Sea Slug Penises Fall Off After Sex and Grow Back Next Day

Sea slugs have disposable penises that fall off after sex then grow back 24 hours later.

A team of Japanese scientists from Osaka City University and Nihon University in Tokyo went scuba diving off the coast of Okinawa to collect the sea slugs in order to look at their mating habits.

Nudibranchs sea slugs, or Chromodoris reticulata, are hermaphrodites, in that they have both male and female reproductive organs.

Their reproductive organs are on the right side of their bodies and to mate, two sea slugs position themselves side by side so both can inseminate one another with sperm - the process involves two penises and two vagina-like organs at the same time.

However, what happened post-copulation astounded the researchers. They found that once the sea slugs had mated three times, their penis fell off and was discarded.

The collected sea slug specimens were placed in an aquarium in pairs. In some instances, both members had been isolated from others for 24 hours.

In other cases, a sea slug that had recently mated was placed with one that had been isolated.

Back-up penis

During the study, the team observed 108 pairings. They noticed that sea slugs that had recently mated were unable to do so again for at least 24 hours.

On closer inspection, they noticed that the sea slugs discarded their penises about 20 minutes after mating.

This suggests they do not mate for 24 hours because they do not have the anatomical ability to do so.

Sea slugs that had not recently mated had a tightly coiled spiral structure inside them, which was not present in those that had mated - suggesting it is a back-up penis, stored internally.

Published in the journal Biology Letters, the authors wrote: "Although the penises of individuals that had been fixed and cleared at least 24 hours since their last copulation had a spiral structure, those that had been fixed immediately after copulation did not.

"This suggests that the spiral structure is important in the autotomy and replenishment of the penis."

Explaining why they think the slugs dispose of their penises, the authors suggest it may be to remove any sperm left by previous sea slug partners.