The unexpected sophistication of a comb jelly’s defecation

There was a time, long, long ago, before there were mammals, dinosaurs or even fish, when your ancestors had to poop out of their mouths. This wasn’t exactly a proud period for life on Earth, but considering the sponges and jellyfish we’re talking about don’t really have recognizable brains, they probably weren’t worried about appearances, but rather the logistical problems this created. As much as nobody admit wants to admit to eating on the toilet, animals from earthworms to humans at least have the option! While we’ve been able to feel smug about our ability to multitask for years, it turns out this may not be as exclusive a trait as once thought, correcting around 130 years of misconceptions about some species’ butts.

Benefits of separate butts

Having two ends to our digestive tract actually affords more advantages than a chance to eat non-stop. Sending food on a one-way trip means your body doesn’t need any kind of pumping mechanism to move the food back to your mouth, and can instead invest in structures like the papillae you find in camels, passively shoving food in the right direction. So for the small investment of an anus, multitudes of animals have allowed themselves new ways to gather calories.

Unexpected anuses

For a long time, it was assumed that digestive tracts like ours were a straight upgrade, improving over what was found on more primitive creatures like sea sponges. This was backed up by early observations of animals like jellies, which were seen regurgitating waste out of their mouths. A recent experiment has shaken those assumptions, as a comb jellyfish was fed fluorescent protein from genetically engineered zebrafish, making the entire digestive journey plainly visible. After two to three hours of watching food travel through the jelly’s translucent body, waste was observed being expelled not from the mouth, but from two small and previously mysterious pores on the animal’s rear end. A closer look showed that the pores actually have sphincter-like muscles, leaving no doubt that comb jellyfish not only have anuses, but they have two of them.

The confusion has therefore been shifted to the possible evolution of anuses themselves. If comb jellyfish do have such anatomy, then perhaps they evolved earlier than we’d realized, and the reason sea sponges and anemones lack them is because they didn’t need them. Maybe when you’re attached to a rock, voiding waste from your mouth makes sense because the back of your body is boxed in. Or the comb jellyfish’s butt might be a case of convergent evolution, where their anuses evolved independently of the one animals like us inherited. At this point its only clear that the history of butts isn’t as neat and tidy as we once thought.

Source: Why watching comb jellies poop has stunned evolutionary biologists by Amy Maxmen, Science Magazine