The vast majority of animals, like humans, appear to exhibit mirror symmetry - not perfect, obviously, but the two halves generally match pretty well, at least on the outside.
I assume this is a function of the way they develop - although the innards seem to be less symmetrical, at least in humans, so it’s presumably not a necessity.
So, two questions really: why the symmetry, and what are the most notable exceptions to it? Off the top of my head I can think of fiddler crabs, although they’re still pretty symmetrical. Some flatfish, too, end up pretty crooked looking but you can see they’re based on a simple mirror-symmetrical form.
Are there any higher creatures that lack any such symmetry?
Well, if you’re only talking about **mirror-**symmetry, there are always star-fish (five-way radial [“pentaradial”] symmetry) - although apparently even they still have vestigial mirror-symmetry as larvae. Dunno if that’s “higher” enough for you…
Otherwise I’ll leave it to better and wiser posters to come up with real answers.
According to wikipedia’s article on symmetry in biology, “The notable exception among animals is the phylum Porifera (sponges) which have no symmetry”.
“Mirror Symmetry” means there is an axis of symmetry – e.g., from head to toes through a human. Starfish don’t really have that kind of symmetry - they have radial, or rotational symmetry – stick a (virtual!) pin through the center of their body and rotate them around that pin, one, two, three-fifths of a full rotation.
Most of the asymmetrical examples you mention are still modifications of a symmetrical body plan. Fiddler crabs have the same general pattern of claw growth on either side, one just grows bigger. Same goes for vertebrate organs – our heart is basically symmetrical except one side is much bigger since it has more work to do. As far as I know, sponges are the only animals with a completely assymetric body plan.
It depends on what you mean by mirror symmetrical. If you simply mean that something can be split into two halves that are mirror images of each other, then yes – but in that case, “mirror symmetrical” simply means “symmetrical”. It seems like the OP may have been using the term to specifically mean bilateral symmetry, in which there is a single plane that splits the organism into symmetrical halves… most “higher animals” (e.g., chordates and arthropods) have this. Echinoderms (sea stars and their relatives), by contrast, have radial symmetry – they are symmetrical about a central point, so any plane that includes that point and is perpendicular to the organism will split it into symmetrical halves.
There are probably some mollusks that might appear pretty asymmetrical from the outside, although they might be symmetrical inside.
Actually, mirror symmetry means there is a plane of symmetry. Which starfish do have. As a matter of fact, they have 5. Think about it…one passes through each point and the opposite notch to divide the sea star into mirror halves.
They also have 1 axis of symmetry, a fivefold one.
Humans, on the other hand, just have the one plane.
Because the ancestral animals were presumably radially symmetrical. However once an organism starts crawling along the ocean floor it needs to concentrate the sense organs in the direction of travel. So you immediately have a font and a back. If you have a front and a back on a radially symmetrical animal then you have bilateral symmetry.
Some of the parasitic barnacles have lost pretty much all traces of symmetry in the adult form. Look at figure 2F herefor an example. These animal start out bilaterally symmetrical, but you wouldn’t guess it from the adult form. Once they enter the host they grow into an amorphous blob that sends out growths that wrap around the host’s organs.
I do think it’s worth noting that the symmetry is mostly external. Internal organs are all over the place. with humans, you’ve got heart on the left and liver on the right and not a whole lot of plan to the intestines (at least not a symmetry-type plan). Sometimes you have pairs of organs and sometimes you don’t.
In terms of addressing the “why” question, this suggests that movement and issues related to movement (balance and sensory organs, for example) really are the driving factors, since internal organs don’t play much of a role in movement.
Anyone who has watched a starfish try to walk can attest to how lousy radial symmetry is for walking. And sponges, the notable exception to any kind of symmetry, do not move at all.
(per wiki, I understand that there’snot complete agreement on htis): there’s two subkingdoms of animals: parazoa and eumetazoa.
Parazoa are the sponges and the enigmatic trichoplax and they generally have no discernible symmetry.
Eumetzoa is divided into radiatiata (jellyfish and allies) and bilateria (all animals ‘higher’ than jellyfishes). Radiatiata generally exhibit radial symmetry and bilateria bilateral symmetry and are guessed to evolved from ancestors who exhibit those symmetries.
So the answer to the question is that the only animals without any real symmetry and sponges and the trichoplax all other animals are evolved from ancestors who were symmetrical and the evidence of that symmetry is still there.
It’s possible to have symmetry without having mirror symmetry. Not to Godwinize the thread, but consider a swastika: It has fourfold rotational symmetry, but no mirror symmetry.
While we’re at it, many mollusks are most recognizable by their spiral shells. There, you get a combination of rotational, translational, and scaling symmetry which results in no symmetry at all. The gooey parts inside are probably still somewhat symmetrical, though.
There are several types of symmetry, and they result from physical factors.
Spherical symmetry results because there is a difference between the inside and the outside of an organism. Lots of microorganisms are spherically symmetrical.
Take a spherically symmetrical ball, and scale it up to the point where gravity is a factor, and you now have radial symmetry. The organism now has a top and a bottom. Once side faces the ocean surface, the other faces, or rests on, the substrate.
Now the organism starts to move about. Which way does it move? Well, it makes sense to cluster sense organs in the direction of movement. And it makes sense to put a mouth there as well. And it makes sense to put excretory organs on the other side, so the organism moves away from its waste. And so this create bilateral symmetry–different structures both up and down and forward and back.
However, going the next step–different structures for left and right–doesn’t usually make sense. There isn’t any overall systematic physical difference between things that happen on the right side of the organism vs things that happen on the left side. And so this is why true asymmetry is rare in mobile creatures. Yes, there is some asymmetry, but it’s usually a tweak or variation on an otherwise bilaterally symmetrical organism, like in fiddler crabs and snails.
True asymmetry seems to be confined to sessile organisms.
