By Josie Glausiusz | October 12, 2010 | Comments Off
A few years ago a friend of mine gave a party and screened the movie Microcosmos for the revelers. Perhaps it was the punch I had imbibed, but I seem to recall that the film – a montage of mesmerizing bug scenes including ants drinking from a dewdrop and caterpillars moving in single-file – had a strangely psychedelic effect on my brain. Especially hypnotic was the sight of two snails mating, their slimy bodies roiling and coiling around each other on a carpet of moss.
I thought of that snail sex scene when I read about some spiffy mollusc research recently published by a group of Swedish marine ecologists at the University of Gothenburg. This group of intrepid scientists have discovered that the females of the marine snail species Littorina saxatilis, or rough periwinkle, conceal their gender identity in order to avoid mating too much. They do this by refusing to label their mucus trails with chemical signals indicating their sex.
Females of most snail species secrete a substance in their mucus trails that enables males to track them. To quote from the paper published by Kerstin Johannesson and her colleagues, on finding a female, a male littorinid snail “mounts the female and after a counter-clockwise movement on the shell, he stops at the right-hand side of the shell and inserts the penis under the shell of the partner.”
So what’s with the disguise? Turns out that mating carries a cost to female L. saxatilis, since she has to carry the male during mating and risks being dislodged from intertidal rocks by waves, thus increasing her risk of being eaten by crabs and fishes. To quantify this cost, the researchers glued male L. saxatilis shells onto females of the same species (to simulate mounting males), allowed the females to attach themselves to a PVC platform, and then towed the platform at increasing speeds along the bottom of a six-meter tank. The trials showed that females with a glued “pairing male” were dislodged from the platform at significantly lower water speeds than single females.
Despite concealing their sex, female rough periwinkles have no trouble reproducing: they live in densely-packed colonies, with hundreds of individuals slowly crawling across a single square meter of wet rock. Data show that each female simultaneously carry offspring sired by 10 to 20 males. By contrast, female periwinkle snails of other species that live at low densities are harder for males to find, so they must expend the energy advertising their sex via chemical cues in their mucus trails.
Male L. saxatilis snails often follow the trails of other males, so they end up spending twice as long looking for a female. But as Johannesson explains, mating is costly for females “and they already achieve more copulation than is required to fertilize all of their eggs.” Female rough periwinkles give birth to live young, known as “crawlaways” which are already housed in their shells at birth. Survival while carrying their offspring, it seems, is more important to these female snails than the constant lure of sex.
Knitted snail image courtesy of moon angel
Mating snail image courtesy of the University of Gothenburg