A scientist has stumbled upon a creature with a “transient anus” that appears only when it is needed, before vanishing completely.

The warty comb jelly, or sea walnut, looks a lot like a jellyfish, but the new finding reveals it may represent a critical stage in evolutionary history.

While regular jellyfish eat food and release waste out of the same opening, these creatures have an anal pore that ejects digested food once the gut has filled up. 

Dr Sidney Tamm of the Marine Biological Laboratory, who made the discovery, said it was a “really spectacular finding”.

“There is no documentation of a transient anus in any other animals that I know of,” he told New Scientist.

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Most other comb jellies have a separate mouth and anus, demonstrating they are a separate group from jellyfish.

However, after examining samples of the warty comb jelly, Dr Tamm could not initially find any trace of an anus on this species. 

“It is not visible when the animal is not pooping,” he explained. 

“There’s no trace under the microscope. It’s invisible to me.”

As the animal’s gut accumulates waste, Dr Tamm suggests that it swells and pushes up against its skin. At this point, an opening appears and the waste is released.

The discovery was published in the journal Invertebrate Biology.

As the comb jelly family is over 500 million years old, the biologist suggests it may shed light on the evolution of a permanent anus in the animal lineage.

The temporary anus may serve as evidence that these ancient creatures are a kind of missing link between animals that evolved proper anuses, like humans, and those that did not, like jellyfish.