With the exception of sperm whales鈥� off-centre blow-holes and some crabs鈥� single large claw, all complex organisms I can think of are effectively symmetrical along one plane of their body. What is the least symmetrical organism?
Flatfish received the largest vote, but there are plenty of other strange candidates out there 鈥� Ed
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鈥� The least symmetrical organism is the halibut, which has both eyes on the same side of its head.
Donald Windsor, Norwich, New York, US
鈥� There are different symmetries in nature. We tend to assume bilateral symmetry is normal because that is what we and most of the organisms we notice (vertebrates and arthropods) display. But bilateral symmetry is the exception rather than the rule; many creatures exhibit radial or even spherical symmetry. Some alter their symmetry over time 鈥� for example, a starfish will start out as a bilaterally symmetrical larva and become radially symmetrical as it ages. Humans, a few days after conception, are basically a spherically symmetrical organism called a morula.
鈥淲e tend to assume bilateral symmetry is normal because that is what vertebrates display鈥�
Many organisms do not have any clear geometrical symmetry but demonstrate some kind of fractal symmetry, where their structures look similar at a variety of scales. Many plants and fungi are a bit of both: think of the leaves and the apples on an apple tree.
Humans are not quite bilaterally symmetrical: our liver is on the right side, our spleen on the left, while our right lung has three lobes and the left two. We even slip into fractal symmetry when it suits the purpose: take a close look at the capillaries which transport blood to the tissues. We are not even superficially symmetrical. Next time you get out of the bath ask yourself, 鈥淒o they both hang the same?鈥� This works for either sex.
We are all changed and shaped by both our genes and our environment. To put it another way, we all conform to a pattern while being eccentric. Heck, that鈥檚 life.
David Hopkins, Smethwick, West Midlands, UK
鈥� Members of genus Histioteuthis (see two photos below), a group of squid that live up to 1000 metres down, are unique in the animal kingdom as their left eye is two to three times the size of the right. The reasons for this trait, which gives rise to its common name of the cock-eyed squid, are unclear. There is also a corresponding asymmetry in the optic lobes of the squid鈥檚 brain. The specimen pictured was filmed on board ship after being caught off the coast of California.
Ron Douglas, Department of Optometry & Visual Science, City University London, UK
One suggestion is that the depth at which cock-eyed squid live is about as far down as sunlight can penetrate. The squid trains one eye on the illuminated water above while the other looks down into blackness 鈥� Ed
鈥� Asymmetry is commonest among organisms that have little need of well-defined structures in their bodies. Some algae, fungi and sponges never developed much symmetry, while parasites can abandon symmetry when they grow opportunistically to secure food. An example of the latter is Sacculina, a barnacle that injects its soft body through a crab鈥檚 shell and then grows a lump of reproductive tissue plus a tangle of feeding filaments throughout the crab鈥檚 body. And some members of the group of tiny crustaceans known as copepods form shapeless reproductive sacs within cysts in the flesh of fishes. Such creatures need no symmetry.
Jon Richfield, Somerset West, South Africa
鈥� There is a wonderfully quirky group of asymmetric barnacles called the (see photo, bottom). They are either 鈥渞ight-handed鈥� or 鈥渓eft-handed鈥� 鈥� apparently a random choice 鈥� their form being determined by the loss of calcareous plates from either the right or left side of the shell wall, plus a reduction in the number of plates in the shell鈥檚 lid to two from the usual four. Why they have adopted this asymmetrical form is uncertain, particularly as both forms occur together. Their soft tissue, as it happens, retains bilateral symmetry. These and other asymmetrical barnacles were first comprehensively described by Charles Darwin in a monograph published in 1854.
John Buckeridge, RMIT University, Melbourne, Australia