 | This image shows: (a) a "lefty" snail, (b) a "righty," (c) a "righty" scarred by a crab's claw; (d) a crab's claw showing the "can opener" tooth. Photo of claw (d) by Eric Lazo-Wasem; courtesy Yale Peabody Museum |
Contrary to traditional wisdom, being a "lefty" promotes survival from attacks -- at least in the world of snails and crabs -- according to a report by researchers at Yale and Cornell in the Biology Letters of the Royal Society, UK.
While rare in nature, "lefties" are found in many organisms, including humans. Author Gregory P. Dietl, postdoctoral fellow in geology and geophysics at Yale has explored the survival advantages and disadvantages of being a "lefty" by studying snails and predatory crabs.
The overwhelming majority of snail species are "right-handed" -- that is, their shells coil clockwise. Dietl studied a species of snails that are "lefties" -- those with shells that coil counterclockwise.
"One of the conclusions we draw from the interaction between crabs and these snails is that in the adversarial mode, 'lefties' have a competitive advantage as long as they remain rare," says Dietl. This parallels some sporting competitions in which left-handed players enjoy an advantage over their right-handed opponents.
The functional advantage of this rare reversal in shell-coiling direction has evaded explanation. Scientists do know that snails mate most effectively with snails whose shells coil in the same direction -- thus, the more common "right-handed" snails have a mating advantage, keeping "righties" prevalent and "lefties" rare.
"Left-handed" snails, however, have an advantage against predators of opposite "handedness." Some predatory crabs are "righties" -- they have a specialized tooth on their right claw that acts like a can opener to crack and peel snail shells.
"The 'sinistral advantage,' or advantage to being 'left-handed,' is that it would be like using a can opener backwards for the crab to crack and peel the snail shell," explains Dietl.
The scientists measured the frequency of unsuccessful crab predation -- as indicated by repair scars on the snail's shell -- in specimens from the fossil record. Ten of the 11 species pairs showed more scars on the "right-handed" shells, suggesting that crabs are attacking them in preference to their "left-handed" counterparts. (A "right-handed" crab often abandons a "left-handed" snail before it breaks the shell, explains Dietl.) These traces of failed predation indicated to the researchers that "left-handed" coiling increases survival from attacks by "right-handed" crabs.
The authors' conclusion: "In the balance of selection processes, the survival advantage of sinistrality may be as important a selection pressure as mate selection. It is not all about sex all of the time."
-- By Janet Rettig Emanuel
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