Venomous animals are able to inject their toxins into another organism while poisons are ingested, inhaled and absorbed. The ability to deliver venom into another animal has distinct evolutionary advantages such as in defence, prey capture and even sexual selection.
Amphibians secrete a wide variety of compounds from their skin glands. Generally, mucous glands help provide a moist coating on their skin to facilitate cutaneous respiration while granular glands secrete substances that amphibians use as a chemical defence against predators (eg. toxic and noxious compounds) and microorganisms (eg. antimicrobial peptides). This is sometimes displayed with bright, aposematic colouration (Duellman and Trueb 1996).
Many different toxic secretions have been found in amphibian skin which act in numerous ways to disrupt the physiology of potential enemies (Daly et al. 2005, Savitzky et al. 2012). For example, newts in the genera Taricha and Notophthalmus synthesise tetrodotoxins in high concentrations and are co-evolving in an evolutionary arms race with Thamnophis garter snakes where toxicity is a selective pressure (Brodie et al. 2005, Mailho-Fontana et al. 2019, Hague et al. 2020). Fire salamanders secrete samandarin alkaloids through their paratoid glands which can cause convulsions, hypertension and respiratory paralysis in potential predators. Frogs can sequester an array of toxins such as the potent neurotoxin Zetekitoxin in the Panamanian golden frog and the batrachotoxins in highly toxic Phyllobates poison frogs (Duellman and Trueb 1996). The bright yellow Australian corroboree frogs of the genus Pseudophryne synthesise their own pseudophrynamine toxins as well as sequestering pumiliotoxins from their environment to deter predators (Smith et al. 2002). Fossorial caecilians are also known to produce defensive toxins with poison glands being discovered on the tails of Siphonops annulatus ringed caecilians as a possible defence against predators as they burrow into the soil (Jared et al. 2019).
Venom, however, is rare in amphibians with only a few species possessing a system to deliver their toxins into another organism.
The Iberian ribbed newt (Pleurodeles waltii) is a fascinating salamandrid from Spain and Portugal with an incredible defence behaviour. They have the ability to use their ribs to protrude through the skin to envenomate and ‘sting’ a predator. When distressed these newts can flatten themselves or arch their backs in an antipredator posture. They will then rotate their ribs 65° forwards which increases the angle of the spine to allow its ribs to penetrate through the skin wall and project as ‘spines’. This allows them to coat their ribs with a viscous, milky substance from their skin tubercles and inject it into the mouth of the predator making them unpalatable and allowing them to escape (Heiss et al. 2010). This defence mechanism doesn’t cause any permanent damage as antimicrobial peptides are able to prevent infection in the lacerated skin and their tissue is able to regenerate remarkably quickly. The tip of the ribs are also covered with a periosteum layer which is also thought to prevent microbial infection when the ribs puncture the skin.
The Echinotriton genus of crocodile newts are a sister group to the sharp ribbed newt and are also able to use their ribs to pierce their body wall when attacked by predators (Brodie et al. 1984).
Brazil is home to two tree frogs which have incredible cranial morphologies and venomous defensive mechanisms. The Greening’s frog Corythomantis greeningi live in the semi-arid caatinga ecosystem of Eastern Brazil and conceal themselves in tree hollows and rock crevices to stay moist and evade predators. Bruno’s casque headed frog Aparasphenodon brunoi is another endemic Brazilian hylid with a fascinating skull morphology. They inhabit lowland tropical forests and shrubland and like C. greeningi, will hide in water-filled tree or bamboo hollows and bromeliad phytotelmata.
Both of these peculiar frogs have flattened, casqued heads with their skin co-ossified with underlying bones. They use their heads to aid in a behaviour known as phragmosis (Jared et al. 2006, Blotto et al. 2020). Phragmosis occurs when an animal enters a hole and blocks the entrance with their head to defend themselves from predators. In the lab, these frogs will exhibit this behaviour by entering test tubes backwards and blocking themselves off with their casqued heads. This phragmotic behaviour along with their venomous spines means these frogs have never been observed being predated in the wild. It is also thought that cranial ossification and phragmosis also indirectly reduces water loss and prevents desiccation by creating a humid microclimate within their tree holes (Jared et al. 2006).
These frogs have bony spines, ridges and protrusions on their skulls in areas with high concentrations of granular and mucous glands which secrete a potent venom. Their mobile heads allow the frogs to deliver the venom into animals via head-butting and jabbing with their spines which are coated with the toxic secretions as the spines pierce their venom glands. This provides a highly effective chemical defensive mechanism as the toxin coupled with the wound caused by the head spines ensures would-be predators have a bad time when they attempt to ingest these frogs (Jared et al. 2015). Their cutaneous secretions include alkaloids and steroids which can induce oedema and intense pain in predators (Mendes et al. 2016). The venom contains both proteolytic and fibrinolytic agents as well as hyaluronidase which aids the toxins in diffusing around their enemies’ bodies (Jared et al. 2015).
The venom of Greening’s frog is thought to be twice as lethal as fer-de-lance snakes of the genus Bothrops while Bruno’s casque headed frog secretes a venom 25x as toxic as these notorious neotropical vipers with an LD50 of 94.8µg in mice. A single gram of A. brunoi venom could kill 300,000 mice or 80 humans (Jared et al. 2015)! However, A. brunoi has smaller spines and granular glands than C. greening and so may not be able to inject as much venom when defending against a predator.
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Top: Bruno’s casque headed frog (Aparasphenodon brunoi) L: Renato Augusto Martins R: Carlos Jared
Bottom: Greening’s frogs (Corythomantis greeningi) Carlos Jared
There are many other frogs with complex cranial morphology including immense variation in skull shape and hyperossification which often relate to their interesting and diverse ecologies (Paluh et al. 2020). With an array of other anurans having mineralised and spiny skulls, it is possible that there are a few more venomous frogs which are waiting to be studied. Contenders include the fascinating shovelhead tree frog Triprion, the crowned tree frog Anotheca spinosa and Polypedates ranwellai (Jared et al. 2015).
In a recent paper by Mailho-Fontana et al. 2020, a new set of specialised dental glands were discovered in Brazilian ringed caecilians (Siphonops annulatus) that may produce venomous enzymes – but further research is needed to confirm this. These enzymes were demonstrated to have gelatinolytic, caseinolytic and fibrinogenolytic properties. This incredible discovery may allow researchers to rethink the evolution of venom in vertebrates (since it could have evolved independently in both amphibians and reptiles) and inspire new studies about caecilian toxinology.
Through histological and biochemical analysis of saliva samples, researchers found A2 phospholipase enzymes which could mean that some fossorial caecilians inject venomous saliva via these dental glands into their earthworm prey in order to incapacitate and digest them. These enzymes are found in many other venomous creatures such as scorpions, snakes and insects. It is also worth noting that many venoms have originated as saliva such as in komodo dragons, shrews, bats and slow lorises making the prospect of venomous gymnophiones very exciting!
Other caecilians including the basal genus Rhinatrema showed similar dental glands to the ringed caecilians which could suggest that caecilians evolved to inject oral venom early on in their evolution (Jared et al. 2020).
Written by Xavier Mahele
References
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