The classic children’s book Everyone Poops ends with a rear-view drawing of a child and some animals, underscoring what we all know intuitively: poop comes out the back. But we take our digestive systems for granted. The very first animals used the same hole for eating and defecating, and some still do. And as one marine biologist recently discovered, at least one has an anus that only appears when it poops.
This “transient” anus, which disappears after pooping, was discovered in a marine animal called the warty comb jelly by Boston University professor emeritus and Woods Hole Marine Biological Laboratory researcher Sidney L. Tamm, Ph.D., who published his study in Invertebrate Biology recently. These transparent animals, known also as “sea walnuts,” belong to a group called ctenophores (ka-ten-o-phores) and swim in the western Atlantic. They muddle our ideas about anus evolution because we only know two types of animals: those with one hole, and those with two.
"You can observe a lot just by looking. No one really looked carefully before.
The thing is, ctenophores have been studied since the 19th century. It was then that scientists observed they have a “through-gut,” meaning they poop and eat out of different holes, like us. Other animals, like jellyfishes, have a one-hole “bottle gut,” through which it eats and releases waste through the same opening. Somehow, over 160 years of observations, nobody seemed to notice that one side of the warty comb jelly through-gut only appeared when it needed to.
“There’s a Yogi Berra saying that explains that,” Tamm tells Inverse. “That is, you can observe a lot just by looking. No one really looked carefully before.”
The Importance of Observation
Tamm has been studying these animals for a long time but had “never been interested” in how they poop until a recent “mix-up” in the ctenophore field. In 2016, scientists at a conference called Ctenopalooza were amazed by footage showing that the animals had both a mouth and an anus. “If people don’t see this video, they won’t believe it,” gushed marine biologist George Matsumoto, Ph.D., in Science. But this was not surprising at all, said Tamm, writing a heated retort pointing out that scientists had known that since the 1850s.
“I looked at it more closely because of this mix-up, and that’s why I found this come-and-go anus,” he says.
"They swirl around and circulate, as if they’re waiting for something to happen.
Tamm watched individual warty comb jellies for hours, marking the interlude between each defecation, “to get the poop rhythm.” With the timing right, he knew when to zoom in with a powerful microscope. When he zoomed in on the anal canal, the transparent tube accumulating particles of waste, he discovered something unprecedented.
“They swirl around and circulate, as if they’re waiting for something to happen,” he says, “and all of a sudden you see an opening appear — a pore.” All at once, the animal’s inner tissues fuse with its skin, creating a passage to the outside world. “It gets bigger and bigger until everything that was in that canal is out,” he continues. “And then it starts to close. And it closes and closes until it finally disappears and you no longer see it.”
His discovery is a reminder of what biology is all about: patient observation.
“Just be careful,” he says. “And that’s what the other people either missed, or didn’t have time to do, or just didn’t bother.”
Between One and Two Holes
As far as Tamm can tell, this is the first time anyone’s observed a transient anus. It complicates our view of how the anus evolved, adding what he thinks might be an “intermediate step” in the evolutionary process.
Having both a mouth and an anus, he says, “is an advance for getting bigger and more complicated” in terms of evolution. Without it, animals couldn’t get too long, eat and digest at the same time, and literally not shit where they eat. We’ve come a long way since our bottle-gut ancestors. But at some point in our evolution, we had to make the transition from one hole to two.
“Getting an anus is the question,” says Tamm, explaining that in an embryo, the anus forms through a fusion of tissue, not unlike what he saw in the warty comb jellies. The difference is that, in most animals, that fusion is permanent: Once the anus forms in the embryo, the animal has it for life. Tamm’s strange creatures, for whatever reason, seem to be less decisive.
“This temporary fusion might be a step in a permanent anus in ctenophores and other animals,” he says.
To confirm whether actual fusion is taking place when these animals poop, he’s already planning to use the powerful electron microscopes at Woods Hole Marine Biological Laboratory. More valuable than any machine, however, is Tamm’s gift of patience. And when your’e studying anuses, a sense of humor can’t hurt, either.
“If you just look, carefully, you can see a lot,” he says. “Yogi Berra, even though it sounds funny, he was right. He has lots of other sayings. Like if you see a fork in the road, take it. And you don’t know if he’s talking about a utensil.”
Defecation in the ctenophore Mnemiopsis leidyi is a stereotyped sequence of effector responses that occur with a regular ultradian rhythm. Here I used video microscopy to describe new features and correct previous reports of the gastrovascular system during and between defecations. Contrary to the scientific literature, individuals defecated through only one of the two anal canals which possesses the only anal pore. The anal pore was not visible as a permanent structure as depicted in textbooks, but appeared at defecation and disappeared afterward. Time intervals between repeated defecations in individual animals depended on body size, ranging from ~10 min in small larvae to ~1 hr in large adults. Differential interference contrast microscopy revealed that both the opening and closing of the anal pore resembled a reversible ring of tissue fusion between apposed endodermal and ectodermal layers at the aboral end. Individuals of M. leidyi thus appear to have an intermittent anus and therefore an intermittent through‐gut that reoccur at regular intervals. The temporality of a visible anal pore in M. leidyi is novel, and may shed light on the evolution of a permanent anus and through‐gut in animals. In addition,mirror image dimorphism of the diagonal anal complex occurs in larval ctenophores but not in adults, indicating developmental flexibility in diagonal symmetry of the anal complex.