A tiny worm which procreates by jabbing a needle-like penis into its own head has left biologists in Europe stunned.

Journal Proceedings of the Royal Society B reported that the bizarre beast is a microscopic, water-dwelling flatworm dubbed Macrostomum hystrix.

The species is a transparent worm about one millimetre long and a hermaphrodite with both male and female reproductive organs.

The experts theorised it may have evolved the unconventional method of self-impregnation in order to procreate "under conditions of low mate availability".

Normally, they exchange sperm with others of their kind using a needle-like protrusion called a stylet, which pierces the partner's outer body membrane in a method known to scientists as "traumatic wounding".

But when sex partners are scarce, it seems the worms can turn their stylets, located in the tail section, on themselves.

Biologists from the University of Basel in Switzerland and Bielefeld University in Germany studied the creatures in petri dishes in the lab.

Some of the worms were kept in small groups while others were isolated.

The studies involved measuring how much sperm each worm contained and the results indicated "strikingly different distributions".

The worms kept in groups contained larger volumes of sperm, mainly in the tail region, while the lone worms were found to have more sperm in the head region.

The team of biologists said the findings indicated that when deprived of a mate, the worm self-injects sperm.

"Including or even exclusively into their own head region," the team said.

The sperm migrates from there to the site of fertilisation in the centre of the tiny body.

AFP