Biology concepts – toxin, venom, cnidarians, kleptocnidae
If even the most primitive animal phylum has members that are venomous, then all phylums probably do – right? Well, no – birds are the exception. We know of no venomous bird; NO bird makes or sequesters a toxin that is then delivered by bite or talon scratch or other natural mechanism.
This is odd, since the toxins involved are from the batrachotoxin family, just about the most potent neurotoxins on the planet. They work by binding to the ion channels in neurons and holding them open. This prevents the neuron from recovering after it sends an electrical signal. Therefore, the neuron can’t fire anymore, and whatever it is connected to can't be stimulated. If it is connected to a touch receptor, you will feel numb there. If it is connected to a muscle, the muscle will be paralyzed. Batrachotoxins seem to work in every animal that has this type of neural system, so why isn’t the beetle the bird’s last meal?
When triggered by mechanical pressure on a hair cell sticking out, and sometimes when accompanied by a chemical signal that a prey organism is near (they “taste” the water), the pressure inside is increased, reaching more than 2000 pounds per square inch, and the cell bursts.
The third class of cnidocytes are the ptychocysts. These are restricted to a group of sea anemones called tube anemones. They are used to build the tubes that these animals live inside. The threads of the ptychocysts are mixed with mucus and debris and become fibrous houses for the animals inside. As such, they are mainly for defense, not for catching food. Yet they do have nematocysts for defense as well.
There are two hypotheses as to how B. coerulescens can consume the nematocytes and then place them into its cerata on its back. One hypothesis is that they coat them with mucus and that keeps them from firing as they are eaten and moved through the digestive system. The other hypothesis states that they mature nematocytes do discharge, but the immature nematocytes cannot; they are then sequestered and mature while being it held in the cerata.
Gong, E., Martin, L., Burnham, D., & Falk, A. (2009). From the Cover: The birdlike raptor Sinornithosaurus was venomous Proceedings of the National Academy of Sciences, 107 (2), 766-768 DOI: 10.1073/pnas.0912360107
Greenwood, P. (2009). Acquisition and use of nematocysts by cnidarian predators Toxicon, 54 (8), 1065-1070 DOI: 10.1016/j.toxicon.2009.02.029
In our discussions of venoms and toxins we have looked at many groups (phylums) of animals. In each phylum we have identified at least one venomous animal. We have talked venomous amphibians (frogs, salamanders), venomous reptiles (lizards, snakes), venomous arthropods (insects and spiders), and even venomous mammals. Even though we haven’t talked about them in this series, there are also venomous sponges, and sponges are the most primitive animals on Earth.
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Sinornithosaurus was a raptor dinosaur with feathers; a very early proto-bird. It was still a reptile, but with features that would come to be typical of birds. One thing that wasn’t typical was its teeth. A 2009 study indicated that one of its long fangs had a groove down the side – channel for venom! So, while no modern birds are/were venomous, maybe an ancient ancestor was. |
Birds are the evolutionary descendents of the reptiles; they diverged from the reptiles about 240 million years ago. The toxicofera hypothesis says that all reptiles were at one time venomous, so why aren’t the birds? It may have something to do with the timing of the divergence. The oldest venom genes and delivery systems are associated with the lizards, about 200 million years ago, AFTER the divergence of birds. Mystery solved. Of course, mammals and arthropods had diverged hundreds of million years earlier, and they have some venomous species. If they could do it on their own, why couldn’t birds?
Well, a few birds have found a way to use the toxins produced by other organisms. Members of the pitohui family of birds in Papua New Guinea tend to feed on toxin producing choresine melyrid beetles. The toxins remain in the bird’s tissues and feathers for some time before they are broken down or excreted.
Just rubbing against the feathers can induce numbing, and consuming a bird would be lethal for many animals, including humans. The Hooded Pitohui seems to know this and is very social and loud. It suspects that predators know it is a bad meal and will leave it alone. An added benefit - lice that usually live in the feathers are also affected by the toxin, so these birds are relatively parasite-free.
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This is the spur winged goose. It lives in the wetlands of Saharan Africa. It also feeds on toxic insects, so it comes by its toxins in the same way that the pitohuis do in New Guinea. Not all the geese are toxic, since they have a large range. Only some live near the blister beetles of Gambia that are poisonous and render the birds toxic. |
Birds are certainly an exception to the rule that at least some organisms in each phylum developed venom. How about the other end of the spectrum? It would also be an exception if we had a phylum of animals that were ALL venomous. Well, we do.
The cnidarians are the phylum of animals that include the anthozoans (corals and sea anemones), and the medusozoa (jellyfish, box jellyfish, and the hydras). It turns out that EVERY species of cnidarian is venomous, though some might not be venomous enough to harm humans.
Cnidarians all have cnidocytes (cnida = nettle, like plant nettles that stick you and cyte = cell); cnidocytes are the secret handshake required for membership in the cnidarian club. There are three main flavors of cnidocytes; nematocysts, spirocysts, and ptychocysts. It is the nematocysts that make cnidarians venomous.
Nematocytes are the cells that house the actual stinging apparatus, called nematocysts. They have a barbed shaft that together looks like a harpoon end. This is housed in a cavity filled with venom and covered by a trap door (operculum). There are about 35-40 different shapes and lengths for nematocysts, but they all work basically the same way.
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The left image is a cartoon of a typical cnidarian nematocyte. You can see the operculum, the trap door on top, as well as how the barb is packaged inside the cnida. The entire volume is filled with venom that is sprayed out under pressure when it fires. The right image is a photomicrograph of a fired nematocyst. The scale shows how small they really are ( a micron - µm - is a millionth of a meter). |
The operculum opens and the shaft is everted at the prey in just 700 nanoseconds (about 700 billionths of a second), with an acceleration of more than 5,400,000 x gravity. It isn’t a surprise that the prey’s skin is pierced by the shaft! The pressurized venom is then injected into the wound through the hollow shaft and/or hollow tubule.
For some cnidarians, like the sea wasp (Chironex) or the Portuguese man-of-war (Physalia), the venom is important because their prey is strong, including large fish. Most cnidarians, especially jellyfish, are fragile animals; they don’t have a strong internal skeletal and can be ripped apart easily. Therefore, it is important for them to immobilize their prey quickly. The venom does the job. It works very well, for some jellyfish it works well enough to severely harm (Irukandji jellyfish) or kill (sea wasp) humans.
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Inside the vial is an Irukandji jellyfish. It is small, but it packs a wallop. This is one of the few jellyfish that possess nematocysts on its bell (the round part at top) as well as on its tentacles. The lower image shows that while the body s about 5 mm long, the tentacles of the Irukandji jellyfish can be up to a meter long! One, you can hardly see it, and two, it can nail you from long distance. |
Spriocysts and ptychocysts are the other types of cnidocytes. I was worried about making the statement that all cnidarians are venomous, on the off chance that some cnidarians possess only spirocysts and/or ptychocysts. I contacted several researchers that study cnidarians, and they all stated that as far as they know, all cnidarians possess nematocysts, while only some have spirocysts and/or ptychocysts.
Many cnidarians rely primarily on spirocysts. These cnidocytes are very similar to nematocysts, except that they don’t have an associated venom. Spirocysts are used primarily by cnidarians that prey on less vigorous animals, animals that aren’t as able to pull them apart at the seams. Most corals, for example, prey on small invertebrates, so they rely less on venom and more on entanglement. This is the function of spirocysts, they substitute adhesive for venom.
The bubble tip anemone (Entacmaea quadricolor), for example, relies on a combination of nematocysts and spirocysts to bring in its prey and for defense. But it doesn’t have to hunt much to gather food. It has symbiotic relationships with other animals that help out. The bubble tip is often green colored, because it has intracellular photosynthetic dinoflagellate organisms that provide it with carbohydrates.
The bubble tip also has a relationship with the clownfish (think Finding Nemo). The fish clean away parasites and devour any dead tentacles, while they also scare off predators and provide the anemone with scraps from its meals. Though the bubble tip does have nematocysts, it seems that the clownfish is immune to the toxin, so living amongst the tentacles provides the clownfish with protection from its predators.
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Entacmaea quadricolor is the scientific name for the bubble tip anemone. It comes in four different colored varieties, pink, red, orange, and green – hence the name “quadricolor.” And the “Nemo” fish it protects is actually called a cinnamon anemonefish. I’m wondering who decided it tasted like cinnamon. Sometimes it is called a fire clownfish – so does it taste like fire too? |
Some people think that there are some cnidarians that have lost the ability to sting using nematocysts. The TV show, Survivor, went to Palau for a season, including an episode where the winners of some challenge were rewarded with a chance to swim in the lakes with the jellyfish. These golden jellyfish and moon jellyfish are related to the species that live in the nearby ocean, have been separated geographically for thousands of years.
This separation has led to the misconception that they have lost their nematocysts due to a lack of predators. But it is not so, moon jellyfish stings in the lake will be noticed, just not as much as those fro the ocean. Perhaps a genetic drift is taking place, but swimmers do report numbness around their mouths and fingers, so the jellyfish in the lake do still have venom.
Venom from cnidarians protects cnidarians, but it also protects others. Nudibranches are a type of sea slug, related to snails and other molluscs. They eat cnidarians, but not only do they eat them, they use them as well. The use of cnidarian nematocysts by nudibranches is discussed in a 2009 review by Paul Greenwood.
Berghia coerulescenslikes to eat sea anemones; it may or may not be susceptible to the venom. But that doesn’t really matter since the nudibranch eats the nematocytes whole, perhaps without triggering them to fire.
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Nudibranch sea slugs are some of the most colorful animals in the world. You can see the cerata on its back, like so many dreadlocks. These are where the nematocysts are housed for defense. They should make a Disney movie about a sea slug – Where’s Nudi? |
Either way it occurs, when the nudibranch is threatened, it stiffens its cerata and the musculature moves the nematocytes to a pore. When they contact the seawater, they fire. This is supposed to keep the predators at bay. The review of Greenwood discusses whether this defense is effective – maybe, maybe not. It needs more study.
We just scraped the surface of the weirdness that is the cnidarians, so we will talk more about them in the future. However, next week we will finish the stories on venoms and toxins by looking at the poisonous plants. Did I say, poisonous? Well at least one is venomous.
For more information or classroom activities, see:
Cnidarians –
Nematocysts –
Sinornithosaurus –
Toxic birds –
Kleptocnidae -