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Back-scratching. Each of two hermaphrodite slugs extends its penis to fasten a sperm package (not visible) on its mate's back.

Slug Sex Shocker

Sexual selection, which tends to favor animals that win many mates, has spiced up the world with stag antlers, cicada concertos, and other wonders. Biologists have long assumed that sexual selection is possible only with separate sexes. But recent studies on mollusks reveal that, paradoxically, sexual selection acts even in the absence of males and females.

That conclusion is backed up by a new study of the hermaphroditic sea slug Aeolidiella glauca. Each individual possesses both male and female organs, and although capable of self fertilization, they've developed a bizarre mating process with each other. Intrigued by this courtship, marine biologist Anna Karlsson of the University of Uppsala, Sweden, and evolutionary biologist Martin Haase of the University of Basel, Switzerland, closely watched the affair.

First, as Karlsson and Haase will reveal later this year in Animal Behaviour, the slugs circle one another until their sexual openings, found on their heads, touch. Then each extends a massive penis, about one-third the length of its body, and deposits a package of sperm on its partner's back. After copulation, sperm begin to escape from this so-called spermatophore and slither along the skin to the sexual opening and the eggs that lie within. A few hours later, the slug disrupts the sperm parade by ingesting the spermatophore.

This curious method of insemination might have evolved to help males avoid putting their sperm in competition with those of other slugs, the researchers said at the World Congress of Malacology last month in Vienna. When offered the choice of mating with a partner that carried a spermatophore on its back or with one from which the sperm capsule had been removed, the slugs preferred the "vacant" partner. By leaving its sperm on top of the mate, says Haase, a slug may be signaling to competitors that its partner is spoken for.

Other players in the burgeoning field of hermaphrodite sex are taken aback by such unorthodox copulation. Zoologist Ronald Chase of McGill University in Montreal, Canada, who studies the use of "love darts" in hermaphroditic land snails (ScienceNOW, 6 July) and sees his share of bizarre mollusk mating, calls this version "very strange." He adds that snails, slugs, and their relatives may have many more surprises in store for evolutionary biologists.

Related sites

University of Basel's page on sexual selection in hermaphrodites
Anna Karlsson's home page
Current issue of Animal Behaviour