By Amy MaxmenMar. 23, 2016
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.