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. 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?
The direction of asymmetry can be fixed or random. Fixed asymmetries include a narwhal’s tusk – an elongated left tooth with a left-handed spiral in 100 per cent of individuals. By contrast, studies of American lobsters show that the dominant crusher claw forms from whichever side is preferred as a juvenile, so the proportion of left to right ends up 50:50. Generally, random asymmetry is determined by chance and influenced by environmental cues, whereas fixed asymmetry is largely inherited and programmed by genes.
Is asymmetry common?
Yes. Asymmetric features can be striking, but many are subtle. The vast majority of animals have fixed asymmetries that aren’t visible externally. The position of the human heart (as well as the stomach and spleen) is off-centre to the left of the body, while the liver and gall bladder are to the right, for instance. Lungs typically differ in many vertebrates: humans have two lobes on the left and three on the right side; mice have one lobe on the left and four on the right; and most snakes have only one functioning lung on the right side.
In rare cases, genetic mutations or accidents during development can create reversed mutants: having organs in an
WITH EVOLUTIONARY BIOLOGIST JV CHAMARY abnormal, mirror-image arrangement (heart on right) gives the rare condition ‘situs inversus’. It causes health problems for some individuals with the condition, but most suffer no ill effects.
What causes asymmetry to develop? Forming a symmetrical body is the default path during the development of an embryo, so the process of establishing distinct left and right sides is called ‘symmetry breaking’. Details vary among animal groups, but it’s triggered by the chirality (handedness) of molecules that interact with the cell’s scaffolding system, the cytoskeleton. Just as being left or righthanded affects how people manipulate objects, chiral molecules transfer their asymmetry to the body by shaping cells.
Why does asymmetry evolve?
Natural selection has favoured some asymmetries directly, like flatfish adapted to life as bottom-feeders. Other features are indirect by-products of evolution, like the arrangement of internal organs. According to one theory, having a gut longer than the body enabled vertebrates to absorb nutrients efficiently. But to pack in that extra length the gut had to start by twisting either left or right, which had knock-on effects within the limited space of the body cavity.
What’s special about snails?
A snail’s asymmetry is clear from the direction of the coil of its shell. Snails are the only animal group in which individuals flip chiral form without causing disease.
Compared to common ‘righties’, rare leftie garden snails have genitals on the other side of their heads. That causes difficulty in mating, unless you’re internet ‘shellebrity’ Jeremy the Snail, who produced offspring after geneticist Angus Davison recruited citizen scientists to find other lefties for Jeremy to mate with.