Called Chromodoris reticula, the thumb-sized slugs are hermaphroditic, possessing both male and female genitalia. During procreation, each slug inserts its thread-like, spine covered penis (located on its side) into its partner's vagina and then discards the appendage after expending its semen. The slug then stores the sperm for later insemination.
“No other animal is known to repeatedly copulate using such ‘disposable penes,'” the researchers wrote in the Royal Society Journal Biology Letters.
The slugs were captured during scuba dives in the warm waters of Southeast Asia. Scientists observing the creatures estimate that the slug requires 24 hours to regrow a new sex organ after copulation. Upon dissection, the team observed an internal structures, shaped as a spiral which they believe would eventually become the replacement penis. Each slug is able to regrow the organ a minimum of three times.
The team theorizes that the backward pointing spines on the penis may help the slug eliminate its partner's previous stores of sperm. “Chromodoris reticulata may compensate for the short-term cost of decreased reproductive opportunities caused by the loss of a penis with the reproductive advantage gained by sperm displacement,” they wrote.