A sea slug's unusual mating behavior -- it discards its penis after copulation, grows a new one, and has sex the next day -- is reported in the latest issue of Biology Letters.

The sea slug, Chromodoris reticulata, is a type of soft-bodied marine mollusk, and its disposable penis is very rare in the animal kingdom.

As for what happens after sex ... "The penis just falls off," lead author Ayami Sekizawa told Discovery News.

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Sekizawa, a researcher at Osaka City University, and colleagues made the discovery after studying C. reticulata individuals that they collected during scuba diving trips in shallow coral reef areas near Okinawa, Japan.

The researchers set up an experiment tank and watched as the sea slugs copulated 31 times.

These animals are "simultaneous hermaphrodites," meaning each performs both the "male role" of donating sperm to a mating partner and the "female role" of receiving sperm from the partner simultaneously during copulation.

A typical mating episode involves two individuals touching each other with their genital orifices. They then "project" their penises and each insert them into the other’s vagina and start copulation. After a short time, one removes its penis from the partner. Later, the other mate removes its penis too.

Both individuals then crawl away, with their elongated penises still dangling. The sexual organs, which feature backward-pointed spines for possibly trapping rival sperm, would then suddenly sever from their bodies and float away.

"The sea slug sheds 1/3 of the internal penis length after each copulation," Sekizawa said. "The sea slug is able to grow the penis gradually to its original length."

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The loss and regrowth doesn’t seem to hamper the sea slug’s active sex life.

"In one case," the researchers wrote, "we observed three successive copulations each separated by approximately 24 hours."

Sekizawa said that "we have no idea about the evolutional conditions for this unique mating behavior."

Only a few other animals have been found to "dispose" of their penis, or male reproductive appendages.

One other is Argonauta, a type of octopus. Some orb-weaving spiders will also shed organs used for mating.

Earlier studies on the periwinkle, a type of edible sea snail, found that they shed their penises after the reproductive season "probably to save the cost of maintenance," Sekizawa and his colleagues believe.

That tactic, however, may not apply to the sea slug, since it has to keep regrowing its penis.

Janet Leonard, a research associate at the University of California at Santa Cruz Institute of Marine Sciences, told Discovery News that she agrees with the new paper’s conclusions and that "similar phenomena (occur) in other species."

"Little is known about mating behavior in simultaneously hermaphroditic animals," Sekizawa said. "The disposable penis in our nudibranch (sea slug) study is merely one case of peculiar mating behavior" in these animals."