Sea slug carries disposable penis, plus spares

A bristly hermaphroditic sea slug mates employing a use-it-then-lose-it penis, and carries one or two extras for future use, researchers have discovered.

Some 20 minutes after copulating, the still-stretched-out penis of a Chromodoris reticulata sea slug “just falls off,” says evolutionary biologist Ayami Sekizawa of Osaka City University in Japan. The sea slug then cannot mate for a matter of hours. But when researchers waited 24 hours to offer the slug a second partner, a backup organ segment appeared in place of the discard, she and her colleagues report February 13 in Biology Letters.

“New tissue emerges like lead in a mechanical pencil,” Sekizawa says.

The source of that emerging lead turned out to be lengths of spare penis tissue coiled in reserve below the currently operative segment, Sekizawa found in studying tissue samples under a microscope. Given a little time between matings, the sea slug can essentially advance the pencil lead to extend three usable sections before having to replenish the reserves by regrowing the whole structure.

A detachable penis may be especially useful when animals compete with other potential fathers. On the surfaces of used slug penises, Sekizawa found abundant backward-pointing barbs, some of which had snagged masses of sperm. She and her colleagues propose that the barbed surface scours out sperm from a previous mating partner. But the same scouring barbs may make the penis difficult to retract, and all in all easier to simply discard.

A string of three “tear-off” penises is how Janet Leonard of the University of California, Santa Cruz describes the sea slug’s genitalia. The banana slugs she studies, of the genus Ariolimax, also part with genitals but in a different way. “On rare occasions, an Ariolimax acting as a female — these are hermaphrodites — will chew off its partner’s penis, a process caused apophallation,” she says. “On even rarer occasions an Ariolimax will chew off its own penis.” Yet these events are rare, so the report of sea slugs’ routine genital disposal with back-ups is unique, as far as she knows, and “quite exciting,” Leonard says.

Sekizawa and her colleagues got the first hint of the Chromodoris detachable penis when they noticed a sea slug with a truncated organ during an earlier experiment. Watching 31 matings, the researchers determined that each slug extends its penis into the reproductive tract of the other, with mutual sperm delivery lasting nine minutes on average. As the slugs disengage, they each crawl around, penis extended. Finally the penis, with no preliminary shrinking or pinching inward, simply detaches.

Several evolutionary paths could lead to detachable penises, says evolutionary ecologist Nico Michiels  of the University of Tübingen in Germany. In many species, a male plugs the female reproductive tract and stymies a rival, at least temporarily, Michiels says. An abandoned penis could certainly serve as a plug.

Also, a left-behind penis could autonomously inject sperm after the donor flees the dangers or constraints of more-intimate mating. “Not as wild as it sounds,” Michiels says. “Male bees have their whole male copulatory system ripped out during copulation, and it continues to pump sperm into the queen even after the male has gone to die.”