Spitting Snakes

Naja sumatrana spraying venom.

Some animals have developed the ability to spray chemicals on perceived attackers as a form of defence. While many of the species abilities are very efficient such as the skunk and bombardier beetle (which sprays a liquid at 100°C; Eisner, 1958), the capability of some snake species to accurately spray venom up to 2m takes the evolutionary cake (Young et al, 2004).

Some species of Naja (cobras) and the Rinkhals of Hemachatus genus have separately evolved unique fang structure to produce an accurate ranged defence. Some vipers, primarily the Mangshan Viper (Zhaoermia mangshanensis) of China have been reported to “spit” venom. These cases are put down to a larger venom yield and hissing action, they have not been shown to employ “intentionally aimed spitting” unlike the Spitting Cobras and Rinkhals (Young et al, 2004).Spitting is only used as a defence mechanism and not for hunting. These snakes still use their fangs normally to deliver a fatal bite whilst hunting (Wüster & Thorpe, 1992).

Fig 1. Fang structure of Spitting and Non-spitting cobras. Note the size and shape of the exit orifice
(Wüster & Thorpe, 1992).

Venom is produced as per normal in the venom glands but the ability comes from the small elliptical exit point at the terminal end of the fang (Fig 1.). The venom is pushed into the fang under great pressure, formed by the contraction of muscles around the venom gland (Young et al, 2004; Wüster & Thorpe, 1992).The head is tilted back and often with a shaking motion of the head the venom is pushed out the smaller rounder exit hole (Wüster & Thorpe, 1992).

 The venoms from most of the spitting Naja contain Neurotoxins and Cytotoxins (Wüster & Thorpe, 1992).

References

Eisner, T. (1958). "The protective role of the spray mechanism of the bombardier beetle, Brachynus ballistarius Lec." Journal of Insect Physiology,Vol:2, No:3, pp.215-220.

Wüster, W. and Thorpe, R.S. (1992). "Dentitional phenomena in cobras revisited: spitting and fang structure in the Asiatic species of Naja (Serpentes: Elapidae)." Herpetologica, Vol: 48, No: 4,  pp.424-434.

Young, B.A., Dunlap, K., Koenig, K. and Singer, M. (2004). "The buccal buckle: the functional morphology of venom spitting in cobras." Journal of Experimental Biology, Vol: 207, No: 20, pp.3483-3494.

Image

Naja sumatrana, Nick Weigner

Ontogenetic Shifts in Venom Composition

When venomous snakes are born they already possess the apparatus and venom to deliver a toxic bite. It is an essential ability in order for the young to feed and defend themselves immediately as this is when they are most susceptible to predation. As growth from young to old is closely linked to preferential prey it is likely a that this change corresponds to more accurately target its prey at that age (Gibbs et al. 2011).

Venoms are an efficient and targeted tool to aid in feeding, this specialisation of venoms leads to the diverse forms we see today. As the diet of the snakes change so must its venom to ensure it is effective. Investigations into the composition shifts in venoms over a snakes lifetime has been carried out on many species particularly the Crotaline snakes (rattle snakes and relatives). Many of these snakes feed on frogs or small lizards when young. In the case of Crotalus oreganus a higher percentage of myotoxins are found in the venom of older snakes that have progressed to feeding on small mammals.The younger specimens showed more neurotoxic venom while still at a similar overall toxicity to the adults (Mackessy et al. 2003). 

This adaptation can cause greater difficulty in producing an effective treatment for bites. The most effective antivenom, polyvalent antivenom, is produced to target specific component of the venom.Should the venom be varied due to ontogenetic shifts a bite from a juvenile may not be effectively treated by an antivenom produced for adults venom. Natural variation in populations and between populations in different localities and conditions also presents the same concern (McCue. 2006).


References 

Mackessy, S.P., Williams, K. and Ashton, K.G., (2003). “Ontogenetic variation in venom composition and diet of Crotalus oreganus concolor: a case of venom paedomorphosis?.” Copeia, Vol: 2003 No: 4, pp.769-782.
McCue M.D. (2006). “Cost of Producing Venom in Three North American Pitviper Species.” Copeia, Vol: 2006, No: 4, pp. 818-825.
Gibbs, H.L., Sanz, L., Chiucchi, J.E., Farrell, T.M. and Calvete, J.J., (2011). “Proteomic analysis of ontogenetic and diet-related changes in venom composition of juvenile and adult Dusky Pigmy rattlesnakes (Sistrurus miliarius barbouri).” Journal of proteomics, Vol: 74 No:10, pp.2169-2179.

Image 
http://www.stuartdahnephotography.com/keyword/crotalus%20tigris%20the%20tiger%20rattler/i-xttv4cn accessed: 13/4/16

Toxicofera

Phylogenetic of the venomous reptiles is a broadly debated. Many different classifications previously presented used morphological features to define clades. The Toxicofera theory was born after genetic testing of the Squamata order showed serious discrepancies between this morphological grouping and their genetic relationships (Vidal & Hedges. 2005). Further testing of this group gave greater support for the new classification of toxicofera based on the genetic similarities, primarily the presence of venom coding genes within three of the Squamata groupings (Fry et al. 2009a). 

The extant taxa forming this clade includes all Ophidia (snakes), Anguimorpha (lizards including monitors) and the Iguanians (lizards including dragons, chameleons and of course iguanas). All the known venomous reptiles belong to this group, however some families in the group are not venomous. It is thought that the non-venomous members have simply lost the venom production ability to some degree (Fry et al. 2009a).

New evidence is constantly being uncovered to support this clade. A study on anguimorphs showed a toxin homologous with those of snakes and functional venom glands have been discovered in the jaw of Komodo dragons Varanus komodoensis dispelling the previously held belief that it was only bacteria that forms the toxic bite by these gargantuous lizard (Fry et al. 2010; Fry et al. 2009b).

The common ancestor to all toxicoferan had a host of core venom genes. Along with other toxin recruitment, these are the original genes which have diversified into the venoms we see in many species today (Fry et al. 2009a).

References 

Fry, BG, Winter, K, Norman, JA, Roelants, K, Nabuurs, RJA, et al. 2010, ‘Functional and structural diversification of the Anguimorpha lizard venom system’, Molecular & Cellular Proteomics, Vol: 9, No: 11, pp. 2369-2390.

Fry, BG, Vidal, N, van der Weerd, L, Kochva, E & Renjifo, C. 2009a, ‘Evolution and diversification of the Toxicofera reptile venom system’, Journal of Proteomics, Vol: 72, No: 2, pp. 127-136.

Fry, BG, Wroe, S, Teeuwisse, W, van Osch, MJP, Moreno, et al. 2009b, ‘A central role for venom in predation by Varanus komodoensis (Komodo Dragon) and the extinct giant Varanus (Megalania) priscus’, Proceedings of the National Academy of Sciences of the United States of America, Vol:106, No: 22, pp. 8969-8974.

Vidal, N & Hedges, SB. 2005, ‘The phylogeny of squamate reptiles (lizards, snakes, and amphsbaenians) inferred from nine nuclear protein-coding genes’, Comptes rendus – Biologies, Vol: 328, No: 10, pp. 1000-1008.

Image

http://www.bbc.com/earth/story/20160226-the-islands-where-dragons-are-real, accessed 5/4/2015 

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.