Thanks to Skillshare for supporting this episode of Journey to the Microcosmos.

The first 1,000 people to click the link in the description can get a 1 month free trial of Skillshare’s Premium Membership. Our oceans and lakes are filled with copepods, a myriad of small crustacean species that might float as plankton or infect other creatures.

And as they’re living in whatever manner best suits them, some copepods—like our friend here—become more than just their own creature. They become a surface, a place for someone else, or something else, to settle down upon. In the case of this copepod, the residents that have taken root on its carapace are many, many ciliates, their single-celled bodies accumulating into a microscopic bouquet.

And before we get into the specifics of what’s going on with this meeting of crustacean and ciliate, I want to ask you a quick question. Don’t overthink it, just go with your gut instinct: Who would you rather be in this scenario? The copepod?

Or the ciliate? Now I promise you that there’s no right answer to this question. It’s just something that’s interesting to think about whenever we peer into the microcosmos and see creatures with habits that seem quite different to our own.

Though we should note that we do have some things in common with the microcosmos in this context. Our skin is covered with its own array of microbes, an invisible community of bacteria and fungi and other creatures that affect the way our bodies interact with the world around us. So if you were asked who you’d rather be, a human or a microbe living on a human, we would not blame you at all if you stuck with what’s familiar and instinctively went with “a human being please.” But the microbiome on our skin has a different experience, than say, these ciliates growing on a rotifer.

For one, they spend their lives in water, which means they have a completely different environment shaping their needs and microbial relationships. Plus, they are a whole lot smaller than us. So that’s what we’re going to focus on today: what happens when one tiny creature decides to live on another tiny creature.

There’s a name for this kind of relationship: epibiosis, and it’s not limited to the microcosmos or to water environments. It might help to think of it as the flip side of another relationship we’ve watched play out in the microcosmos: endosymbiosis, where one creature decides to live on the inside of another one, like these algae growing inside of a hydra. But the relationships between endosymbionts and their host can look different depending on the species.

But in cases like these, it might be the host providing shelter in the form of their bodies in exchange for nutrients made by their endosymbiont. In contrast, epibiosis is when one creature decides to live on the outside of another one. And sometimes epibiosis is super obvious, like it was with the copepod we started out with.

But take a look at this dinamoeba. This organism was first described by the American naturalist Joseph Leidy in the 19th century. And from a distance, it doesn’t seem like the dinamoeba has much going on.

There’s certainly no ornate, flowery growths protruding from it. But when you zoom in, you can see all sorts of bacteria and archaea arranged on its surface. In this case, those bacteria and archaea are epibionts, which is the name that we give to organisms growing on top of the other organism.

And the dinamoeba is the basibiont, the organism that the epibionts are growing upon. And in the microcosmos, sometimes you are the basibiont, and other times you are the epibiont. This leafy ciliate zoothamnium, for example, can be found growing on copepods, similar to how we found our ciliates at the beginning of this video.

But we have also found zoothamnium with euglenoids growing on them like little green sprinkles that stay glued to their stalks. And we’ve even seen strange sights like these prokaryotes growing on top of a larger prokaryote, though we’re not sure what species they are and what they might be getting out of this nearness to one another. And given that the diversity in epibionts, the way they attach themselves to a basibiont varies a lot as well.

For the smallest microbes, their path towards their new surface is shaped by the viscous forces around them, that creates a wall of water they must break through before they can latch on to their new home6. And for unicellular eukaryotes, or even multicellular ones, the decision to attach itself to another creature might be just be another part of a longer life cycle, a shift from a free-swimming self that ages into something willing to settle down using an adhesive molecule to glue them together. And if you are the basibiont, welcoming these epibionts might work out well for you.

These new residents might provide you with some nutrition that you were lacking, supplying metabolic byproducts that are now very close in proximity. These epibionts might even help to hide you from predators, masking the chemical cues that give your location away. But there are trade-offs as well.

For one, you are literally shouldering the weight of other organisms. In water, that can make all the difference, affecting your speed and buoyancy. And epibionts can even get in the way of important needs, blocking off the sun or impeding reproductive organs from their usual functions.

Meanwhile, for the epibiont, there are pros and cons as well. Life on another creature means that you’ve got free transportation wherever your basibiont chooses to go. And these surfaces often make for a great place to grab the remains of whatever your basibiont is eating.

But life is also unstable on a living surface. When your basibiont grows or sheds or experiences some other major life change, you the epibiont will find the very ground beneath you changing as well. There’s this disadvantage inherent to both epibiont and basibiont life.

In one paper, we found the term they used was “shared doom,” which is a more poetic way to say this, “if they get eaten, you get eaten too.” And it is a strange consequence to think about, that this partnership makes its individuals vulnerable to new threats. The basibiont now faces the epibiont’s predators, and vice versa, as if their identities have co-mingled and blurred together. And yet they are still two distinct entities: one lived upon, the other living upon.

So to ask the question again: which would you rather be? The epibiont, or the basibiont? Or maybe there is little difference between the two, each one shaped by the whims of the other.

Because ultimately, in some way, every organism on earth, it has to be said, is lived upon and lives upon just as we are shaped by the microbes on our skin and the earth under our feet. But next week, we’re going to focus on one case of epibiosis where we think the answer is perhaps a bit cleaner. Thank you for coming on this journey with us as we explore the unseen world that surrounds us.

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