'Why Some Snails Are Left–Handed'

Hands–down (ooh — my bad) the best headline of the week past.

It appeared in the March 25 issue of the Economist over an article explaining just why snails with sinister tendencies tend to live longer.

Here's the story.

On the one hand

Why some snails are left-handed

In the human world, the left-handed seem at quite a disadvantage.

Indeed, a whole industry has sprung up to supply this oppressed minority with everything from left-handed computer mice to mugs and boomerangs.

The world of snails, too, has its sinister minority.

Hold a snail with the tip of its shell pointing upwards and the opening towards you, and the shell will normally coil away to the right.

Less commonly, it will coil to the left.

Until now, biologists have been at a loss to explain what advantage these rare reversals in shell-coiling offer.

But Gregory Dietl, of Yale University, and Jonathan Hendricks, at Cornell, think they know the answer.

Dr Dietl and Dr Hendricks, who have just published their results in Biology Letters, looked at the question by examining fossil sea snails.

They studied 1,722 specimens from six different species and found that right-handed snails were far more likely than left-handers, on a per-snail basis, to show evidence of a lethal confrontation with a crab.

What they suspect is happening is that left-handed snails avoid the attentions of right-handed crabs because these dexterous crustaceans find it tricky to eat lefties.

To appreciate this theory fully, you need to understand a little about crab table manners.

Modern-day right-handed crabs (below, a right–handed flame box crab)



orient their prey with the shell pointing away from their body and the opening on the right-hand side.

This allows the crab to insert a large toothlike appendage on its right-hand claw into the opening, thus cracking the snail's shell.

The rest of the shell is then rolled towards the tooth in an anticlockwise direction, in order to remove it.

The process is rather like peeling a potato with a right-handed peeler. Peeling in the other direction is awkward.

Thus, a left-handed snail presents a right-handed crab with a challenge—and one that it usually cannot be bothered to accept.

Indeed, when Dr Dietl and Dr Hendricks tested their hypothesis they observed crabs picking up left-handed snails and then abandoning them.

For humans, the equivalent is probably those really annoying pistachio nuts that accumulate at the bottom of the bag.

They are simply more trouble to open than they are worth, and are thus likely to be tossed aside.

It all seems to parallel ideas about left-handedness in human cultures.

Left-handers enjoy an advantage in some sports, such as tennis.

In the past, it is theorised, they enjoyed a similar advantage on the battlefield.

But that raises a further question: if left-handedness is beneficial, then why is it so rare?

Here, Dr Dietl and Dr Hendricks, like many others before them, admit they are stumped.

One possibility is that lefty snails have difficulty mating with their right-handed relations.

Here, at least, parallels with humans appear to end.

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Can't get enough?

Here's a link to an excellent BBC story on the findings.