Dangerous Discussions: Part Two – November 2012

The Reptile Times

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

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 (Lötters 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.

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.

fire walking frog

The fire walking frog secretes a substance that can cause intense burning sensations in humans, and death in other amphibians.

Venoms

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.

vine snake

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.

fangs

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.

scorpion

The telson of a scorpion contains the venom gland and delivery system in one specialized evolutionary development.

Conclusions

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 readers’ 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.

Lötters, Stefan, Karl-Heinz Jungfer, Friedrich Wilhelm Henkel and Wolfgang Schmidt. 2007. Poison Frogs: Biology, Species and Captive Care. Frankfurt: Edition Chaimaira.

Dangerous Discussions: Part One – October 2012

The Reptile Times

DANGEROUS DISCUSSIONS

PART I

By Kevin Scott

Over the last couple of months, I’ve had the opportunity to discuss with various people the differences between poisons, toxins and venoms a surprising number of times. Having studied chemistry, carried out research in a biochemistry and molecular biology laboratory, and having worked in the reptile industry for close to a decade, I find the topic especially interesting when it pertains to herpetofauna.

Depending on whom you ask, the precise definitions for poison, toxin, and venom will differ slightly. There are, however, major differences between these terms, and often the terms are erroneously interchanged. The following is a brief discussion of these differences. Let’s start off by taking a look at and comparing definitions from The Oxford English Dictionary and Stedman’s Concise Medical Dictionary.

Oxford English Dictionary

Stedman’s Concise Medical Dictionary

Poison

A substance that, when introduced to or absorbed by a living organism causes death or injury. Any substance, either taken internally or applied externally, that is injurious to health or dangerous to life.

Toxin

An antigenic poison or venom of plant or animal origin. A noxious or poisonous substance that is formed or elaborated during the metabolism and growth of certain microorganisms and some higher plant and animal species.

Venom

Poisonous fluid secreted by animals such as snakes and scorpions and typically injected into prey or aggressors by biting or stinging. A poisonous fluid secreted by snakes, spiders, scorpions and other cold-blooded animals.

POISON

According to the way that we have defined these terms here, the word poison behaves as sort of an umbrella term for things that can harm biological systems. Poisons include substances that range from household cleaners and pesticides to large organic molecules found in frogs and protein complexes found in snake venom. While the small, brightly colored South American dart frogs are commonly called Poison Frogs, or Poison Arrow Frogs (family Dendrobatidae), it is generally considered incorrect to refer to a venomous snake as a poisonous snake (I will explain why in a moment), although this happens rather often.

TOXINS & VENOM

According to these definitions, a toxin is a type of poison that is produced through a biological pathway. Although this particular medical dictionary’s definition does not make it explicit, toxinologists generally agree that toxins must be taken into the body by absorption or consumption. Venoms, in contrast, must be ‘injected’ into the body by way a specially evolved mechanism, for instance, a stinger or fangs. (I use the word ‘injected’ loosely here, more on this in part II).

In addition to the differences in the mode of application, toxins and venoms are comprised of substances that are inherently different from one another. Toxins tend to consist of comparatively simple organic molecules while venom is usually comprised of an array of peptides and proteins that possess enzymatic activity. In general, venoms are extremely complex mixtures of different compounds while toxins are chemically well defined, pure, and homogenous substances (Mebs 2002).

All amphibians secrete ‘toxic’ substances through their skin that act as anti-bacterial and anti-fungal agents to help them avoid contracting illnesses (Clark 2007). Many species secrete more potent compounds as anti-predatory mechanisms. A particularly well-characterized group is the Arrow Frogs. Several species belonging to this group possess extremely potent toxins, many of which are alkaline steroids. The frogs use these compounds as a defense, and the mechanism through which they work are part of the definition of what a toxin is. Because there is no delivery system for these compounds, they must be consumed by a predator in order for them to be employed. Venom is usually secreted from specialized gland or tissue and is subsequently stored in specialized sacs until it is used. Technically venom can be referred to as being toxic or poisonous, but an animal, a snake for example, that is venomous, is not poisonous, because it wouldn’t harm you to eat it.

ETYMOLOGY

The origin of words is a topic that I find interesting and illuminating. Sometimes you can quickly see the Latin or Greek root of a word just by looking at it, but sometimes the derivation is not immediately obvious. The origin of the word toxin for example, I found surprising.

Poison

Comes from Middle English (probably 15th century) denoting a harmful medical drink, which comes from Old French poison, a potion or poisonous drink (14th century), previously simply a drink (12th century), but originally from Latin, potare, to drink.

Toxin

Comes from the Latin toxicus or toxicum, meaning poisoned or poison, respectively, from the Greek wordtoxikon, or (poison for) arrows, from the Greek toxon, or bow. I found it interesting that the source of the word toxin comes from poisons that were extracted from plants and invertebrates to coat the tips of arrows by ancient Greeks and Romans. The Poison Arrow Frogs obviously got their common name because theirtoxins were used for the same purpose.

Venom

Comes from Middle English, from the Old French venim, a variation of venin from an alteration of the Latinvenenum, or poison.

CLOSING REMARKS

In closing, I would like to point out that I am by no means offering precise definitions for any of the terms used. In fact, I would go so far as to argue that there are no watertight definitions for any of these words. For one thing, biology and biochemistry are so incredibly complex that it is near impossible to precisely define these things by placing them neatly into some well-defined little boxes, there always seem to be exceptions to every rule. Language is always changing and words usually have more than one meaning, so even a precise definition can be open to interpretation. The language discussed here is only relevant within the English language, but there are at least thirteen dialects of English, each with its own differences from modern American English.

When setting out to writing this article I had two points in mind:

  1. To explore some of the fascinating issues that arise when language is used to describe biological systems. Language is inherently obscure and biology is inherently complex. I feel that both are important to understand and interesting to study.
  2. To clear up some of differences between terms in an attempt to at least tighten up the definitions already in place. Even though biology always seems to offer exceptions, we can at least attempt to avoid some of the common errors in terminology.

In part I of this article I have played with the etymology of poisons, toxins, and venom. In part II of this article I will probe deeper into the biology and chemistry of toxins and venom and the evolutionary impact on delivery systems, and I will discuss various types of each.

Mebs, Dietrich. 2002. Venomous and Poisonous Animals: A Handbook for Biologists, Toxicologists and Toxinologists, Physicians and Pharmacists. Stuttgart: Medpharm.

Clark, BT. 2007. “The Natural History of Amphibian Skin Secretions, Their Normal Functioning and Potential Medical Applications.” Biological Reviews. (3):365-379.