This Bizarre Organism Has A Disappearing Anus

Mnemiopsis leidyi close to the surface in the Caribbean sea, Costa Rica. Seaphotoart/Shutterstock

Everybody poops. But not everyone seems to poop in quite the same way. And one species of comb jelly, known as Mnemiopsis leidyi, might be unique in having an anus that appears when it’s needed and then disappears. The findings are reported in the journal Invertebrate Biology.

Mnemiopsis leidyi, also known as the warty comb jelly or sea walnut, seems to be the only one of the comb jellies to have such a configuration. Other species have a mouth, a through-gut, and a permanent anus. Other simple animals like jellyfish have only one opening, so waste goes out from where the food came in.

The warty comb jelly appears to be an intermediate stage between animals with and without a dedicated anus. Observations conducted by Sidney Tamm of the Marine Biological Laboratory in Woods Hole, Massachusetts, showed that once the little critter has expelled waste, its anus seem to vanishes.

“That is the really spectacular finding here,” Tamm told New Scientist. “There is no documentation of a transient anus in any other animals that I know of. It is not visible when the animal is not pooping. There’s no trace under the microscope. It’s invisible to me.”

Tamm used video recordings of the animals to study their digestive cycle. The earliest sign that defecation was about to happen was the change in the shape of their stomach. As the amount of waste increases in the animal, its gut gets larger and larger until it touches the outer layer, the epidermis, on the rear of the body. When they touch, they fuse together and suddenly an anus is born. Once the waste is expelled, the gut shrinks down, separates itself from the epidermis, and the anal opening once again ceases to exist.

Both the gut and the outer layer of the animal are just one cell thick, so the process can be done relatively quickly – once an hour for the adults, which can reach 5 centimeters (2 inches) in length, and six times as fast in the larvae. Tamm also noted that the diameter of the anal pore varied both during single defecations and over successive ones. For the larvae, who are 2 to 4 millimeters in diameter, the largest anus seen was around 40 microns.

Tamm hopes to study more comb jellys to prove that the anus pore really goes away and doesn't simply become too small to be seen by cameras. 

The sea walnut is a carnivore feeding on zooplankton, other comb jellies, eggs, larvae of fish, and even smaller individuals of its own kind. The species is capable of self-fertilization and are hermaphroditic. It is native to the Western Atlantic but due to naval trade, it has become an established invasive species in the Black Sea, the North and Baltic Sea, and even the Caspian Sea.

[H/T: New Scientist]

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