Tiger Keelback

Image of Rhabdophis tigrinus including insets of feed behaviour and chemical structure of  commonly sequestered toxin.

The primary goal for most organisms is to reproduce. However passing on genes to the next generation is pointless if they don’t survive the onslaught of predators in their new world. Animals that are pregnant and support the development of young internally also become susceptible to predation due to a greater size and changed behavior, eg sun baking for longer (Mori & Burghardt. 2001).
The Tiger Keelback (Rhabdophis tigrinus) along with others of the same genus have developed an unusual strategy to enhance survive rates of both it and its young by having the best of both worlds being venomous and poisonous (for an explanation see Venomous Anurans ;Hutchinson  et al. 2007).
The Keelbacks (Rhabdophis genus) of Asia have developed a row of paired glands along the back of the neck. These glands sit just below the surface and release their toxin if pressure is applied. The poison is variable as it is sequestered by the snake from toads which they feed on (Hutchinson et al. 2007).

These snakes are oviparous meaning they produce eggs. It has been shown that pregnant snakes with ready access to the toxin producing toads in their diet consume a higher proportion of these toads.This leads to an increase in poison availability which provides two benefits;

-A female often left more susceptible to predation now has a better defensive capability.

-The female is also enabled to pass on the poison to the young, which can in turn use them for defence. This gives them a greater chance for survival after birth when they are most vulnerable.

The Keelbacks also biosynthesise (as apposed to sequester) as venom which is delivered through the rear mouth fangs.The venom is produced in the Duvernoy's gland and the most significant effects of envenomation include coagulopathy and renal failure.


Reference

Hutchinson, D.A., Mori, A., Savitzky, A.H., Burghardt, G.M., Wu, X., Meinwald, J. and Schroeder, F.C., 2007."Dietary Sequestration of Defensive Steroids in Nuchal Glands of the Asian Snake Rhabdophis tigrinus." Proceedings of the National Academy of Sciences, Vol:104 No:7, pp.2265-2270.

Mori, A. and Burghardt, G.M., 2001."Temperature Effects on Anti‐Predator Behaviour in Rhabdophis tigrinus, a Snake with Toxic Nuchal Glands." Ethology, Vol;107, No:9, pp.795-811.

Image

http://modernsteroid.blogspot.com.au/2016/01/sequestration-of-defensive.html

Pseudophryne Toxins

Pseudophryne covacevichae

When most people think of poisonous frogs, images of brightly coloured frogs of the South American jungles spring to mind. Whilst these attractive frogs do take all the fame, Australia's Pseudophryne genus despite being less toxic, does contain equally charming and brightly coloured poisonous frogs.

Like the dart frogs of South America the many species in the Pseudophryne genus exhibit bright warning colours as a predator deterrent, a trait know as aposematism. In the case of these Toadlets (Pseudophryne genus) these colours alert the prey to the poisons excreted on the skin (Santos et al. 2003).

Two main toxins have been found on the skin of Pseudophyrne frogs. Pseudophrynamines (PSs) and Pumiliotoxins (PTX) (Smith et al. 2002). Pumiliotoxins are found on the skin of many frog around the world most notably the Phyllobates (South American Poison Dart Frogs) and the Mantilla genus of Madagascar (Santos et al. 2003). The toxin is obtained by the frogs by consuming specific invertebrates in particular beetles. Other animals including some birds have been known to also acquire this toxin from beetles (Daly et al. 2002).

Pseudophryne corroboree

The PSs are unique to the Pseudophryne genus. Studies comparing the toxins present on the skin of wild caught specimens and their captive bred young has shown that the toxin is biosynthesised by the frog instead of being sequestered. Captive animals had a controlled diet constraining any toxin excretions to those that are biosynthesised (Smith et al. 2002). PTXs are preferentially excreted by the frogs probably because sequestering the toxin may be metabolically less taxing, preventing the need to expend energy producing PSs toxin (Smith et al. 2002). 


Refernces

Daly, J.W., Kaneko, T., Wilham, J., Garraffo, H.M., Spande, T.F., Espinosa, A. and Donnelly, M.A. (2002). “Bioactive alkaloids of frog skin: combinatorial bioprospecting reveals that pumiliotoxins have an arthropod source.”, Proceedings of the National Academy of Sciences, Vol: 99, No: 22, pp: 13996-14001.

Santos, J. C., Coloma, L. A., & Cannatella, D. C. (2003). Multiple, recurring origins of aposematism and diet specialization in poison frogs. Proceedings of the National Academy of Sciences of the United States of America, Vol: 100, No: 22, pp: 12792–12797.

Smith, B.P., Tyler, M.J., Kaneko, T., Garraffo, H.M., Spande, T.F. and Daly, J.W.(2002). Evidence for biosynthesis of pseudophrynamine alkaloids by an Australian myobatrachid frog (Pseudophryne) and for sequestration of dietary pumiliotoxins.”, Journal of natural products, Vol: 65, No: 4, pp: 439-447.

Images

P.covacevichae by Nick Weigner

P.corroboree http://www.australiangeographic.com.au/blogs/australian-endangered-species/2014/05/endangered-southern-corroboree-frog ,22/3/16.

Duvernoy's Gland

 

The three largest snake families that contain “Venomous” species are;

-Elapidae (containing species such as Cobras, Mambas and Taipans. Sea snakes are also tentatively placed in this family; Cogger 2014)

-Viperidae (including Rattlesnakes, Puff adders and Saw-scaled vipers)

-Colubridae (including Rat snakes, Tree snakes and Garter snakes)

Pictures: Top right: Ringed Brownsnake (Elapidae)
Opposite: Bornean Keeled Green Pit Viper (Viperidae)
Below: Brown Tree Snake (Colubridae)

The Colubridae family actually consists of several independent clades awaiting further reclassification (Pyron et al. 2011).This large group consisting of nearly 2000 species includes species of opistoglyphs (rear fanged) and aglyphs (no fangs) (Kardong et al. 2009). Not all species produce toxins and of those that do, most are not considered medically significant. There are some notable exceptions including the Dispholidus(Boomslang), Philodryas, Rhabdophis (Asian Keelbacks) and Thelotornis(Twig snake) genra. Unlike the members of Elapidae and Viperidae, the opistoglyphs don’t possess true “venom glands”, but a pair of Duvernoy’s glands(Figure 1.). These snake deliver their toxins, produced in this gland, through a grooved tooth in the back of the mouth (Kardong 2002).

Figure 1. Oral glands of Colubrids.(Note, not all species have each gland; Kardong 2002)

Opistoglyphs are often described as having an “inefficient delivery apparatus”, referring to the often symptomless bites recorded in humans, however, the Duvernoy’s gland is an homologous structure (sharing common structure but a different function) with venom glands (Kardong 2002). The toxins excreted by this oral gland likely serve other purposes as very few species toxins display rapid prey death (the foremost biological purpose of elapid and viper venom),other roles of the excretions may play include; defence, post-strike prey tracking, digestion, lubrication and immobilisation of prey (Kardong 2002).

The Duvernoy’s gland is different to “true venom glands” in that it doesn’t contain a large storage area for the toxin. “True Venom glands” expel the venom through a series of ducts under pressure. This allows the immediate transfer of venom into the target upon penetration (Kardong et al. 2009). The Duvernoy’s glands excretions however use capillary action to be moved to the fang, therefore require a prolonged bite or chewing action to envenomate (Kardong et al. 2009).

References

Cogger H.G., 2014, "Reptiles and amphibians of Australia, 7th edn, CSIRO Publishing, Collingwood, VIC.

Kardong V.K., 2002, Colubrid Snakes and Duvernoy’s “Venom” Glands.", Journal of Toxicology: Toxin Reviews, Vol: 21 No: 1-2, pp: 1-19.

Kardong, V.K., Weinstein, S.A., Smith, T.L. and Mackessy, S.P., 2009. "Reptile venom glands: form, function, and future.", Handbook of venoms and toxins of reptiles, pp:65-91.

Pyron R.A, Burbrink F.T., Colli G.R., De Oca A.N.M., Vitt L.J., Kuczynski C.A. and Wiens J.J.,  2011, "The phylogeny of advanced snakes (Colubroidea), with discovery of a new subfamily and comparison of support methods for likelhood trees.", Molecular Phylogenetics and Evolution, Vol: 58, No: 2, pp: 329–342.
Images by Nick Weigner

                

Headbutting Anurans

Something not out of line with a B-grade SCI-FI film has been discovered in Brazil, the venomous ability of the two previously described species has been unearthed.

Until the discovery of Aparasphenodon brunoi’s and Corythomantis greeningi’s unique venom delivery system there were no definitive examples of venomous frogs known to science, despite a plethora of poisonous species (Jared et al. 2015).  This unique system combines both the toxin and skull structure of the two species to form the venomous apparatus. Despite often being confused venoms and poisons are distinct from each other.Venoms must be produced in specialised tissues or glands and must be delivered through a mechanism such as stingers, fangs or spines (Meier & White. 1995).

Both C. greeningi and A. brunoi possess highly specialised glands in the skin, these folded structures produce a strong toxin. When threatened, spines in the head and lip area protrude through the epidermis and are coated in the venom upon exposure (Figure 1.).It is thought that the venom is used as a defense during phragmotic behaviour (When an animal uses its own body as a barrier whilst retreating in a burrow) (Jared et al. 2005).

Figure 1. Images of two venomous frogs, Aparasphenodon brunoi (A, C and E) and Corythomantis greeningi (B, D and F).(Jared et al. 2015).

The frogs have evolved a greater flexibility in the neck, allowing it to pierce the attacker with a side to side sweeping motion of the head.Whilst collecting specimens for toxin analysis, a researcher was jabbed in the hand by a C.greeningi.The envenomation resulted in intensive pain in the limb for 5 hours (Jared et al. 2015).

The lethal dose 50 (LD50 is a scale commonly used to compare toxicity, here all are intraperitoneally injected unless stated otherwise) of the toxin from the head of A. brunoi was found to be 3.12μg and 51.94 μg from C. greeningi (Jared et al. 2015).For comparison Some LD50’s of other amphibians and reptiles is included (Table 1.). Despite C. greeningi being less toxic, it was found to produce a greater volume of the toxin (Jared et al. 2015). 

Table 1. List of LD50 ratings for reptiles and amphibians.

Scientific Name	Common Name	LD50 (μg/20g)
Bitis atriens	Puff Adder	17.4
Naja haje	Egyptian Cobra	4.1
Oxyuranus microlepidotus	Inland Taipan	0.025
Phyllobates aurotaenia	Colombian Arrow Poison Frog	0.002 (subcutaneous)

Adapted from (Daly & Witkop. 1971), (Oukkache et al. 2014) & (Meier & White. 1995).

It is likely that more research of Hylidae frog species will likely expose their venomous nature (Jared et al. 2015).

References

Jared, C., Antoniazzi, M. M., Navas, C. A., Katchburian, E., Freymüller, E., Tambourgi, D. V., and Rodrigues, M. T. (2005), ''Head co-ossification, phragmosis and defence in the casque-headed tree frog Corythomantis greeningi.'' Journal of Zoology, Vol: 265, No: 1, pp: 1-8.

Jared, C., Mailho-Fontana, P.L., Antoniazzi, M.M., Mendes, V.A., Barbaro, K.C., Rodrigues, M.T. & Brodie, J., Edmund D. (2015), "Venomous Frogs Use Heads as Weapons", Current biology, Vol: 25, No:16, pp: 2166-2170.

Oukkache, N., Jaoudi, R.E., Ghalim, N., Chgoury, F., Bouhaouala, B., Mdaghri, N.E. and Sabatier, J.M. (2014), “Evaluation of the lethal potency of scorpion and snake venoms and comparison between intraperitoneal and intravenous injection routes.” Toxins, Vol: 6, No: 6, pp.1873-1881.

Daly, J. and Witkop, B. (1971), “Batrachotoxin, an extremely active cardio-and neurotoxin from the Colombian arrow poison frog Phyllobates aurotaenia”, Clinical toxicology, Vol: 4, No: 3, pp: 331-342.

Meier, J. & White, J. (1995), “Handbook of clinical toxicology of animal venoms and poisons.” CRC Press, Boca Raton.