They’re zombies, Jim, but not as we know them…
NOTE: Before I begin the post proper, I’d like to admit to committing a bit of a biological crime for the sake of accessibility. In my previous post, I separated ‘viruses’ and ‘parasites’ as two separate groups. I did this mostly because, as far as I’m aware, popular culture references (e.g. Resident Evil 6) do differentiate the two. In reality, the biological definition of parasite is ‘any organism that lives in/on another organism, its host, and derives nutrients from its host, at its host’s expense’. A virus meets all of these criteria, and can thus be considered a parasite too.
The Importance of Being Parasitic
I’ll confess that I’m probably one of a relative few that truly admire parasites. Even beyond zombifying parasites in fiction (and fact, a bit later in the post) I am fascinated by everything about the parasites that exist on our planet, that I know of and continue to gain knowledge on. I could go on about why I admire them so much but it would probably take me a good while to get through all of the reasons why. I will say that it is astonishing that over 50% of ALL life on this planet is parasitic. From viruses to tapeworms, from wasps to fungi, a diverse range of organisms have adopted this successful lifestyle. It is terrifying, yet in some respects admirable, that parasites can have such massive impacts on life on Earth. I acknowledge that human superiority is challenged when we are in a constant war with parasitic organisms. A war in which we have lost many battles. These organisms are to be respected, admired and, yes, even feared. The following post contains just three examples of zombie parasites I didn’t pick them based on any criteria of awesomeness or terrifying prowess…just three that I find personally fascinating (though there are many more, rest assured!). I’ll leave you to decide what you think about them!
1) Cordyceps – fungal parasite of several insect species
If you’re David Attenborough fan, or just enjoy watching the weirdest footage of nature on offer, you may have heard of this particular group of fungi. Numbering around 400 species in Asia alone, they exist in tropical rainforest environments worldwide. They are all ‘entomopathogenic’, meaning that they specifically parasitise on insect species, notably ants. Essentially, the fungal spore(s) land on the body of the insect, penetrating the insect’s hard exoskeleton. Once inside the insect’s body, the fungus replaces its host’s non-vital organs. Now here’s where things get interesting in a most ‘zombifying’ way! To illustrate the mode of action, I’ll use a Cordyceps fungus that has an ant host. When ready to sporulate (release spores for reproduction), the fungus invades the ant’s brain and forces it to climb to a high point on a plant, attaching itself there before it dies. Once the ant dies, the fungal ‘fruiting body’ (the part that actually produces and releases spores) grows and grows until its mature enough to release the spores. And since the fungi is so high up, the spores are released over a wider radius, meaning a significantly higher chance to come in contact with another host and begin the cycle anew.
Now, what’s the ecological importance of these parasites? After all, it seems cruel to decimate large numbers of insect populations, right? From a human point of view, possibly (unless you hate insects, which I’d imagine a majority of people do). From an ecological point of view, its essential for ecosystem functioning. Each species of Cordyceps specifically parasitises on one host. They regulate their host populations, ensuring that no one species dominates, a result that would seriously reduce tropical biodiversity.
Hard to imagine such graphic imagery? As the saying goes, seeing is believing:
2) Apocephalus borealis – fly parasite of honey and bumble bees
I personally classify this particular parasite as terrifying, for the sole reason that honey and bumble bee populations are already threatened due to other factors, such as habitat loss and fragmentation. Especially where honey bees are concerned, their loss would be catastrophic both ecologically and economically. Nevertheless, let’s take a look at what this parasite does…
Female wasps lay their eggs inside the bodies of their bee hosts. As the larvae mature and develop, they attack the internal organs and brains of their hosts, causing odd behavioural changes. Notably, these include the bees becoming disorientated and flying during the night. Eventually, this behaviour results in the death of the bees. Then, as if adding insult to biological injury…er…mortality, the mature fly larvae (pupae) decapitate the bees. These pupae eventually become adult wasps and the cycle begins anew. What makes this species even deadlier, if that was even possible, is that they are thought to be vectors (carriers) of other diseases affecting honey bees…meaning that the destructive effect they have on honeybee colonies is magnified. Nasty.
3) Nematomorph (hairworm) species – parasite on grasshoppers
I remember sitting in a lecture on ‘Invertebrates’ when I was in the second year of my degree course. This particular lecture was on Nematode/Nematomorphs. I was instantly hooked when the lecturer – who since that lecture fed my growing curiosity and fascination with parasitic organisms – wowed us with the fact that, if you took all the species of these worms away from the face of the planet, we would see a very bare picture indeed. That very lecture also planted the seed of the idea for my dissertation in my head, which I realised fully much later – and yes, my dissertation was, of course, also on parasites!
And that is the same lecture where I found about this particular ‘hairworm’ (the species name escapes me) and its startling effect on grasshoppers. Although its unclear how the larvae get into their grasshopper hosts, a working theory is that they invade their hosts’ bodies when the grasshoppers drink infested waters. Once inside, the larvae mature into adult worms. Of course, they need a fair bit of space to do this…so they replace pretty much all of the grasshopper’s internal organs with their own body mass. Essentially, the grasshopper’s shell becomes a Halloween costume of sort for the hairworm. Once the parasite is ready to release the next generation of grasshopper-guzzlers, it does something interesting. It releases chemicals which cause the grasshopper to suicidally leap into the nearest body of water. The reason? The next generation of parasitising hairworms need to be in water to disperse and find another gamete (sexual cell) to fertilise. That’s life!
The moral of this post? Don’t even try to assert your superiority over the world of parasites. They will outnumber you. They will outwit you.
And if you’re particularly unlucky, they will zombify you.
Any other examples of beloved zombifying parasites? Feel free to share them! 🙂