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If we were to write a fable to get this moral across, it would have to star the freshwater cnidarian called the hydra. Because in the hydra, the question of butts connects to the ambiguities of immortality, which in turn relates to the befuddling matter of sexual reproduction.

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Go to to save 10% off your first purchase of a website or domain. Within the little droplets of water we study each week on Journey to the Microcosmos, we have seen all the chaotic aspects of life that we experience on a macro scale unfold between the tiniest of organisms.

And through them, we’ve learned plenty of lessons as stories of resilience and adaptation unfold before us, like little microscopic Aesop’s fables. But Aesop probably would not have come up with one of our favorite morals of the microcosmos: never let anyone convince you that butts, immortality, or sex are easy to define or describe. Now if we were to write a fable to get this moral across, it would have to star the freshwater cnidarian called the hydra, one of our favorite stars of the microcosmos.

Because in the hydra, the question of butts connects to the ambiguities of immortality, which in turn relates to the befuddling matter of sexual reproduction. And in some species, it builds to a fatalistic reality where sex spells the beginning of the end. But that’s jumping ahead.

Let’s start with what's usually the end of the tale: the butt, which in the hydra also double as the mouth. And for hydras, defining the mouth and the butt is a bit of a complicated affair because much of the time, there is no opening at all. The area where the mouth belongs is a continuous sheet of cells that ruptures open and expands when the time comes to eat or to poop.

Imagining for a moment our own bodies working this way is, horrifying. There’s the fact that food would enter and exit our bodies from the same location, and then the wound inflicted from within to make that possible. But for hydras, this is casual, a part of their daily habit of recycling cells and making new ones that makes their bodies capable of handling all sorts of things.

Their ability to do this is rooted in populations of stem cells that are able to endlessly divide to make different types of cells that can patch the hydra right up. Theoretically, a hydra could replace all of the cells along its surface within a week. Now there’s a number of philosophical questions you could ask about this.

Is the hydra like the hypothetical ship whose every part has been replaced by a new piece? And if so, is a hydra with new cells still even the same hydra? It seems like we would have to say yes, especially because it maintains the same sequence of DNA —that molecular source of identity that so often seems to define us.

But then what to make of a hydra that reproduces asexually, budding off new little clones of itself that swim away carrying that same DNA? Are those clones part of the same original hydra, or are they their own entities? We could argue those questions all day, and perhaps some of you will.

But we’re asking them not to get to an obvious answer, but to instead have a little fun with the fact that our definitions inevitably fall short of what nature actually reveals. As far as murkiness in nature goes, hydra butts might be a little unusual. But they are practically straightforward compared to the question of immortality.

We’ve actually done an episode about immortality before, and in it we said the following: These stem cells may make the hydra biologically immortal. Because its cells are always being replaced, the hydra is able to avoid the accumulation of damage that other multicellular organisms experience. The hydra doesn’t age.

And so until another force or predator intervenes, it doesn’t die. Well, we sure did say something that sounded uncomplicated there. But, it turns out, we were a little bit wrong.

Not entirely wrong, just not entirely right either. And this is where the complicated question of sex comes in. This is Hydra viridissima, a species found throughout the world and that’s notable for the green color it gets thanks to the Chlorella algae living symbiotically within it.

And, along this hydra’s body, you can see tiny round structures with something wiggling inside. Those wiggling bits are immature sperm cells, waiting to burst from the organ that encases them. Eventually the sperm are released by the hydra into the water in search of eggs to be fertilized.

Most of the year, none of that is happening. The Hydra viridissima will reproduce asexually, budding off its little clones. But when the weather gets warmer, the population shifts.

Individuals develop gonads that mark it as male, female, or hermaphroditic. And for the female hydras, asexual reproduction is put on pause. Now that all sounds very simple and like it follows a clear cycle.

But it’s also very specific to this species. If you ever want to feel your brain turn completely to mush, read a few scientific papers about hydra sex. It’s not that hydra sex itself is particularly complicated in its mechanics.

It’s just that it is very difficult to say any kind of generality about the strategy and effect of sexual reproduction in hydras across species. Some species spend most of their life reproducing asexually. Some alternate between asexual and sexual reproduction.

Some rely on populations of hermaphroditic individuals, others have a mix, and still others have individuals that change their sex. And that’s just a few of the details. Hydra viridissima, for example, are called “warm crisis” hydras because they tend to shift towards sexual reproduction when the waters get warmer.

In general, a shift from asexual to sexual reproduction marks an attempt to survive through variation, creating eggs or offspring that might be able to weather the challenges of an environment that a population of clones might not. And so, for the hydra viridissima, as waters get warmer, the possibility of drought or shifts in their food might signal the need to become sexual reproducers. But for other species of hydra, it’s not warmth that triggers the switch.

It’s cold, the oncoming winter that makes their environment harder and informs their decision to change to sexual reproduction. The species Hydra oligactis, for example, has become a useful model for researchers to study because all the hydras need to be convinced to reproduce sexually is a little chill in the water. And yet, hydras are nothing if not wildly inconsistent with one another, which means there is another species, Hydra vulgaris, that can begin reproducing sexually within a few days of its birth with little care for temperature of the water at all.

However, the most meaningful inconsistency of all ties into one of the long-standing debates about hydras: are they immortal? So what does immortality have to do with sex? Well, for some hydras, nothing.

But for others, it’s everything, including the end. As we said before, hydras can of course die. There are predators and forces of nature they cannot escape.

But aging, well, that seems like something they’ve never had to deal with before because of that pool of constantly renewing stem cells within them. And for some of the species we’ve talked about today, like Hydra viridissima and Hydra vulgaris, those pools of cells just keep on churning, keeping the animals ageless. But for Hydra oligactus, our cold-weather reproducer, a switch to sexual reproduction puts a halt to all of that.

When the temperature drops and the hydra begins to produce its sperm or eggs, the rest of the animal begins to decline as well. The cells within it begin to descend into disarray, and over the time the hydra will start to eat less and less. In the lab, they will usually die within months.

But why? There are a myriad of pathways within the animal that seem to describe Hydra oligactis’ sudden but slow shift from an immortal to aging animal. And yet these changes in gene expression and behavior don’t fully explain why this species is so affected by the switch to sexual reproduction while others— like hydra viridissima— are able to switch to and from different methods of reproduction without any aging at all.

Buried in that mystery are the answers to so many of our questions about how aging affects our own bodies. And perhaps some day we will find those answers, nestled in between contradictions created by our own attempts to define the world, to understand it as best as we can with the tools we have available to us, imperfect as they might be. Thank you for coming on this journey with us as we explore the unseen world that surrounds us.

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