Among the enormous diversity of cichlid fish living in Lake Tanganyika in eastern Africa, one group in particular has evolved a most unusual feeding habit: they sneak up behind other fish and pick their scales off, approaching every time either from the left or right side. You can easily see whether a fish is a lefty or righty by the shape of its mouth, which is hugely lopsided, bending around to one side or the other, like a pair of tweezers with the ends bent over.
Now researchers Thomas Stewart and Craig Albertson from Syracuse University in the US, have discovered that these fish are genetically programmed to be left or right -jawed. The reason both lefty and righty fish are found in a population, rather than one dominating, is because there will always be an advantage for the minority form. If there are more righty cichlids around, the prey fish learn to expect an attack from the right, and so lefty fish can easily sneak in and get a bite to eat.
But, it turns out that the situation is more complicated that was originally thought, because some scale eaters start out life with a straight mouth, pointing neither left nor right. More studies are now needed to work out what happens to these young straight-mouths that don't seem to make it to adulthood. It could be that they aren't good enough at hunting and so don't survive. Or as they grow up, their mouths could bend around to the left or right.
It just goes to show that from flat fish with eyes that migrate from one side of their head to the other, and human beings who have hearts usually on the left side, not everything in life is neat and symmetrical.