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
added .
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.
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.
