Why watching comb jellies poop has stunned evolutionary biologists

Comb jellies such as Mnemiopsis leidyi have a through-gut, challenging when this evolutionary innovation arose.


Why watching comb jellies poop has stunned evolutionary biologists

No buts about it, the butthole is one of the finest innovations in the past 
540 million years of animal evolution. The first animals that arose seem to have literally had potty mouths: Their modern-day descendants, such as sea sponges, sea anemones, and jellyfish, all lack an anus and must eat and excrete through the same hole. Once an independent exit evolved, however, animals diversified into the majority of species alive today, ranging from earthworms
 to humans.

One apparent advantage of a second hole is that animals can eat while digesting a meal, whereas creatures with one hole must finish and defecate before eating again. Other possible benefits, say evolutionary biologists, include not polluting an animal’s dining area and allowing an animal to evolve a longer body because it does not have to pump waste back up toward the head.

However, several unprecedented videos of gelatinous sea creatures called comb jellies, or ctenophores, now threaten to upend the standard view of the evolution of the so-called through-gut. On 15 March, at the Ctenopolooza meeting in St. Augustine, Florida, evolutionary biologist William Browne of the University of Miami in Florida debuted films of comb jellies pooping—and it wasn’t through their mouths.

Browne’s videos elicited gasps from the audience because comb jellies, whose lineage evolved long before other animals with through-guts, had been thought to eat and excrete through a single hole leading to a saclike gut. In 1880, the German zoologist Carl Chun suggested a pair of tiny pores opposite the comb jelly mouth might secrete some substance, but he also confirmed that the animals defecate through their mouths. In 1997, biologists again observed indigestible matter exiting the comb jelly mouth—not the mysterious pores.

Browne, however, used a sophisticated video setup to continuously monitor two species that he keeps in captivity, 
Mnemiopsis leidyi and Pleurobrachia bachei. The movies he played at Ctenopolooza capture the creatures as they ingest tiny crustaceans and zebrafish genetically engineered to glow red with fluorescent protein. Because comb jellies are translucent, the prey can be seen as it circulates through a network of canals lacing the jellies’ bodies. Fast-forward, and 2 to 3 hours later, indigestible particles exit through the pores on the rear end. Browne also presented a close-up image of the pores, highlighting a ring of muscles surrounding each one. “This is a sphincterlike hole,” he told 
the audience.

“Looks like I’ve been wrong for 30 years,” said George Matsumoto, a marine bio
logist at Monterey Bay Aquarium Research Institute in Moss Landing, California, after he saw Browne’s talk. “If people don’t see this video, they won’t believe it,” he added. Matsumoto said he, as well as the bio
logists before him, likely missed the bowel movements because they did not observe their animals long enough after a meal. Jellies seen to expel waste from their mouths might have been, in effect, vomiting because they were fed too much, or the 
wrong thing. 

According to recent DNA analyses, comb jellies evolved earlier than other animals considered to have one hole, including sea anemones, jellyfish, and possibly sea sponges. (Some studies suggest sponges arose first.) Consequently, Browne’s as-yet unpublished findings disrupt the stepwise progression of digestive anatomy from one to two holes early in animal evolution.

One possibility is that the comb jellies evolved through-guts and anuslike pores on their own, independent of all other animals, over hundreds of millions of years. Alternatively, a through-gut and exit hole may have evolved once in an ancient animal ancestor, and subsequently became lost in anemones, jellyfish, and sponges. Perhaps if you’re an anemone or a sponge stuck to a rock, suggests Matsumoto, it’s better to push waste back into the current rather than below.

Browne is currently exploring the latter theory by seeing whether comb jellies activate the same genes when developing their pores that other animals do when growing an anus. If he finds that the genes are different, the evolution of our most unspeakable body part will no longer be considered the singular event zoologists long supposed. “We have all these traditional notions of a ladderlike view of evolution, and it keeps getting shaken,” says Kevin Kocot, an evolutionary biologist at the University of Alabama, Tuscaloosa.