Parasites have it tough. They’re usually ignored, and when they are mentioned, it’s never positive. They’re repulsive primitive creatures that are an annoyance at best and terrifying at worst. At the end of the day, they’re just marginal players at the edge of an ecosystem, and can probably be ignored.
True, a parasite worming its way into a host may not be quite as awe-inspiring as a crocodile seizing a hapless zebra in its jaws. But that doesn’t mean it’s any less ecologically important. My fiancé Matthew and I went on a bike trip near Walla Walla recently, where we saw a white-tailed deer. However, that was the only wild ungulate we saw – no other deer, no moose, no elk, nothing. We didn’t just get unlucky. Around 80% of white-tailed deer are infected by a parasitic nematode (Parelaphostrongylus tenuis). They can survive with it without too many issues. But if the nematode makes its way into other ungulates, it causes severe neurological damage, usually killing the host. Because of this, on a local level, white-tailed deer habitat practically never overlaps with other ungulates. We didn’t know it at the time, but our bike trip – and the whole ecosystem we traveled through – was shaped by a humble parasite!
To really understand how parasites work, you need to think like a parasite. And if you’re a parasite, you’ve got a pretty tough life. Your hosts are your only possible habitat, islands of safety in a vast, vast ocean of death. Your odds of making it are maybe one in a couple million. So naturally, you want to improve those odds, even if by just a bit. And you’ve got two main options. There’s the usual evolutionary path – modify your body or your behavior. But since their environment is a living being, parasites have an interesting alternative option. They can modify their host.
That first route may not be unique to parasites. But they have found some ingenious paths to take. As an example, let’s look at some freshwater mussels in the genus Lampsilis.
That weird-looking flap kind of looks like a minnow, right? Well, fish sure think so. They see an easy meal, but as they swoop in for the kill, those lips open up and spit a cloud of larvae in its face. Those larvae attach to the gills of the fish, where they have an easy meal and a safe home as they mature.
For the parasitic flatworm Dicrocoelium dendriticum, tricking a potential host isn’t enough. In fact, just one host isn’t enough! This particular species has to find a way to get from a terrestrial snail into an ant, and then from an ant into a sheep. To do that, it employs a bit of trickery and a bit of manipulation. It starts in the snail gut, where it gets released from the snail in sugary slime balls. Ants are attracted to the sweetness and obliviously munch away. As the parasites develop in the ant, one migrates into the ant’s brain, and something strange happens. That evening, the ant develops an uncontrollable urge to climb. It makes its way all the way up to the very tip of a blade of grass, and waits. As the sun rises the next morning, the urge fades, and the ant descends to feed and work. This repeats for a few days until an unlucky sheep swallows the ant along with a bite of grass. From there, the flatworm matures in the sheep’s immune system, ejecting its eggs out to infect a snail and start the whole cycle over again.
We’ve seen parasites that can trick their hosts, and ones that can change their host’s behavior. But some can go even further, and change their host’s appearance. Even better, they tend to be parasites of our favorite critters – crabs!
Rhizocephalan barnacles are the leather-jacketed rebel of the barnacle world. They aren’t content to stay put on a rock and feed on plankton. They have their sights on much, much bigger prey. Crabs. An infected crab will have a big, obvious sac, usually under its abdominal flap. And inside, the parasite creates a long, tangled root-like network through the crab.
Now, the rhizocephalan isn’t just randomly attaching to the abdominal flap. That’s where females keep their eggs. So if it infects a female, it just forces the female to keep protecting the “eggs” as usual. But what if it’s a male crab? Well, the barnacle has a solution for that too. Just turn the males into females. An infected male crab will see its abdominal flap widen and its claws get thinner. It’ll even start caring for its “eggs”, pushing water over them to keep them oxygenated and healthy. And just like an infected female, when the parasite is ready to spawn, the crab will climb up high and shake its abdomen around as if it’s sending its own eggs out into the current.
Clearly, parasites aren’t backwards, degenerated, marginal creatures. They’re key parts of their ecosystems, doing far more than we realize to structure the world around us. And they’re shockingly specialized, able to trick potential hosts and deftly manipulate their current hosts. They might be gross, but they’re also worthy of a whole lot of attention.
Postscript: Many of the examples in this post came from Claude Combes’ book “Parasitism: The Ecology and Evolution of Intimate Interactions”. It’s not just a fantastic book on parasites, it’s a fantastic book period – one of my absolute favorites ever. It’s chunky (630 pages without references), but if you really want to dive into the world of parasitism, there’s nothing better.
– Fahzan H., Waiho K., Wee H.B., Surzanne M.A., Ma H., Ikhwanuddin M. 2018. “Predicting the sacculid Sacculina beauforti infection status of the orange mud crab Scylla olivacea by discriminant analysis”. Aquaculture 491: 128-134
– Noever C., Keiler J., & Glenner H. 2016. “First 3D reconstruction of the rhizocephalan root system using MicroCT”. Journal of Sea Research 113: 58-64