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.
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.
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.
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.
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.
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 coerulescens likes 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.
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:
Toxic birds –