Asymmetry: why it exists in animals

All but the simplest animals have a body with two distinct sides – one left and one right – that are roughly mirror images, at least in the embryo.

- JV CHAMARY

That ‘bilateral symmetry’ can even be seen in starfish before the larvae develop into adults with multiple arms radiating from the centre (‘radial symmetry’). But while left and right may look similar, the bodies of most creatures are only superficially symmetrical.

How are animals asymmetric?

Asymmetry can appear across the whole body. One example is flatfish: a juvenile starry flounder swims vertically and has eyes on opposite sides of its head, but one eye will migrate across its skull as it matures so both eyes end up on either the left or right of a lopsided adult. The male fiddler crab, meanwhile, has one major claw – used to fight rivals and impress females – that can reach half the animal’s total size.

Each body part can have asymmetry too. Parts can exist in mirror-image forms that can’t be superimposed on one another – like our right and left hands, for example. Known as ‘chiral’ structures, they can take either right-handed (dextral) or left-handed (sinistral) forms.

What determines the direction of the asymmetry?