Ok ok. This post is already starting off like a bad 3rd-grade-boy joke. But seriously (butt seriously?)–that beautiful jellyfish pictured above, well, you’re looking right at its anus. Or is its mouth? As my friend loves to call it, it’s actually a “manus”. That’s because jellyfish doesn’t technically have mouths or anuses, they just have one hole for both in things and out things, and for biologists, that’s kind of a big deal.

See, most animals have two holes, and the evolution of the dual-hole system has consumed the minds of many a scientist for centuries. In fact, just recently a friend of mine published a whole paper on the butt problem, titled–and this is no joke–“Getting to the Bottom of Anal Evolution” (You can read it here). Butts are serious stuff. They mean you can digest food in a linear fashion, which allows you to extract all kinds of nutrients in different specialized parts of your digestive tract. Jellyfish have no such linear movement o’ poo. There’s just one hole for taking things in, and then moving ’em out again–are they throwing up or pooping? For now, let’s call it throoping. And that makes them really interesting.

In the future, I hope to do a bit more work on jellyfish throoping. For example, when food moves around this very two-way digestive system, does it all get digested evenly all around, or does coarse digestion take place in one part of the gut, and fine digestion take part in a different place? Does it all circulate chaotically, or is there some systematic process?

At the moment, all of these are open questions. Though a few studies suggest there is some order to this seeming internal chaos. There do appear to be particular areas for coarse digestion and fine digestion. But is this true for all species? How about for size? And are there particular “in” and “out” parts of the manus? For now at least, the best we can say is that jellyfish have a single hole, and how exactly it works for them is something of a mystery. In the meantime, let’s all be thankful for our wonderful one-way digestion, complete with a front door AND a back!

Feature photo by Alexander Semenov.