Snails with left-handed shells can have a big advantage in life - predators may find it impossible to eat them.
That is the conclusion of research just published in the Royal Society's journal Biology Letters.
Scientists from the US examined whelks and cone shells preyed on by the crab Calappa flammea.
They found the crab is unable to open left-handed shells because it only has a tool for peeling them on its right claw; so it discards them.
"The crabs have a special tool on their claw, a tooth that's used like a can-opener," said Gregory Dietl from Yale University.
"So, if you imagine trying to use a right-handed can-opener with your left hand - it's very hard to do," he told the BBC News website.
Old food
The fossil shells which Dr Dietl and colleagues used in their study date from between 1.5 and 2.5 million years ago.
The scientists identified 11 whelks and cone shells which exist, or existed, in both right- and left-handed forms.
"The best way to visualise it is to imagine you have the shell in your hand with the pointed end upwards and the opening towards you," said Dr Dietl.
"If the opening is on the right-hand side, it's a right-handed shell or a dextral shell; if it's on the left, it's left-handed or sinistral."
Many bear the scars of attempted evisceration by crab.
Ten out of the 11 pairs showed higher rates of scars on dextral shells, suggesting that crabs are attacking them in preference to their left-handed counterparts.
If you assume that left-handed and right-handed whelks and cone shells would be equally tasty, something else must be causing the crabs to prefer those of one orientation.
In the behaviour of C. flammea, more commonly known in North American waters as the flame box crab, researchers found a clue.
Sinister difficulties
Typically a crab grasps a shell with the pointed end away from its body.
With a dextral shell, this means the opening is on the right, and the special "tooth" on its claw can break in. But with a sinistral shell, either the opening is on the left, or it has to grasp the pointed end towards its body.
Both solutions are apparently too much trouble; the crab simply moves on.
"They picked them up, and they just dropped them," said Dr Dietl.
"If we left them for a long period of time they would probably figure it out; but in nature these left-handed shells are really rare."
The evolutionary question is why these left-handed forms have remained so rare - some have even gone extinct - if they escape death by crab more easily.
In humans, left-handers make up about 10-13% of the population; but in some competitive situations, including such sports as tennis, cricket and boxing, they are much more prevalent and dominant than that figure would suggest.
At the last cricket World Cup, left-handed batsmen scored more runs, batted for longer and were more likely to bat in the top of the order than right-handers.
It is the relative rarity of left-handed batsmen which seems to confer advantage. One theory holds that right-handed bowlers struggle against them because they do not face them that often; if they did, they would learn how to get them out quicker.
Presumably, if left-handed marine snails became more common, crabs would eventually evolve apparatus or techniques for eating them, and their advantage would disappear.
But that cannot explain why in some populations they persist only in extremely low proportions, about 1%, or why in others they have gone extinct; other factors must be at play.
Sinistral snails apparently find it much harder to find a mate, and so may be doomed to remain rare or die out completely, whether or not they evade can-opening crabs.
Shell images courtesy G Dietl/J Hendricks/Biology Letters
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