If you're the kind of sea slug that cuts off its penis after use, that's no excuse for turning down a new partner the next day — and the day after that, Japanese researchers have found.

Ayami Sekazawa, a biologist at Osaka City University, and his scientific collaborators believe the ability of a certain sea slug to quickly regenerate a disposable penis is unique in the animal kingdom.

"No other animal is known to repeatedly copulate using such 'disposable penes,'" they reported in an article published Wednesday in the journal Biology Letters.

The paper explores in detail the bizarre sex life of a type of reddish sea slug known by the scientific name Chromodoris reticulata.

The researchers collected the creatures —  which look a bit like a foccaccia loaf with a pair of horns on one end and a flower stuck in the other — from shallow coral reefs off Okinawa, Japan, during their mating season, which runs from April to June.

The researchers paired the animals up in tanks in the lab and observed their mating behaviour.

Like most other sea slugs, individuals of this species have both male and female sex organs, located on opposite ends of their body. The researchers observed that the animals would line themselves up sideways, facing opposite directions, so that each could perform both the male role and the female role at the same time.

Once mating was completed, they would separate, and each partner would cut off and discard its penis.

The researchers found that if they paired one of the post-coital sea slugs with a new partner less than nine hours after its previous tryst, it sometimes mated again, but only in the female role.

However, if the researchers waited about 24 hours, the sea slugs typically had a new penis available to perform the male role and could once again mate "reciprocally."

In order to figure out how that was possible, the researchers examined the anatomy of the sea slug penis. They found that the penis discarded after mating was on average one centimetre long. However, a closer look showed that this part was just the tip of a much longer penis that was partly spiraled up inside the sea slug's body, with a total length of about three centimeters — making it "sufficiently long for at least three copulations."

That was consistent with the observation that one of their sea slugs was able to mate three times in a row, with each event separated by roughly 24 hours.

When the outer end of the penis "elongates to be used for copulation, part or all of the spiral structure seems to be uncoiled at the same time," the study said.

However, the newly uncoiled part did not seem to be immediately ready for use, as about 24 hours was needed for the tissue to mature.

Why throw out a perfectly good penis?

Of course, one of the researchers' big questions was why the sea slug cuts off and throws away its penis in the first place. The answer appears to have to do with the backward-facing spines visible on the discarded penises under a microscope, which had "a large amount of sperm" entangled in them.

It turns out that sea slugs can afford to be rather non-discriminating about their partners because they have two pouches where they can store sperm. That may allow them mate with multiple partners and then wait until later to choose which sperm to use among the samples they have collected.

The researchers propose that the penis spines are used to remove stored sperm from their partner's previous partners, but may make it difficult for the penis to be withdrawn again.

"This difficulty must be the essential reason" that the sea slug cuts it off after use, the researchers concluded.