Multitasking Fish Pee Out of Their Intestines

The stickleback may not look like much, but this little fish is quite the overachiever.

They’re the "geniuses of the fish world" and some of the animal kingdom’s best dads. They welcome robots into their ranks without missing a stroke. They’re skilled nest-builders. And now, it turns out, they can pee from their intestines.

Let’s go back to that nest. To entice the ladies, a male stickleback constructs a comfy green blanket out of plants and sand. If a female stickleback likes it, she’ll swim inside and lay her eggs for the male to fertilize. Quality matters—a better nest means a better chance of making babies.

“Gasterosteus aculeatus,” by Alexander Francis Lydon. Image Credit: Wikimedia Commons // Public Domain

But plants and sand don’t just naturally stick together. The male stickleback binds his materials with a special fish glue that he makes himself.

Here’s where things get tricky. The glue is made of slimy, spider-silk-like threads called “spiggins” that the fish brews up in his kidney. The kidney is not a multitasking organ. It can either spin fish glue or make pee—not both.

Every organism, from nanobacteria to Nobel Prize winners, must get rid of the waste that builds up in its body. For most animals, this takes the form of urine and feces. If something stops us from peeing and pooping, the toxins and fluids will back up in our systems, and we’ll die.

The whole time a male stickleback is building a nest, his kidneys are too busy to make urine. “He ought to swell up like a sponge, absorbing water, and eventually explode,” Stockholm University’s Steffen Madsen said in a press release.

Instead, Madsen discovered, the fish delegate pee production to their intestines. Water and waste are funneled through channels called aquaporins in the fish’s intestinal walls and come out the other end as urine. The stickleback gets to keep building and not explode.

This type of organ repurposing has never before been seen in the animal kingdom. It’s a great example, Madsen says, of how animals can adapt to almost any situation.