The warty comb jelly has an anus like no other imageBROKER / Alamy Stock Photo

A jellyfish-like creature has a neat trick that makes it unique among animals: its anus forms only when it needs to defecate, then disappears without a trace.

“That is the really spectacular finding here,” says Sidney Tamm of the Marine Biological Laboratory in Woods Hole, Massachusetts, who made the discovery. “There is no documentation of a transient anus in any other animals that I know of.”

Tamm thinks the discovery might represent an intermediate stage in evolution.


In some simple animals, such as jellyfish, the gut has only one opening, which functions as the mouth and anus.

It has been known since 1850 that comb jellies – which superficially resemble jellyfish, but belong to a separate group called ctenophores – have a through-gut, with a separate mouth and anus. Some even have more than one anus.

Missing anus

But when Tamm studied the warty comb jelly (Mnemiopsis leidyi), he could not find its anus. Only when the animals are actually defecating does a tiny opening appear – and it disappears again straight afterwards.

“It is not visible when the animal is not pooping,” says Tamm. “There’s no trace under the microscope. It’s invisible to me.”

His observations show there is no permanent connection between the gut and the rear of the body. Instead, as waste accumulates, part of the gut starts to balloon out until it touches the outer layer, or epidermis.

The gut then fuses with the epidermis, forming an anal opening. Once excretion is complete, the process is reversed and the anus vanishes.

Because both the gut and epidermis are just a single cell layer thick, this can happen relatively easily and quickly. The animals defecate at regular intervals: once an hour in the 5-centimetre-long adults, and once every 10 minutes or so in the larvae.

Tamm thinks this process of the gut bulging and fusing with the epidermis to form an anus is how the anus evolved in the first place. The intermittent anus may represent an intermediate stage in which the structure has yet to become permanent.

Today, the transient anus may be unique to the warty comb jelly. Tamm is now looking at other species of comb jelly, but so far they appear to have permanent anuses.

Journal reference: Invertebrate Biology, DOI: 10.1111/ivb.12236