Tuesday 27 May 2014

Venom: A Summary

As we know, venom is a secretion of proteins and peptides that is produced by a specialised gland, and is injected into a target animal through the infliction of a wound (Fry et al., 2009). Venom is used in many families of animals to subdue prey, deter predators and outcompete other members of the species. The venom is delivered through many types of apparatuses, including fangs, stingers, barbs, spines, teeth and tentacles.

A Mozambique Spitting Cobra (Image 1).

Evidence of venom systems has been found in fossils of Sinornithosaurus (Gong, et al., 2010) and an ancestor of the shrews (Fox & Scott, 2005). These animals had grooves in there teeth that conducted venom from a gland to the tip of the tooth so that it could be injected into the prey. Gila Monsters and Bearded Lizards also have grooves in their teeth that conduct venom; however, their venom is used to defend themselves against predators, not to subdue prey (Beck, 1990). The venom of stonefish is also used entirely for defence. The venom is injected into the predator through dorsal spines when the stonefish is stood on or bitten. Stingrays also inject venom into their predators through a barb on their tail when they are stood on (Fenner, 2004). Other venomous animals use their venom as a defence, but it is primarily for killing prey.

Stonefish spines (Image 2).

Snakes and spiders use fangs to inject their prey with venom (Calvete, et al., 2009). The fangs have an opening on the dorsal side of the tip and venom flows from the gland to this opening to be injected into the prey. Scorpions have a similar structure on their tails (Politi, et al., 2012). Cone shells use barbs soaked in venom to kill their prey and to attack predators. The venom of some cone shells leads to respiratory paralysis and death (Fenner, 2004). Box jellyfish hunt by using nematocysts that discharge venom when they come into contact with an animal. The death of prey is instantaneous to avoid harm coming to the jellyfish (Sutherland & Nolch, 2000). Blue-ringed octopuses bite their prey and inject them with a salivary toxin that acts as venom (Fenner, 2004). Vampire bat venom isn't used to subdue prey, but it is used to make feeding easier by anaesthetising the area and thinning the blood (Ligabue-Braun, Verli, & Carlini, 2012). There is a struggle between venomous animals and their prey and predators to get the upper hand. The prey and predators of venomous animals become more tolerant to venom, causing venomous animals to increase the potency of their venom (Casewell, et al., 2013). Another use of venom is for male-male competition during mating season.

A blue-ringed octopus (Image 3).

Male platypi fight for the right to mate with females during the breeding season. They use their barbs to puncture the other males and to inject them with a pain-inducing venom (Ligabue-Braun, Verli, & Carlini, 2012). Slow lorises also use venom for this reason, as well as deterring predators and killing external parasites (Nekaris, et al., 2013).

A platypus (Image 4).

Venom is used by us to create antivenom to treat venomous attacks and for medical purposes, such as the development of heart attack and stroke medication (Ligabue-Braun, Verli, & Carlini, 2012). Hopefully, by studying venomous animals and their venom, more uses of venom can be established (Calvete, et al., 2009). 

References

Beck, D. (1990). Ecology and Behavior of the Gila Monster in Southwestern Utah. Journal of Herpetology , 54-68.

Calvete, J. J., Sanz, L., Angulo, Y., Lomonte, B., & Gutierrez, J. M. (2009). Venoms, venomics, antivenomics. FEBS Letters , 583 (11), 1736-1743.

Casewell, N. R., Wüster, W., Vonk, F. J., Harrison, R. A. & Fry, B. G. 2013. Complex cocktails: the evolutionary novelty of venoms. Trends in Ecology and Evolution 28: 219-229.
Fenner, P. (2004). Venomous marine animals. South Pacific Underwater Medicine Society Journal, 34(4), 196-202.

Fox, R., & Scott, C. (2005). First evidence of a venom delivery apparatus in extinct mammals. Nature, 435, 1091-1093.

Fry, B.G., Roelants, K., Champagne, D.E., Scheib, H., Tyndall, J.D., King, G.F., Nevalainen, T.J., Norman, J.A., Lewis, R.J., Norton, R.S., Renjifo, C., de la Vega, R.C., 2009. The toxicogenomic multiverse: convergent recruitment of proteins into animal venoms. Annu. Rev. Genomics Hum. G, R., & Scott, C. (2005). First evidence of a venom delivery apparatus in extinct mammals. Nature, 435, 1091-1093.

Gong, E., Martin, L., Burnham, D., & Falk, A. (2010). The birdlike raptor Sinornithosaurus was venomous. PNAS , 107 (2), 766-768.

Ligabue-Braun, R., Verli, H., & Carlini, C. (2012). Venomous mammals: A review. Toxicon , 59, 680-695.

Nekaris, K., et al. (2013). Mad, bad and dangerous to know: the biochemistry, ecology and evolution of slow loris venom. Journal of Venomous Animals and Toxins including Tropical Diseases , 19 (21).
Politi, Y., Priewasser, M., Pippel, E., Zaslansky, P., Hartmann, J., Siegal, S., et al. (2012). A Spider´s Fang: How to Design an Injection Needle Using Chitin-Based Composite Material. Advanced Functional Materials , 22, 2519-2528.

Sutherland, S., & Nolch, G. (2000). Box Jellyfish. Australasian Science , 21 (8), 39-41.

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Tuesday 20 May 2014

Venomous Animals and their Prey

There is a constant tug-of-war between venomous animals and their prey and predators. Most venoms are used for prey capture, and in some species, the venom is targetted towards a specific species of prey. This is seen in a wide range of animals, including spiders, scorpions, cone snails, and snakes. For example, there is a correlation between the diet of Malayan pit vipers (Calloselasma rhodostoma) and their venom. The prey and predators have a evolved a resistance to the venom of these species. The venom resistance in prey and the evolution of novel venom composition exert reciprocal selective pressures on each other, causing both venomous animals and their prey to race to gain the upper hand (Casewell, et al., 2013).

A Malayan pit viper (Image 1).

Venom synthesis appears to carry an appreciable metabolic cost. Some animals display behavioural adaptation to optimise venom expenditure. Rattlesnakes, for example, change the amount of venom that they inject into their prey depending on the size of their prey. Scorpions and spiders inject more venom when the intensity and/or duration of prey movement is increased, or when dangerous prey is encountered (Casewell, et al., 2013).

A rattlesnake (Image 2).

Defence is a common secondary function of venom (in the animals that don't use venom primarily for defence) and animals have defence-specific morphological and behavioural adaptations. There is currently little evidence for defence-related selective pressures on venom composition. Some snakes have evolved to eat undefended prey (such as eggs) or use constriction as prey subjugation, resulting in the disappearance of the venom apparatus and degeneration of toxin genes. This suggests that prey capture is the principal selective force acting on venom and the retention of the venom apparatus (Casewell, et al., 2013).

A snake eating an egg (Image 3).

There is an 'overkill' hypothesis that selection for venom potency is unlikely because the amount of venom injected into prey is often greater than 100 times the lethal dose required. This hypothesis overlooks the fact that laboratory animals might not reflect the response of natural prey to venom. The target-specific venom could be an explanation of why venom is so excessively lethal to labratory animals. Specific venoms have evolved among natural prey and predators, many of which have also evolved a natural resistance to the venom. A variation in venom composition leads to differential venom effectiveness against different prey; therefore, laboratory animals would not have this resistance and the venom would seem to be excessively potent (Casewell, et al., 2013).

References

Casewell, N. R., Wüster, W., Vonk, F. J., Harrison, R. A. & Fry, B. G. 2013. Complex cocktails: the evolutionary novelty of venoms. Trends in Ecology and Evolution 28: 219-229.

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