Disposable Penis Helps Sea Slug Ensure Clean Mating
February 13, 2013

Disposable Penis Helps Sea Slug Ensure Clean Mating

Lawrence LeBlond for redOrbit.com - Your Universe Online

Disposable razors, disposable diapers, disposable cameras, disposable penises... err; wait a minute, disposable penises?

Yes disposable penises. That´s what´s new in the world of science today.

It seems a team of researchers from Osaka City University and Tokyo's Nihon University in Japan have discovered the first instance of creatures than can grow their own ℠disposable penises.´ This trait came to light after scientists had studied the mating habits of a particular sea slug called the red-and-white slug (Chromodoris reticulate). They found that this sea creature would lose its appendage after sex, only to grow a whole new one shortly thereafter.

"No other animal is known to repeatedly copulate using such 'disposable penises'," the team wrote in the Royal Society journal Biology Letters, describing the behavior as “extremely peculiar.”

This thumb-sized, non-shelled mollusk, which inhabits the warm tropical waters of Southeast Asia, needs only 24 hours after copulating to unroll an internally coiled and compressed juvenile penis that replaces the old, discarded unit, the team explained. They found that the creature can repeat this process at most three times.

But having a disposable organ is not the only oddity of this creature; its sex life is just as peculiar. Nearly all slugs of this family, collectively known as nudibranchs, are believed to be “simultaneous hermaphrodites,” meaning they have both male and female sexual organs and can use them both at the same time. During copulation, the slug gives sperm to a mating partner while simultaneously receiving sperm, which is stored for later insemination.

The team captured several of these creatures during a scuba diving mission and placed them in a tank to observe their mating habits up close. They watched as the slugs coupled for up to two minutes, and then upon completing coitus each slug crawled away with their penises dragging behind them, only to fall off about 20 minutes after copulation ended. A closer examination of the creatures´ anatomy revealed a tightly coiled spiral inside the slug´s body that was missing in those who had not recently had sex.

“We think this spiral is effectively a penis-in-waiting that will uncoil rapidly after penis disposal and grow into a new penis,” Ayami Sekizawa, leader of the study, told Mail Online's Harriet Arkell.

In fact, the team observed that the sea slugs were not able to mate again for 24 hours after copulation, suggesting it takes about that long for the new penis to uncoil and regrow into a new sexual organ. But why these creatures have such peculiar equipment is a mystery. One interesting feature the team discovered was that the discarded penises had backwards-facing spines. It´s possible these spines may make it difficult for the penis to be withdrawn after copulation, potentially explaining why the organ is disposable.

Nils Anthes, an evolutionary biologist from the German University of Tubingen, said it is possible that the penis is discarded after copulation to ensure a clean pairing in the future.

“If genetic work confirms that the penis spines are really loaded with rival sperm after retraction, then it is best for the animals to dispose of the penis like a dirty syringe,” Anthes said in an interview with Nature. “Otherwise they may risk delivering some rival sperm mixed with their own during later sexual encounters.”

“I haven't seen anything like this before,” Bernard Picton, a marine invertebrate specialist with National Museums Northern Ireland, said in an interview with the BBC.

Sea slugs are not the only creatures in the animal kingdom who discard their penises after sex. Orb weaver spiders are known to lose their organ after copulation, as well as the periwinkle and other land slugs in the genus Ariolimax.

However, the researchers believe this is the first evidence of a creature that can actually regrow and replace its old sexual organ, giving the sea slug a sexual advantage. With the initial penis abandoned along with any rival sperm, the slug then uses the new appendage to inject another dose of its own sperm, ensuring that its genes are the ones being passed on.

"They do have very, very complicated biology - and a lot have awfully complicated things in terms of reproduction," said Picton.