A scientist has discovered a new type of sea creature that has a "transient anus" - one that only appears when it is needed.

The creature is a warty comb jelly, or sea walnut, and looks similar to a jellyfish. However, jellyfish eat and expel digested food out of the same opening while this new creature has an anus that appears when it needs to expel digested food, before disappearing again.

Other comb jellies have separate mouths and anus.

This is thought to be the first animal ever discovered with this type of anus that becomes invisible when the animal is not defecating.

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"There is no documentation of a transient anus in any other animals that I know of," Dr Sidney Tamm of the Marine Biological Laboratory told New Scientist.

He called it a "really spectacular finding".

As the creature accumulates waste in the gut, it swells and pushes against its skin until an opening appears and the waste is released.

Scientists are looking at this as an important discovery that can shed light into the evolution of digestive systems.