved through a pre-adaptation to living in arid environments. However, this does not appear to be the case for all amphibians inhabiting salt water. The natterjack toad (Epidalea calamita) is known to inhabit both freshwater and brackish habitats both in the UK and across northern Europe (Beebee, 1985; Gomez-Mestre & Tejedo, 2005) (Figure 3). Through genetic studies Gomez-Mestre & Tejedo (2005) found no support for this hypothesis and suggested that salt and aridity tolerance has evolved independently. They conclude that the mechanisms for resisting salt are fundamentally different in embryos and larvae compared to juveniles and adults. Therefore, amphibians which are able to resist salt whilst in the embryonic or larval stages require unique physiology which has not evolved in arid tolerant species (Gomez-Mestre & Tejedo, 2005). Other theories for the evolution of salt tolerance in amphibians are selection pressures from freshwater predators, forcing amphibians into new habitats or diversification of amphibians into new habitats to allow them to utilise unique prey (Hopkins & Brodie, 2015).

Figure 2. The green toad (Bufotes viridis) of Central Europe is able to tolerate extremely high temperatures using a similar physiological mechanism as species inhabiting salt water. By Umberto Salvagnin – originally posted to Flickr as European Green Toad, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=11010130

Figure 3. Typical coastal habitat of the natterjack toad. Breeding ponds in certain areas can easily become saline. As a result, populations can adapt to living in salt water at low concentrations. Inset: male natterjack toad (Epidalea calamita). Natterjack toad habitat: By Gary Rogers, CC BY-SA 2.0, https://commons.wikimedia.org/w/index.php?curid=13571929 Inset image: By Thomas Brown – Natterjack Toad (Epidalea calamita). Uploaded by mgiganteus, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=27451130.

In Australia, many amphibian species are threatened with a range of anthropogenic factors including habitat loss, fragmentation and pollution. Salinization of freshwater habitats has been cited as being one of the factors in causing declines of Australian amphibian species but evidence is lacking (Kearney et al., 2012).  In experimental studies, Kearney et al. (2012) examined the effects of elevated salinity on growth, metamorphosis and survival of the tadpoles of three species of native Australian frog. Kearney et al. (2012) found that tadpoles of the green and golden bell frog (Litoria aurea) raised in salt conditions had higher survival than those in freshwater. In contrast, a number of studies on other anuran larvae have found that larvae suffered abnormalities when raised in saline conditions. The authors propose that L. aurea has adapted to salt water by rapidly increasing development when in these conditions to avoid the potentially harmful effects of salt. However, other amphibians native to Australia such as the spotted grass frog (Limnodynastes tasmaniensis) and the painted burrowing frog (Neobatrachus sudelli) are unable to exhibit such plasticity in development and consequently are negatively affected by increases in salt water in the environment (Kearney et al., 2012).

Overall, it appears that an increasing number of amphibian species are being recognised as being able to tolerate salt water in the natural environment. However, species vary hugely in their tolerance of salt water habitats and our knowledge of the degree to which amphibians have adapted to salt water is poorly understood, especially those in human dominated habitats. Further in-depth investigations are required to increase our understanding of the mechanisms behind the evolution of salt tolerance in amphibians and their response in a potentially changing environment.

 

References 

AmphibiaWeb (2018) http://amphibiaweb.org. University of California, Berkeley, CA, USA. Accessed 11 Sep 2018.

Beebee, T.J.C. (1985) Salt tolerances of natterjack toad (Bufo calamita) eggs and larvae from coastal and inland populations in Britain. Herpetological Journal, 1: 14-16.

Gomez-Mestre, I. & Tejedo, M. (2005) Adaptation or exaptation? An experimental test of hypotheses on the origin of salinity tolerance in Bufo calamita. Journal of Evolutionary Biology, 18: 847–855.

Hopkins, G.R. & Brodie Jr, E.D. (2015) Occurrence of amphibians in saline habitats: a review and evolutionary perspective. Herpetological Monographs, 29: 1-27.

Hua, J. & Pierce, B.A. (2013) Lethal and sublethal effects of salinity on three common Texas amphibians. Copeia, 2013 (3): 562-566.

Kearney, B.D., Bryrne, P.G. & Reina, R.D. (2012) Larval tolerance to salinity in three species of Australian anuran: an indication of saline specialisation in Litoria aurea. PLOS ONE, 7 (8): e43427.

Ren, Z., Zhu, B., Ma, E., Wen, J., Tu, T., Cao, Y., Hasegawa, M. & Zhong, W. (2015). Complete nucleotide sequence and gene arrangement of the mitochondrial genome of the crab-eating frog Fejervarya cancrivora and evolutionary implications. Gene, 441: 148-155.