Dangerous Discussions: Part Two
By Kevin Scott
In Part I of Dangerous Discussions I gave an overview of the definitions of and differences between poisons, toxins and venom. In Part II, I will go into greater detail in describing what toxins and venoms are and where they occur in nature. Of course, it would be impossible to talk about more than a handful of occurrences, so I decided to choose those that I find most interesting.
Toxins are organic molecules that are produced via biological pathways and are often used as defense mechanisms by animals. As mentioned in Part I, amphibians secrete substances that are toxic to bacteria and fungi, as the external part of their immune system. Some amphibians also secrete substances that are toxic to predators in order to prevent becoming prey. Tomato frogs and toads, for example, secrete thick milky substances that serve as irritants to potential predators. Arrow Frogs, as discussed in Part I, also secrete toxic compounds, these often being far more toxic than any produced by other amphibians.
The most potent of these toxins are steroid alkaloids, but nearly all of them are neurotoxic. Batrachotoxin is the most toxic of these, but other common compounds include epibatidine, histrionicotoxin and pumiliotoxin. If you are familiar with the arrow frogs, you can see the names of a few species in the names of these molecules. Batrachotoxin targets sodium ion channels, while epibatidine and histrionicotoxin target nicotinic acetylcholine receptors, and pumiliotoxin targets calcium ion channels.
Some, but not all, of these substances are actually produced by the frogs themselves. Many of the more toxic compounds, however, are actually produced downstream in the food chain. Pyrrolidines like epibatadine, and piperidines that are present in species found in the genera Oophaga and Ranitomeya, and Ameerega, Dendrobates and Ranitomeya (Litters et al 2007), respectively, come from the ants that they eat.
Being that invertebrates and plants are the sources of these toxins, it is not surprising that it is not only the arrow frogs that possess them. Mantellas, the Arrow Frog's Malagasy counterpart in terms of parallel evolution, also possess some of these toxins. One advantage of the fact that these frogs get this defense from their food items is that they are not nearly as toxic in captivity as they are in the wild.
The telson of a scorpion contains the venom gland and delivery system in one specialized evolutionary development.
Another frog that we commonly see in captivity, the Fire Walking Frog (Phrynomantis bifasciatus), also has toxins that can be used as a defense toward predators. This toxin's identity is not known, but wild caught specimens can cause a burning sensation on the skin of a human, and it is strong enough to cause cardiovascular arrest in other frogs.
The fire walking frog secretes a substance that can cause intense burning sensations in humans, and death in other amphibians.
There are many animals that produce venoms, including spiders, scorpions, marine invertebrates, fish, snakes, lizards and even mammals. Of these, a good handful can be found in the reptile industry.
Venoms are made up of mixtures of low-molecular-weight proteins, mucus, salts and organic compounds that include oligopeptides, nucleotides and amino acids (Colis 1990). This mixture can serve a variety of functions that include defense, prey submission and pre-digestion. Some of the types of venom are as they follow: neurotoxins cause neuromuscular paralysis that can result in immobilization and death; presynaptic neurotoxins block the release of the physiological transmitter acetylcholine, destroying the nerve terminal, and postsynaptic neurotoxins competitively inhibit binding of acetylcholine, preventing the transmission of nerve impulses across the synaptic gap; haemotoxins destroy red blood cells, and extreme cases can lead to renal failure; myotoxins damage muscles, especially respiratory muscles; cytotoxins destroy tissue, and these can aid in pre-digestion; nephrotoxins damage the kidneys (O'Shea 2005).
While the toxins that we have discussed in frogs are passively delivered, venom is delivered with an active delivery system. Special apocrine glands are connected to or in the vicinity of specialized hollow teeth or fangs, grooved teeth or a stinger (in the cases of the reptiles, tarantulas and scorpions that are common in the industry) that act as a penetration device that allows the venom to be administered.
The fang of this vine snake can be seen within the red patch of gums behind the eye.
The most advanced delivery systems utilize fangs as an application mechanism. These fangs are specialized hollow teeth, through which the venom is delivered. These are used by vipers (including rattlesnakes) and elapids (including cobras, sea-snakes and kraits). Vipers have long, movable fangs that can be used to alternately progress, "walking" a prey item down during feeding. When not in use, these fangs fold inward, allowing the mouth to close. Elapids are also front-fanged, but they generally have shorter, fixed fangs.
Only relatively few colubrids are venomous, but the ones that are have grooved teeth toward the back of the skull, which is known as being rear-fanged. These teeth are located below or behind the eye socket, and below a specialized salivary gland know as a Duvernoy's gland, which secretes a toxic saliva that is used in subduing prey (O'Shea 2005).
While they may outwardly appear similar, the fangs of a tarantula or centipede are actually not teeth at all. Rather, they are chelicerae. Chelicerae are pointed appendages that are found in all members of the subphylum Chelicerata, that are used for grasping food or for defense. In spiders and venomous myriapods the chelicerae are hollow, and are used to inject venom from the connected venom gland.
The chelicerae of tarantulas, spiders and centipedes can be quite large, and are used for grabbing and envenomating prey items, as well as for defense purposes.
Scorpions have a pretty unique venom delivery system known as a telson, or stinger. At the end of the tail, a specialized anatomical development contains both the venom gland and the sharp point used for injection.
Although there are many other animals that are capable of delivering venom and the systems with which venom is delivered are far too complex to discuss in any depth here, I hope that the topics discussed here were enlightening. Furthermore, I hope that the content was deep enough to hold the majority of the readersâ€™ attention, but straight forward enough so that no reader was excluded due to complicated writing.
O'Shea, Mark. 2005. Venomous Snakes of the World. Princeton: Princeton University Press.
Polis, Gary A. 1990. The Biology Of Scorpions. Stanford: Stanford University Press.
Litters, Stefan, Karl-Heinz Jungfer, Friedrich Wilhelm Henkel and Wolfgang Schmidt. 2007. Poison Frogs: Biology, Species and Captive Care. Frankfurt: Edition Chaimaira.